Linked Lists: Locking, Lock-Free, and Beyond …

Linked Lists: Locking, Lock-Free, and Beyond …
Companion slides for The Art of Multiprocessor Programming by Maurice Herlihy & Nir Shavit

Last Lecture: Spin-Locks
. CS spin lock critical section Resets lock upon exit Art of Multiprocessor Programming

Today: Concurrent Objects
Adding threads should not lower throughput Contention effects Mostly fixed by Queue locks Art of Multiprocessor Programming

Today: Concurrent Objects
Adding threads should not lower throughput Contention effects Mostly fixed by Queue locks Should increase throughput Not possible if inherently sequential Surprising things are parallelizable Art of Multiprocessor Programming 4 4

Coarse-Grained Synchronization
Each method locks the object Avoid contention using queue locks Art of Multiprocessor Programming

Each method locks the object Avoid contention using queue locks Easy to reason about In simple cases Art of Multiprocessor Programming 6 6

Each method locks the object Avoid contention using queue locks Easy to reason about In simple cases So, are we done? Art of Multiprocessor Programming 7 7

Sequential bottleneck Threads “stand in line” Art of Multiprocessor Programming

Sequential bottleneck Threads “stand in line” Adding more threads Does not improve throughput Struggle to keep it from getting worse Art of Multiprocessor Programming 9 9

Sequential bottleneck Threads “stand in line” Adding more threads Does not improve throughput Struggle to keep it from getting worse So why even use a multiprocessor? Well, some apps inherently parallel … Art of Multiprocessor Programming 10 10

Art of Multiprocessor Programming
This Lecture Introduce four “patterns” Bag of tricks … Methods that work more than once … Art of Multiprocessor Programming

This Lecture Introduce four “patterns” Bag of tricks … Methods that work more than once … For highly-concurrent objects Concurrent access More threads, more throughput Art of Multiprocessor Programming 12 12

First: Fine-Grained Synchronization
Instead of using a single lock … Art of Multiprocessor Programming

Instead of using a single lock … Split object into Independently-synchronized components Art of Multiprocessor Programming

Instead of using a single lock … Split object into Independently-synchronized components Methods conflict when they access The same component … At the same time Art of Multiprocessor Programming

Second: Optimistic Synchronization
Search without locking … Art of Multiprocessor Programming

Search without locking … If you find it, lock and check … OK: we are done Oops: start over Art of Multiprocessor Programming

Search without locking … If you find it, lock and check … OK: we are done Oops: start over Evaluation Usually cheaper than locking, but Mistakes are expensive Art of Multiprocessor Programming

Third: Lazy Synchronization
Postpone hard work Art of Multiprocessor Programming

Postpone hard work Removing components is tricky Art of Multiprocessor Programming

Postpone hard work Removing components is tricky Logical removal Mark component to be deleted Art of Multiprocessor Programming

Postpone hard work Removing components is tricky Logical removal Mark component to be deleted Physical removal Do what needs to be done Art of Multiprocessor Programming

Fourth: Lock-Free Synchronization
Don’t use locks at all Use compareAndSet() & relatives … Art of Multiprocessor Programming

Don’t use locks at all Use compareAndSet() & relatives … Advantages No Scheduler Assumptions/Support Art of Multiprocessor Programming

Don’t use locks at all Use compareAndSet() & relatives … Advantages No Scheduler Assumptions/Support Disadvantages Complex Sometimes high overhead Art of Multiprocessor Programming

Linked List Illustrate these patterns … Using a list-based Set Common application Building block for other apps Art of Multiprocessor Programming

Set Interface Unordered collection of items Art of Multiprocessor Programming

Set Interface Unordered collection of items No duplicates Art of Multiprocessor Programming

Set Interface Unordered collection of items No duplicates Methods add(x) put x in set remove(x) take x out of set contains(x) tests if x in set Art of Multiprocessor Programming

List-Based Sets public interface Set<T> { public boolean add(T x); public boolean remove(T x); public boolean contains(T x); } Art of Multiprocessor Programming

List-Based Sets public interface Set<T> { public boolean add(T x); public boolean remove(T x); public boolean contains(T x); } Add item to set Art of Multiprocessor Programming

List-Based Sets public interface Set<T> { public boolean add(T x); public boolean remove(T x); public boolean contains(Tt x); } Remove item from set Art of Multiprocessor Programming

List-Based Sets public interface Set<T> { public boolean add(T x); public boolean remove(T x); public boolean contains(T x); } Is item in set? Art of Multiprocessor Programming

List Node public class Node { public T item; public int key; public volatile Node next; } Art of Multiprocessor Programming

List Node public class Node { public T item; public int key; public volatile Node next; } item of interest Art of Multiprocessor Programming

List Node public class Node { public T item; public int key; public volatile Node next; } Usually hash code Art of Multiprocessor Programming

