Transactional Memory Student Presentation: Stuart Montgomery CS5204 – Operating Systems 1

Transactional Memory Why? Avoid explicit locking Non-blocking: can’t deadlock Higher level abstraction for the programmer “The value of STM is that it allows you to focus on designing applications which happen to scale instead of the mechanisms employed to scale those applications.” CS5204 – Operating Systems 2 Shan Lu, et. al, Learning From Mistakes

Transactional Memory What is TM? Regions of code that manipulate values (memory) atomically ACID: Atomicity, Consistency, Isolation, Durability All or nothing Atomic Regions (Lomet) Hardware TM /1993 (Knight/Herlihy and Moss) Software TM (Shavit and Touitou) CS5204 – Operating Systems 3 apt/projects/TM/LeeRouting/

Transactional Memory HARDWARE TRANSACTIONAL MEMORY CS5204 – Operating Systems 4

Transactional Memory Hardware TM Transactions operate on hardware data cache Changes to memory are committed atomically to other caches A natural extension of Load-Linked/Store-Conditional  LL/SC provides an atomic update to a single word of memory  HTM extends LL/SC to create groups of atomic instructions Generally, HTM systems are bounded  Restricts the size of a transaction! CS5204 – Operating Systems 5

Transactional Memory Hardware TM Concepts Snoopy Cache Coherence – Goodman 1983 Caches listen to the bus to detect changes to their copy of the same cached address CS5204 – Operating Systems 6 Shared Memory Bus addressvaluestatetag cache... CPU

Transactional Memory Hardware TM Instructions LT  Read with NON-exclusive access (normal) LTX  Read with exclusive access  Useful for when we anticipated writing to the read value soon ST  Write to memory Transaction State Instructions: Validate, Commit, Abort XCOMMIT = Old vs. XABORT = New Processor flags TSTATUS and TACTIVE CS5204 – Operating Systems 7

Transactional Memory Quick Example shared int counter; void process(int work){ int success = 0, backoff = BACKOFF_MIN; unsigned wait; while (success < work) { ST(&counter, LTX(&counter) + 1); if (COMMIT()) { success++; backoff = BACKOFF_MIN; } else { wait = random() % (1 << backoff); while (wait--); if (backoff < BACKOFF_MAX) backoff++; } CS5204 – Operating Systems 8 Herlihy and Moss, Transactional Memory: Architectural Support for Lock-Free Data Structures

Transactional Memory Hardware TM Concerns Requires new hardware Possibility of starvation with BUSY signals  Augment with queuing mechanism Separate Transactional Cache or not  1 cache: set size limits transaction size But could use cache emulation in software  Extra abort, etc. logic applies to entire larger cache  Tradeoff between strong atomicity and efficiency Hybrid Systems:  VTM  HASTM  Hybrid TM CS5204 – Operating Systems 9

Transactional Memory Notable HTM Implementations Simulations Sun’s Rock SPARC multicore processor CS5204 – Operating Systems 10 SPARC Rock CPU HTM Test on Rock Surpasses STM

Transactional Memory Performance Simulation results show that transactional memory matches or outperforms the best known locking techniques for simple benchmarks, even in the absence of priority inversion, convoying, and deadlock.  Fewer access to memory Long transactions more likely to abort Large transactions more likely to exceed TM cache size CS5204 – Operating Systems 11

Transactional Memory SOFTWARE TRANSACTIONAL MEMORY CS5204 – Operating Systems 12

Transactional Memory Word-Based STM CS5204 – Operating Systems 13 Shavit & Touitou  Historical  Each word of memory has an ownership record with old values  Transactions can “help” each other Harris & Fraser  Track the deltas in transaction descriptors  Atomically commit transactions to the ownership records

Transactional Memory Object-Based STM CS5204 – Operating Systems 14 Dynamic STM (Herlihy et. al.)  Higher-level TM Objects FSTM (Fraser)  Similar to Dynamic STM Larus and Kozyrakis, Transactional Memory

Transactional Memory Software TM Design Closed nesting  The child commits into the parent Open nesting  The child commits to the world Other considerations:  Direct/Deferred Update  Early/Late Conflict Detection  Conflict Resolution E.g. Abort or Backoff  Nesting  Exceptions Often ignored in STM designs CS5204 – Operating Systems 15 figure adopted from [tcc-mcdonald-isca06]

Transactional Memory Case Study: STM.NET Microsoft’s experiment , then dropped Hook the C# JIT compiler, also investigated C++ Separate Haskell development Joe Duffy’s retrospective on unbounded STM: 1. Applying transactions to intrinsically non-transactional operations (I/O) Reading a block or file from the FS, output to the console, entry in the Event Log, web service calls, etc. 2. Weak vs. Strong Atomicity Weak: Non-TM regions seeing the results of TM regions 3. Privatization 4. “Where is the killer App?”… most applications naturally parallel? CS5204 – Operating Systems 16 “But with the wisdom of age and hindsight, I do believe limited forms of TM could be wildly successful at particular tasks and yet would have avoided many of the biggest challenges with unbounded TM.”

Transactional Memory Other Notable STM Implementations Intel STM Compiler Prototype (C/C++) Sun DSTM2 (Java factories) Several libraries from Harris & Fraser Other languages: Haskell, LISP, Clojure, C#, OCaml, Perl CS5204 – Operating Systems 17

Transactional Memory Summary Hardware TM  Make memory access atomic by holding in a transactional cache  Caches for each CPU cooperate in determining use of memory locations  Faster Software TM  Allows for a larger transactions and more design flexibility than HTM  Both word and object level granularity  Many possible design choices: Strong/Weak Atomicity Granularity Conflict detection Nested (Open or Closed) Real-world implementations continue  Issues with particular design choices of STM.NET CS5204 – Operating Systems 18

Transactional Memory QUESTIONS? CS5204 – Operating Systems 19

Transactional Memory BACKUPS CS5204 – Operating Systems 20

Transactional Memory Weak Atomicity Problem bool itIsOwned = false; MyObj x = new MyObj(); … atomic { // Tx0 atomic { // Tx1 // Claim the state for my use: if (!itIsOwned) itIsOwned = true; x.field += 42; } int z = x.field;... CS5204 – Operating Systems 21