1. More! Multi-threaded Development
Jarrod Hollingworth
ADUG Melbourne, June 2018

2. Overview Threads Delphi Parallel Programming Library Thread Safety
Challenges & Tips
How presentation will benefit audience: Adult learners are more interested in a subject if they know how or why it is important to them.
Presenter’s level of expertise in the subject: Briefly state your credentials in this area, or explain why participants should listen to you.

3. Threads >= 1 per process Different CPUs in same process
Share process resources (heap memory, globals, file handles)
Own stack, program counter, registers
Controlled by OS (time slice/wait, paused, resumed)
Switched at any time
Low level details are important

4. Parallel Programming Library
XE7+, cross-platform
System.Threading
Based on TThread
TParallel
For loop, demo
Basic thread safety
Bonus demo: Conway’s game of life
TTask
Running tasks, demo: Parallel Squares
Waiting for task, demo
Joins, futures, demo
Lesson descriptions should be brief.

5. TParallel class function For(
LowInclusive: Integer; HighInclusive: Integer; IteratorEvent: TProc<Integer, TLoopState>; Sender: TObject; Stride: Integer; Pool: TThreadPool): TLoopResult;
(30 overloads: optional parameters, anonymous methods/procedure of object, …)
Iterations are run in parallel. Returns once all iterations are complete.
TLoopState
procedure Stop;
procedure Break;
property Stopped: Boolean;
property Faulted: Boolean;
TLoopResult
property Completed: Boolean;
property LowestBreakIteration: Integer;
class function Join(const Procs: array of TProc; Pool: TThreadPool): ITask; (several overloads) Procedures are run in parallel. Returns once all procedures are complete.

6. x86 Assembly: total := total + 1;
CPU1:
CPU2:
mov edx,[eax+$0c] total inc edx mov [eax+$0c],edx total …
mov edx,[eax+$0c] total
inc edx
mov [eax+$0c],edx total

7. x86 µOperations: Inc(total);
ASM:
inc dword ptr [eax] µOp.load temp_reg, [eax] µOp.inc temp_reg µOp.store [eax], temp_reg
Total
 total
CPU:

8. x86 µOperations: Inc(total);
ASM:
inc dword ptr [eax] µOp.load temp_reg, [eax] µOp.inc temp_reg µOp.store [eax], temp_reg
CPU1:
CPU2: …
µOp.load temp_reg, [eax]
µOp.inc temp_reg
µOp.store [eax], temp_reg

9. TTask
TTask
class function Create/Run(Sender: TObject; Proc: TProc; APool: TThreadPool): ITask;
class function WaitForAll(Tasks: array of ITask; Timeout: LongWord): Boolean;
class function WaitForAny(Tasks: array of ITask; Timeout: LongWord): Integer; class function Future<T>(Sender: TObject; Func: TFunc<T>): IFuture<T>;
ITask
function Start: ITask;
property Status: TTaskStatus;
function Wait(Timeout: LongWord): Boolean;
procedure Cancel;
TTaskStatus Created, WaitingToRun, Running, Completed, WaitingForChildren, Canceled, Exception
IFuture<T>
property Value: T

10. TThreadPool
TParallel.For/Join(…, ThreadPool: ThreadPool) TTask.Create/Run(…, ThreadPool: ThreadPool)
TThreadPool is self-tuning based on # CPU cores and load Cannot free until all tasks are complete
SetMaxWorkerThreads(Value: Integer) >= # CPU cores
SetMinWorkerThreads(Value : Integer) <= MaxWorkerThreads Note: No guarantees. For total control use TThread.

11. Thread Safety
Thread local variables (global storage) threadvar class threadvar
Co-ordination & communication between threads
(SyncObjs) TThread.Synchronize (immediate) TThread.Queue (when main thread idle)
(SyncObjs) TEvent.WaitFor(Timeout)/SetEvent/ResetEvent
(Windows) PostMessage/SendMessage(Handle)
(Windows) PostThreadMessage(ThreadId) Shared flag/data/queue, shared memory (inter-process), …
Example objectives
At the end of this lesson, you will be able to:
Save files to the team Web server.
Move files to different locations on the team Web server.
Share files on the team Web server.

12. Thread Safety cont. Locking shared data/resource
(SyncObjs) TCriticalSection.Acquire/Release/TryEnter  Fast (System) TMonitor.Enter(Obj, Timeout)/Exit/TryEnter: Boolean  Fast-ish, timeout
(SysUtils) TMultiReadExclusiveWriteSynchronizer  Faster
BeginRead/EndRead/BeginWrite/EndWrite (SyncObjs) TSemaphore  Slow, inter-process Create(InitialCount, AMaximumCount: Integer; Name: string)
Acquire/Release(Count) (SyncObjs) TMutex.Create(Name: string)/Acquire/Release  Slow, inter-process …
Atomic operations (SyncObjs) TInterlocked.Increment/Decrement/Add/Exchange/… (RTL) AtomicIncrement/… (Windows) InterlockedIncrement…
Example objectives
At the end of this lesson, you will be able to:
Save files to the team Web server.
Move files to different locations on the team Web server.
Share files on the team Web server.

13. Challenges & Tips Using the VCL/FMX & RTL
Performance (different constraints, self tuning)
Deadlocks (multiple locks, non re-entrant locks, use timeouts)
Unhandled exceptions
Testing (every computer/device and every run will be different)
Stopping (or not) threads (notify, auto free, TerminateThread)
Debugging (thread status, naming threads, switching threads)
TThread.NameThreadForDebugging(Name: string)

14. Simple communication design
Main thread
Data (locked)
Manager thread
Job list UI
Update
Dispatch
Synchronize/Queue/PostMessage
Worker
thread
Worker
thread
Worker
thread

15. Resources Book: Delphi High Performance - Primož Gabrijelčič
Wiki:
Introduction video:
Advanced techniques: MartinHarvey1.1/ToC.html
Demos: Conway’s Life (TParallel.For) Pascal/RTL/Parallel%20Library/ Delphi Academy (Parallel Squares, TTask, Future)