List Node public class Node { public T item; public int key; public volatile Node next; } Reference to next node Art of Multiprocessor Programming

The List-Based Set Sorted with Sentinel nodes
-∞ a b c +∞ Sorted with Sentinel nodes (min & max possible keys) Art of Multiprocessor Programming

Reasoning about Concurrent Objects
Invariant Property that always holds Art of Multiprocessor Programming

Reasoning about Concurrent Objects
Invariant Property that always holds Established because True when object is created Truth preserved by each method Each step of each method Art of Multiprocessor Programming

Specifically … Invariants preserved by add() remove() contains() Art of Multiprocessor Programming

Specifically … Invariants preserved by add() remove() contains() Most steps are trivial Usually one step tricky Often linearization point Art of Multiprocessor Programming

Interference Invariants make sense only if methods considered are the only modifiers Art of Multiprocessor Programming

Interference Invariants make sense only if methods considered are the only modifiers Language encapsulation helps List nodes not visible outside class Art of Multiprocessor Programming 44 44

Interference Invariants make sense only if methods considered are the only modifiers Language encapsulation helps List nodes not visible outside class Art of Multiprocessor Programming 45 45

Interference Freedom from interference needed even for removed nodes Some algorithms traverse removed nodes Careful with malloc()& free()! We rely on garbage collection Art of Multiprocessor Programming

Abstract Data Types Concrete representation: Abstract Type: {a, b} a b Art of Multiprocessor Programming

Abstract Data Types Meaning of rep given by abstraction map S( ) = {a,b} a b Art of Multiprocessor Programming

Rep Invariant Which concrete values meaningful? Sorted? Duplicates? Rep invariant Characterizes legal concrete reps Preserved by methods Relied on by methods Art of Multiprocessor Programming

Blame Game Rep invariant is a contract Suppose add()leaves behind 2 copies of x remove() removes only 1 Which is incorrect? Art of Multiprocessor Programming

Blame Game Suppose add() leaves behind 2 copies of x remove() removes only 1 Art of Multiprocessor Programming

Blame Game Suppose add() leaves behind 2 copies of x remove() removes only 1 Which is incorrect? If rep invariant says no duplicates add() is incorrect Otherwise remove() is incorrect Art of Multiprocessor Programming

Rep Invariant (partly)
Sentinel nodes tail reachable from head Sorted No duplicates Art of Multiprocessor Programming

Abstraction Map S(head) = { x | there exists a such that a reachable from head and a.item = x } Art of Multiprocessor Programming

Sequential List Based Set
add() a c d remove() a b c Art of Multiprocessor Programming

Sequential List Based Set
add() a c d b remove() a b c Art of Multiprocessor Programming

Coarse-Grained Locking
b d Art of Multiprocessor Programming

b d c Art of Multiprocessor Programming

b d honk! honk! c Simple but hotspot + bottleneck Art of Multiprocessor Programming

Easy, same as synchronized methods “One lock to rule them all …” Art of Multiprocessor Programming

Easy, same as synchronized methods “One lock to rule them all …” Simple, clearly correct Deserves respect! Works poorly with contention Queue locks help But bottleneck still an issue Art of Multiprocessor Programming 61 61

Fine-grained Locking Requires careful thought “Do not meddle in the affairs of wizards, for they are subtle and quick to anger” We can improve concurrency by locking individual entries, rather than locking the list as a whole. Instead of placing a lock on the entire list, let us add a lock to each entry, along with lock() and unlock() methods. As a thread traverses the list, it locks each entry when it first visits, and sometime later releases it. Suchfine-grained locking permits concurrent threads to traverse the list together in a pipelined fashion. Art of Multiprocessor Programming

Fine-grained Locking Requires careful thought “Do not meddle in the affairs of wizards, for they are subtle and quick to anger” Split object into pieces Each piece has own lock Methods that work on disjoint pieces need not exclude each other We can improve concurrency by locking individual entries, rather than locking the list as a whole. Instead of placing a lock on the entire list, let us add a lock to each entry, along with lock() and unlock() methods. As a thread traverses the list, it locks each entry when it first visits, and sometime later releases it. Suchfine-grained locking permits concurrent threads to traverse the list together in a pipelined fashion. Art of Multiprocessor Programming 63 63

Hand-over-Hand locking
b c Art of Multiprocessor Programming

Removing a Node a b c d remove(b) Art of Multiprocessor Programming

Removing a Node a b c d remove(b) Art of Multiprocessor Programming 72 72

Removing a Node a b c d remove(b) Here is a naive approach to removing a node. Each thread locks the node being examined and its predecessor. Art of Multiprocessor Programming

Removing a Node a c d Why lock victim node? remove(b) Here is a naive approach to removing a node. Each thread locks the node being examined and its predecessor. Art of Multiprocessor Programming

Concurrent Removes a b c d remove(c) remove(b) Supposed we only locked one node and not two. Consider two concurrent threads that want to remove adjacent nodes. Art of Multiprocessor Programming

