Re: multithreading

Fyi, i just noticed the array will go in and out of scope for each loop. That might not help with performance...

On November 20, 2022 4:33:14 PM UTC, Jeroen van der Zijp <[email protected]> wrote:
>On Sun, 20 Nov 2022 10:59:16 -0500
>"John Selverian" <[email protected]> wrote:
>
>> Here is the main part of the code, I left out allocate and
>deallocation...
>> 
>> It is slower then numTreads=12 than for numTreads = 1
>> 
>> I have 12 logically processors, the more I use the slower it goes.
>> 
>> 
>> 
>> 
>> 		numThreadsRunning = 0;
>> 
>> 
>> 		for (FXint i = 0; i < numTreads; i++)
>> 		{
>> 			FXArray<Material> materialsArray;
>> 
>> 			for (int list_box_index = firstListBoxIndex[i]; list_box_index <
>lastListBoxIndex[i] + 1; list_box_index++)
>> 			{
>> 				int j_start = _listBoxDefinition[list_box_index].iStart;
>> 				int j_end = _listBoxDefinition[list_box_index].iEnd;
>> 
>> 				for (int j = j_start; j < j_end + 1; j++)
>> 				{
>> 					materialsArray.append(_materialsArray.at(j));
>> 				}
>> 			}
>> 
>> 			// pass data to thread
>> 			checkValuesThreadInitalizedOK[i] =
>checkValuesThread[i].passData(materialsArray,
>_lastArrayIndexInMatArrayBeforeUserMats, dt);
>> 
>> 			if (checkValuesThreadInitalizedOK[i])
>> 			{
>> 				checkValuesThread[i].start();
>> 				hThreadCheckValues[i] = checkValuesThread[i].id();
>> 				numThreadsRunning++;
>> 			}
>> 		}
>> 
>> 
>> 		while (true)
>> 		{
>> 			bool anyThreadsRunning = false;
>> 
>> 			for (FXint i = 0; i < numTreads; i++)
>> 			{
>> 				if (checkValuesThread[i].running())
>> 				{
>> 					anyThreadsRunning = true;
>> 					break;
>> 				}
>> 			}
>> 
>> 			// if all threads are finished
>> 			if (!anyThreadsRunning)
>> 			{
>> 				break;
>> 			}
>> 
>> 			// keep UI responsive
>> 			if (getApp()->peekEvent())
>> 			{
>> 				getApp()->runWhileEvents();
>> 			}
>> 		} // while (true)
>> 
>> 
>> 
>> 		for (FXint i = 0; i < numTreads; i++)
>> 		{
>> 			checkValuesThread[i].join();
>> 		}
>
>Whether multiple threads is faster or slower depends on a number of
>factors:
>
>	1) Is workload big enough to warrant overhead incurred creating
>threads,
>           and does it make up for inter-thread locking etc. needed?
>
>	2) Are workloads localized fighting for common resources that would
>make
>	   things slower [e.g. mutexes, memory, etc.].
>
>	3) Are multiple threads making cache-locality worse?  I've had this
>exact
>	   scenario, where workloads for threads weren't very large, and the
>overhead
>	   of moving "hot" cachelines from main thread to "worker" threads
>wasn't worth
>	   the expense.  The other cores each have their own L1 and L2, and
>thus hot
>	   cachlines from main thread will have to be exchanged to the other
>cores.
>	   This causes some cacheline exchanging with other cores.
>
>	4) Why the busy-looping of the main thread?  Just use FXMessageChannel
>to
>	   let the worker threads know the tasks are done.
>
>You can also try use FXThreadPool.  It can amortize the cost of
>thread-startup by
>having a number of threads already present, and ready to go when
>there's a job.
>
>With FXThreadPool, threads will not have to be started/stopped for each
>parallel
>job, but only once at startup.
>
>Cost of starting a thread, while not huge, is:
>
>	1) Allocate stack space for the thread,
>
>	2) Allocate thread-local variables,
>
>	3) Perform bunch of system calls to launch the thread.
>
>This cost is negligeable if you perform a multi-hour raytrace or
>something
>like that.  If your entire task is a few seconds, the start-stop cost
>could be 
>signifant.
>
>For scientific workloads [and I have some experience with that], which
>are heavy
>matrix number crunching, may have more benefit from vectorization than
>parallelization;
>I actually would argue to vectorize first, and parallelize second.
>
>Enable AVX [or AVX512 if you have it], and write simple loops.  Use
>libraries such
>as FFTW3 and OpenBLAS, or Intel Math Kernel Library.  These are very
>fast and highly
>tricked out to max throughput.
>
>If that still doesn't do it, and you have a GPU, try CUDA [or CUDA
>libraries such
>as cublas etc.].
>
>
>
>		 -- JVZ
>
>
>
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-- 
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