Concurrent Removes a b c d remove(c) remove(b) The red thread looks for the predecessor to c, while the green thread looks for the predecessor to b. Art of Multiprocessor Programming

Concurrent Removes a b c d remove(c) remove(b) Art of Multiprocessor Programming

Concurrent Removes a b c d remove(c) remove(b) Art of Multiprocessor Programming 83 83

Concurrent Removes a b c d remove(c) remove(b) Art of Multiprocessor Programming 84 84

Uh, Oh a c d remove(c) remove(b) But the final effect is to remove b but not c! Art of Multiprocessor Programming

Uh, Oh Bad news, c not removed a c d remove(c) remove(b) But the final effect is to remove b but not c! Art of Multiprocessor Programming

Problem To delete node c Swing node b’s next field to d Problem is, Someone deleting b concurrently could direct a pointer to c a b c a b c Art of Multiprocessor Programming

Insight If a node is locked No one can delete node’s successor If a thread locks Node to be deleted And its predecessor Then it works Art of Multiprocessor Programming

Hand-Over-Hand Again a b c d remove(b) Art of Multiprocessor Programming

Hand-Over-Hand Again a b c d remove(b) Found it! Art of Multiprocessor Programming

Hand-Over-Hand Again a c d remove(b) Art of Multiprocessor Programming

Removing a Node a b c d remove(c) remove(b) Art of Multiprocessor Programming

Removing a Node a b c d remove(c) Must acquire Lock for b Art of Multiprocessor Programming

Waiting to acquire lock for b
Removing a Node a b c d remove(c) Waiting to acquire lock for b Art of Multiprocessor Programming

Removing a Node a b c d remove(c) Wait! Art of Multiprocessor Programming

Removing a Node a b d Proceed to remove(b) Art of Multiprocessor Programming

Removing a Node a b d remove(b) Art of Multiprocessor Programming

Removing a Node a d remove(b) Art of Multiprocessor Programming

Removing a Node a d Art of Multiprocessor Programming

Remove method public boolean remove(T item) { int key = item.hashCode(); Node pred, curr; try { … } finally { curr.unlock(); pred.unlock(); }} Art of Multiprocessor Programming

Remove method public boolean remove(T item) { int key = item.hashCode(); Node pred, curr; try { … } finally { curr.unlock(); pred.unlock(); }} Key used to order node Art of Multiprocessor Programming

Predecessor and current nodes
Remove method public boolean remove(T item) { int key = item.hashCode(); Node pred, curr; try { … } finally { currNode.unlock(); predNode.unlock(); }} Predecessor and current nodes Art of Multiprocessor Programming

Make sure locks released
Remove method public boolean remove(T item) { int key = item.hashCode(); Node pred, curr; try { … } finally { curr.unlock(); pred.unlock(); }} Make sure locks released Art of Multiprocessor Programming

Remove method public boolean remove(T item) { int key = item.hashCode(); Node pred, curr; try { … } finally { curr.unlock(); pred.unlock(); }} Everything else Art of Multiprocessor Programming

Remove method try { pred = head; pred.lock(); curr = pred.next; curr.lock(); … } finally { … } Art of Multiprocessor Programming

Remove method lock pred == head try { pred = head; pred.lock(); curr = pred.next; curr.lock(); … } finally { … } Art of Multiprocessor Programming

Remove method try { pred = head; pred.lock(); curr = pred.next; curr.lock(); … } finally { … } Lock current Art of Multiprocessor Programming

Remove method try { pred = head; pred.lock(); curr = pred.next; curr.lock(); … } finally { … } Traversing list Art of Multiprocessor Programming

Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Art of Multiprocessor Programming

Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Search key range Art of Multiprocessor Programming

At start of each loop: curr and pred locked
Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; At start of each loop: curr and pred locked Art of Multiprocessor Programming

If item found, remove node
Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; If item found, remove node Art of Multiprocessor Programming

Remove: searching Unlock predecessor while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Art of Multiprocessor Programming

Remove: searching Only one node locked! while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Art of Multiprocessor Programming

Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; demote current Art of Multiprocessor Programming

Find and lock new current
Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = currNode; curr = curr.next; curr.lock(); return false; Find and lock new current Art of Multiprocessor Programming

Lock invariant restored
Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = currNode; curr = curr.next; curr.lock(); return false; Lock invariant restored Art of Multiprocessor Programming

Remove: searching while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Otherwise, not present Art of Multiprocessor Programming

Why remove() is linearizable
while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; pred reachable from head curr is pred.next So curr.item is in the set Art of Multiprocessor Programming

while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Linearization point if item is present Art of Multiprocessor Programming

while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Node locked, so no other thread can remove it …. Art of Multiprocessor Programming

while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Item not present Art of Multiprocessor Programming

while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; pred reachable from head curr is pred.next pred.key < key key < curr.key Art of Multiprocessor Programming

while (curr.key <= key) { if (item == curr.item) { pred.next = curr.next; return true; } pred.unlock(); pred = curr; curr = curr.next; curr.lock(); return false; Linearization point Notice that we could also have the linearization points be those when the locks are acquired. Art of Multiprocessor Programming

Adding Nodes To add node e Must lock predecessor Must lock successor Neither can be deleted (Is successor lock actually required?) successor lock not actually required! Art of Multiprocessor Programming

Same Abstraction Map S(head) = { x | there exists a such that a reachable from head and a.item = x } Art of Multiprocessor Programming

Rep Invariant Easy to check that tail always reachable from head Nodes sorted, no duplicates Art of Multiprocessor Programming

Drawbacks Better than coarse-grained lock Threads can traverse in parallel Still not ideal Long chain of acquire/release Inefficient Art of Multiprocessor Programming

Optimistic Synchronization
Find nodes without locking Lock nodes Check that everything is OK Art of Multiprocessor Programming

Optimistic: Traverse without Locking
b d e Aha! add(c) Art of Multiprocessor Programming

Optimistic: Lock and Load
b d e add(c) Art of Multiprocessor Programming

Optimistic: Lock and Load
b d e add(c) Art of Multiprocessor Programming 145 145

What could go wrong? a b d e Aha! add(c) Art of Multiprocessor Programming

What could go wrong? a b d e add(c) Art of Multiprocessor Programming 147 147

What could go wrong? a b d e remove(b) Art of Multiprocessor Programming 148 148

What could go wrong? a b d e remove(b) Art of Multiprocessor Programming 149 149

What could go wrong? a b d e add(c) Art of Multiprocessor Programming 150 150

What could go wrong? c a b d e add(c) Art of Multiprocessor Programming 151 151

What could go wrong? a d e add(c) Uh-oh Art of Multiprocessor Programming 152 152

Yes, b still reachable from head
Validate – Part 1 a b d e Yes, b still reachable from head add(c) Art of Multiprocessor Programming

What Else Could Go Wrong?
b d e Aha! add(c) Art of Multiprocessor Programming 154 154

What Else Coould Go Wrong?
b d e add(b’) add(c) Art of Multiprocessor Programming

What Else Coould Go Wrong?
b d e add(b’) b’ add(c) Art of Multiprocessor Programming 156 156

b d e b’ add(c) Art of Multiprocessor Programming 157 157

b d e add(c) Art of Multiprocessor Programming 158 158

Validate Part 2 (while holding locks)
b d e Yes, b still points to d add(c) Art of Multiprocessor Programming

Optimistic: Linearization Point
b d e c add(c) Art of Multiprocessor Programming

Same Abstraction Map S(head) = { x | there exists a such that a reachable from head and a.item = x } Art of Multiprocessor Programming

Invariants Careful: we may traverse deleted nodes But we establish properties by Validation After we lock target nodes Art of Multiprocessor Programming

Correctness If Nodes b and c both locked Node b still accessible Node c still successor to b Then Neither will be deleted OK to delete and return true Art of Multiprocessor Programming

Unsuccessful Remove a b d e Aha! remove(c) Art of Multiprocessor Programming

Yes, b still reachable from head
Validate (1) a b d e Yes, b still reachable from head remove(c) Art of Multiprocessor Programming

Validate (2) a b d e Yes, b still points to d remove(c) Art of Multiprocessor Programming

OK Computer a b d e remove(c) return false Art of Multiprocessor Programming

Correctness If Nodes b and d both locked Node b still accessible Node d still successor to b Then Neither will be deleted No thread can add c after b OK to return false Art of Multiprocessor Programming

Validation private boolean validate(Node pred, Node curry) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Art of Multiprocessor Programming

Predecessor & current nodes
Validation private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Predecessor & current nodes Art of Multiprocessor Programming

Validation private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Begin at the beginning Art of Multiprocessor Programming

Validation private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Search range of keys Art of Multiprocessor Programming

Predecessor reachable
Validation private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Predecessor reachable Art of Multiprocessor Programming

Validation private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Is current node next? Art of Multiprocessor Programming

Validation private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Otherwise move on Art of Multiprocessor Programming

Predecessor not reachable
Validation Predecessor not reachable private boolean validate(Node pred, Node curr) { Node node = head; while (node.key <= pred.key) { if (node == pred) return pred.next == curr; node = node.next; } return false; Art of Multiprocessor Programming

Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Art of Multiprocessor Programming

Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Search key Art of Multiprocessor Programming

Retry on synchronization conflict
Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Retry on synchronization conflict Art of Multiprocessor Programming

Examine predecessor and current nodes
Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Examine predecessor and current nodes Art of Multiprocessor Programming

Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Search by key Art of Multiprocessor Programming

Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Stop if we find item Art of Multiprocessor Programming

Remove: searching public boolean remove(T item) { int key = item.hashCode(); retry: while (true) { Node pred = head; Node curr = pred.next; while (curr.key <= key) { if (item == curr.item) break; pred = curr; curr = curr.next; } … Move along Art of Multiprocessor Programming

On Exit from Loop If item is present curr holds item pred just before curr If item is absent curr has first higher key Assuming no synchronization problems Art of Multiprocessor Programming

Remove Method try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.item == item) { pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Art of Multiprocessor Programming

Remove Method try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.item == item) { pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Always unlock Art of Multiprocessor Programming

Remove Method try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.item == item) { pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Lock both nodes Art of Multiprocessor Programming

Check for synchronization conflicts
Remove Method try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.item == item) { pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Check for synchronization conflicts Art of Multiprocessor Programming

target found, remove node
Remove Method try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.item == item) { pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} target found, remove node Art of Multiprocessor Programming

Remove Method try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.item == item) { pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} target not found Art of Multiprocessor Programming

Optimistic List Limited hot-spots Targets of add(), remove(), contains() No contention on traversals Moreover Traversals are wait-free Food for thought … Art of Multiprocessor Programming

So Far, So Good Much less lock acquisition/release Performance Concurrency Problems Need to traverse list twice contains() method acquires locks Art of Multiprocessor Programming

Evaluation Optimistic is effective if cost of scanning twice without locks is less than cost of scanning once with locks Drawback contains() acquires locks 90% of calls in many apps Art of Multiprocessor Programming

Lazy List Like optimistic, except Scan once contains(x) never locks … Key insight Removing nodes causes trouble Do it “lazily” Art of Multiprocessor Programming

Lazy List remove() Scans list (as before) Locks predecessor & current (as before) Logical delete Marks current node as removed (new!) Physical delete Redirects predecessor’s next (as before) Art of Multiprocessor Programming

Lazy Removal a a b c d Art of Multiprocessor Programming

Lazy Removal a a b c d Present in list Art of Multiprocessor Programming

Lazy Removal a a b c d Logically deleted Art of Multiprocessor Programming 198 198

Lazy Removal a a b c d Physically deleted Art of Multiprocessor Programming

Lazy Removal a a b d Physically deleted Art of Multiprocessor Programming

Lazy List All Methods Scan through locked and marked nodes Removing a node doesn’t slow down other method calls … Must still lock pred and curr nodes. Art of Multiprocessor Programming

Validation No need to rescan list! Check that pred is not marked Check that curr is not marked Check that pred points to curr Art of Multiprocessor Programming

Business as Usual a b c Art of Multiprocessor Programming

Business as Usual a b c remove(b) Art of Multiprocessor Programming

Business as Usual a b c a not marked Art of Multiprocessor Programming

Business as Usual a b c a still points to b Art of Multiprocessor Programming

Business as Usual a b c Logical delete Art of Multiprocessor Programming

Business as Usual a b c physical delete Art of Multiprocessor Programming

Business as Usual a b c Art of Multiprocessor Programming

New Abstraction Map S(head) = { x | there exists node a such that a reachable from head and a.item = x and a is unmarked } Art of Multiprocessor Programming

Invariant If not marked then item in the set and reachable from head and if not yet traversed it is reachable from pred Art of Multiprocessor Programming

Validation private boolean validate(Node pred, Node curr) { return !pred.marked && !curr.marked && pred.next == curr); } Art of Multiprocessor Programming

List Validate Method private boolean validate(Node pred, Node curr) { return !pred.marked && !curr.marked && pred.next == curr); } Predecessor not Logically removed Art of Multiprocessor Programming

List Validate Method private boolean validate(Node pred, Node curr) { return !pred.marked && !curr.marked && pred.next == curr); } Current not Logically removed Art of Multiprocessor Programming

List Validate Method private boolean validate(Node pred, Node curr) { return !pred.marked && !curr.marked && pred.next == curr); } Predecessor still Points to current Art of Multiprocessor Programming

Remove try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.key == key) { curr.marked = true; pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Art of Multiprocessor Programming

Remove try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.key == key) { curr.marked = true; pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Validate as before Art of Multiprocessor Programming

Remove try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.key == key) { curr.marked = true; pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Key found Art of Multiprocessor Programming

Remove try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.key == key) { curr.marked = true; pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} Logical remove Art of Multiprocessor Programming

Remove try { pred.lock(); curr.lock(); if (validate(pred,curr) { if (curr.key == key) { curr.marked = true; pred.next = curr.next; return true; } else { return false; }}} finally { pred.unlock(); curr.unlock(); }}} physical remove Art of Multiprocessor Programming

Contains public boolean contains(T item) { int key = item.hashCode(); Node curr = head; while (curr.key < key) { curr = curr.next; } return curr.key == key && !curr.marked; Art of Multiprocessor Programming

Contains public boolean contains(T item) { int key = item.hashCode(); Node curr = head; while (curr.key < key) { curr = curr.next; } return curr.key == key && !curr.marked; Start at the head Art of Multiprocessor Programming

Contains public boolean contains(T item) { int key = item.hashCode(); Node curr = head; while (curr.key < key) { curr = curr.next; } return curr.key == key && !curr.marked; Search key range Art of Multiprocessor Programming

(nodes may have been removed)
Contains public boolean contains(T item) { int key = item.hashCode(); Node curr = head; while (curr.key < key) { curr = curr.next; } return curr.key == key && !curr.marked; Traverse without locking (nodes may have been removed) Art of Multiprocessor Programming

Contains public boolean contains(T item) { int key = item.hashCode(); Node curr = head; while (curr.key < key) { curr = curr.next; } return curr.key == key && !curr.marked; Present and undeleted? Art of Multiprocessor Programming

Summary: Wait-free Contains
a b d c e Use Mark bit + list ordering Not marked → in the set Marked or missing → not in the set Art of Multiprocessor Programming

Lazy List a a b 1 d c e Lazy add()and remove() + Wait-free contains() Art of Multiprocessor Programming

Evaluation Good: contains() doesn’t lock In fact, its wait-free! Good because typically high % contains() Uncontended calls don’t re-traverse Bad Contended add() and remove() calls must re-traverse Traffic jam if one thread delays Art of Multiprocessor Programming

Traffic Jam Any concurrent data structure based on mutual exclusion has a weakness If one thread Enters critical section And “eats the big muffin” Cache miss, page fault, descheduled … Everyone else using that lock is stuck! Need to trust the scheduler…. Art of Multiprocessor Programming

Reminder: Lock-Free Data Structures
No matter what … Guarantees minimal progress in any execution i.e. Some thread will always complete a method call Even if others halt at malicious times Implies that implementation can’t use locks Art of Multiprocessor Programming

Lock-free Lists Next logical step Wait-free contains() lock-free add() and remove() Use only compareAndSet() What could go wrong? Art of Multiprocessor Programming

compareAndSet public abstract class CASObject { private int value; public boolean synchronized compareAndSet(int expected, int update) { if (value==expected) { value = update; return true; } return false; } … } Art of Multiprocessor Programming

compareAndSet public abstract class CASObject { private int value; public boolean synchronized compareAndSet(int expected, int update) { if (value==expected) { value = update; return true; } return false; } … } If value is as expected, … Art of Multiprocessor Programming

compareAndSet public abstract class CASOBJECT{ private int value; public boolean synchronized compareAndSet(int expected, int update) { if (value==expected) { value = update; return true; } return false; } … } … replace it Art of Multiprocessor Programming

compareAndSet public abstract class RMWRegister { private int value; public boolean synchronized compareAndSet(int expected, int update) { if (value==expected) { value = update; return true; } return false; } … } Report success Art of Multiprocessor Programming

compareAndSet public abstract class RMWRegister { private int value; public boolean synchronized compareAndSet(int expected, int update) { if (value==expected) { value = update; return true; } return false; } … } Otherwise report failure Art of Multiprocessor Programming

Lock-free Lists Logical Removal a a b 1 c c e Use CAS to verify pointer is correct Physical Removal Not enough! Art of Multiprocessor Programming 239 239

Problem… Logical Removal a a b 1 c c e d Physical Removal Node added Art of Multiprocessor Programming

The Solution: Combine Bit and Pointer
Logical Removal = Set Mark Bit a a b 1 c c e d Physical Removal CAS Fail CAS: Node not added after logical Removal Mark-Bit and Pointer are CASed together (AtomicMarkableReference) Art of Multiprocessor Programming

Solution Use AtomicMarkableReference Atomically Swing reference and Update flag Remove in two steps Set mark bit in next field Redirect predecessor’s pointer Art of Multiprocessor Programming

Marking a Node AtomicMarkableReference class Java.util.concurrent.atomic package Reference address F mark bit Art of Multiprocessor Programming

Extracting Reference & Mark
public Object get(boolean[] marked); Art of Multiprocessor Programming

Extracting Reference & Mark
public Object get(boolean[] marked); Returns mark at array index 0! Returns reference Art of Multiprocessor Programming

Extracting Mark Only public boolean isMarked(); Value of mark Art of Multiprocessor Programming

Changing State public boolean compareAndSet( Object expectedRef, Object updateRef, boolean expectedMark, boolean updateMark); Art of Multiprocessor Programming

If this is the current reference … And this is the current mark …
Changing State If this is the current reference … public boolean compareAndSet( Object expectedRef, Object updateRef, boolean expectedMark, boolean updateMark); And this is the current mark … Art of Multiprocessor Programming

…then change to this new reference …
Changing State …then change to this new reference … public boolean compareAndSet( Object expectedRef, Object updateRef, boolean expectedMark, boolean updateMark); … and this new mark Art of Multiprocessor Programming

Changing State public boolean attemptMark( Object expectedRef, boolean updateMark); Art of Multiprocessor Programming

If this is the current reference …
Changing State public boolean attemptMark( Object expectedRef, boolean updateMark); If this is the current reference … Art of Multiprocessor Programming

.. then change to this new mark.
Changing State public boolean attemptMark( Object expectedRef, boolean updateMark); .. then change to this new mark. Art of Multiprocessor Programming

Removing a Node CAS a b c d remove(c) Art of Multiprocessor Programming

Removing a Node failed CAS CAS a b c d remove(c) remove(b) Art of Multiprocessor Programming

Removing a Node a b c d remove(c) remove(b) Art of Multiprocessor Programming

Removing a Node a d remove(c) remove(b) Art of Multiprocessor Programming

Traversing the List Q: what do you do when you find a “logically” deleted node in your path? A: finish the job. CAS the predecessor’s next field Proceed (repeat as needed) Art of Multiprocessor Programming

Lock-Free Traversal (only Add and Remove)
pred curr CAS pred curr a b c d Uh-oh Art of Multiprocessor Programming

The Window Class class Window { public Node pred; public Node curr; Window(Node pred, Node curr) { pred = pred; curr = curr; } Art of Multiprocessor Programming

A container for pred and current values
The Window Class class Window { public Node pred; public Node curr; Window(Node pred, Node curr) { pred = pred; curr = curr; } A container for pred and current values Art of Multiprocessor Programming

Using the Find Method Window window = find(head, key); Node pred = window.pred; curr = window.curr; Art of Multiprocessor Programming

Using the Find Method Window window = find(head, key); Node pred = window.pred; curr = window.curr; Find returns window Art of Multiprocessor Programming

Using the Find Method Window window = find(head, key); Node pred = window.pred; curr = window.curr; Extract pred and curr Art of Multiprocessor Programming

Art of Multiprocessor Programming© Herlihy-Shavit 2007
The Find Method Window window = find(item); item At some instant, or … pred curr succ Art of Multiprocessor Programming© Herlihy-Shavit 2007

Art of Multiprocessor Programming© Herlihy-Shavit 2007
The Find Method Window window = find(item); item not in list At some instant, curr= null pred succ Art of Multiprocessor Programming© Herlihy-Shavit 2007

Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet(succ, succ, false true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} Art of Multiprocessor Programming

Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet (succ, succ, false, true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} Keep trying Art of Multiprocessor Programming

Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet (succ, succ, false, true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} Find neighbors Art of Multiprocessor Programming

Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet(succ, succ, false, true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} Not there … Art of Multiprocessor Programming

Try to mark node as deleted
Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet(succ, succ, false, true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} Try to mark node as deleted Art of Multiprocessor Programming

If it doesn’t work, just retry, if it does, job essentially done
Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet(succ, succ, false, true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} If it doesn’t work, just retry, if it does, job essentially done Art of Multiprocessor Programming

Remove public boolean remove(T item) { Boolean snip; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key != key) { return false; } else { Node succ = curr.next.getReference(); snip = curr.next.compareAndSet(succ, succ, false, true); if (!snip) continue; pred.next.compareAndSet(curr, succ, false, false); return true; }}} a Try to advance reference (if we don’t succeed, someone else did or will). If we don’t succeed then either someone already removed curr, or someone modified pred, for example, by inserting another node as pred’s successor, in which case we need not worry since some other later thread that will attempt to traverse the list will remove the marked curr node before traversing beyond it. Art of Multiprocessor Programming

Add public boolean add(T item) { boolean splice; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key == key) { return false; } else { Node node = new Node(item); node.next = new AtomicMarkableRef(curr, false); if (pred.next.compareAndSet(curr, node, false, false)) {return true;} }}} Art of Multiprocessor Programming

Add public boolean add(T item) { boolean splice; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key == key) { return false; } else { Node node = new Node(item); node.next = new AtomicMarkableRef(curr, false); if (pred.next.compareAndSet(curr, node, false, false)) {return true;} }}} Item already there Art of Multiprocessor Programming

Add public boolean add(T item) { boolean splice; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key == key) { return false; } else { Node node = new Node(item); node.next = new AtomicMarkableRef(curr, false); if (pred.next.compareAndSet(curr, node, false, false)) {return true;} }}} create new node Art of Multiprocessor Programming

Install new node, else retry loop
Add public boolean add(T item) { boolean splice; while (true) { Window window = find(head, key); Node pred = window.pred, curr = window.curr; if (curr.key == key) { return false; } else { Node node = new Node(item); node.next = new AtomicMarkableRef(curr, false); if (pred.next.compareAndSet(curr, node, false, false)) {return true;} }}} Install new node, else retry loop Art of Multiprocessor Programming

Wait-free Contains public boolean contains(T item) { boolean marked; int key = item.hashCode(); Node curr = head; while (curr.key < key) curr = curr.next; Node succ = curr.next.get(marked); return (curr.key == key && !marked[0]) } The wait-free \mContains{} method of the \cLockFreeList{} is the same as that of the \cLazyList{} with one small change: to test if \fCurr{} is marked we apply \lstinline+curr.next.get(marked)+ and check that \aMarked{0} is true. Art of Multiprocessor Programming

Only difference is that we get and check marked
Wait-free Contains public boolean contains(T item) { boolean marked; int key = item.hashCode(); Node curr = head; while (curr.key < key) curr = curr.next; Node succ = curr.next.get(marked); return (curr.key == key && !marked[0]) } Only difference is that we get and check marked The wait-free \mContains{} method of the \cLockFreeList{} is the same as that of the \cLazyList{} with one small change: to test if \fCurr{} is marked we apply \lstinline+curr.next.get(marked)+ and check that \aMarked{0} is true. Art of Multiprocessor Programming

Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} Art of Multiprocessor Programming

If list changes while traversed, start over
Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} If list changes while traversed, start over Art of Multiprocessor Programming

Start looking from head
Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} Start looking from head Art of Multiprocessor Programming

Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} Move down the list Art of Multiprocessor Programming

Get ref to successor and current deleted bit
Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} Get ref to successor and current deleted bit Art of Multiprocessor Programming

Try to remove deleted nodes in path…code details soon
Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} Try to remove deleted nodes in path…code details soon Art of Multiprocessor Programming

If curr key that is greater or equal, return pred and curr
Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} If curr key that is greater or equal, return pred and curr Art of Multiprocessor Programming

Otherwise advance window and loop again
Lock-free Find public Window find(Node head, int key) { Node pred = null, curr = null, succ = null; boolean[] marked = {false}; boolean snip; retry: while (true) { pred = head; curr = pred.next.getReference(); while (true) { succ = curr.next.get(marked); while (marked[0]) { … } if (curr.key >= key) return new Window(pred, curr); pred = curr; curr = succ; }} Otherwise advance window and loop again Art of Multiprocessor Programming

Lock-free Find retry: while (true) { … while (marked[0]) { snip = pred.next.compareAndSet(curr, succ, false, false); if (!snip) continue retry; curr = succ; succ = curr.next.get(marked); } Art of Multiprocessor Programming

Lock-free Find Try to snip out node retry: while (true) { … while (marked[0]) { snip = pred.next.compareAndSet(curr, succ, false, false); if (!snip) continue retry; curr = succ; succ = curr.next.get(marked); } Art of Multiprocessor Programming

if predecessor’s next field changed, retry whole traversal
Lock-free Find if predecessor’s next field changed, retry whole traversal retry: while (true) { … while (marked[0]) { snip = pred.next.compareAndSet(curr, succ, false, false); if (!snip) continue retry; curr = succ; succ = curr.next.get(marked); } Art of Multiprocessor Programming

Otherwise move on to check if next node deleted
Lock-free Find Otherwise move on to check if next node deleted retry: while (true) { … while (marked[0]) { snip = pred.next.compareAndSet(curr, succ, false, false); if (!snip) continue retry; curr = succ; succ = curr.next.get(marked); } Art of Multiprocessor Programming

Performance Different list-based set implementaions 16-node machine Vary percentage of contains() calls Art of Multiprocessor Programming

High Contains Ratio Lock-free Lazy list Coarse Grained Fine Lock-coupling Art of Multiprocessor Programming

Low Contains Ratio Lock-free Lazy list Coarse Grained Fine Lock-coupling Art of Multiprocessor Programming

As Contains Ratio Increases
Lock-free Lazy list Coarse Grained Fine Lock-coupling % Contains() Art of Multiprocessor Programming

Summary Coarse-grained locking Fine-grained locking Optimistic synchronization Lazy synchronization Lock-free synchronization We saw 4 approaches to concurrent data structure design. Art of Multiprocessor Programming

“To Lock or Not to Lock” Locking vs. Non-blocking: Extremist views on both sides The answer: nobler to compromise Example: Lazy list combines blocking add() and remove()and a wait-free contains() Remember: Blocking/non-blocking is a property of a method We should make sure to notice that being blocking or lock-free or wait-free is a property of a method, and that individual methods of a given item implementation don’t necessarily provide the same properties (as in the lazy list). Art of Multiprocessor Programming

This work is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. You are free: to Share — to copy, distribute and transmit the work to Remix — to adapt the work Under the following conditions: Attribution. You must attribute the work to “The Art of Multiprocessor Programming” (but not in any way that suggests that the authors endorse you or your use of the work). Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license. For any reuse or distribution, you must make clear to others the license terms of this work. The best way to do this is with a link to Any of the above conditions can be waived if you get permission from the copyright holder. Nothing in this license impairs or restricts the author's moral rights. Art of Multiprocessor Programming

Linked Lists: Locking, Lock-Free, and Beyond …

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Linked Lists: Locking, Lock-Free, and Beyond …

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