Re: multithreading

Hi John,
We've observed a similar problem when trying to parallelize a complex
traffic simulation which uses fox (and using FXThread as the technical
basis).
Speedup was only observable after using a big work load with at least 10
threads. To our best guess the drop in efficiency was due to inefficient
cache use in the multi-threaded case and we didn't yet find a way to solve
that.
Our timing comparison numbers show a similar pattern to yours:
https://github.com/eclipse/sumo/issues/9278#issuecomment-946459826

regards,
Jakob


Am Mo., 21. Nov. 2022 um 23:47 Uhr schrieb John Selverian <
[email protected]>:

> Here are the timing results:
>
>
>
> For 1 thread [0]:
>
>
>
>
>
>   0: 116 s
>
> sum: 116 s
>
>
>
> 121 s (2 m : 1 s)
>
>
>
>
>
> The thread took 116 s and the entire routine took 121 s, some other stuff
> minor is also going out outside of the treaded part.
>
>
>
>
>
>
>
>
>
> For 5 threads [0-4] (I have 6 cores)
>
>
>
>   0: 182 s
>
>   1: 181 s
>
>   2: 178 s
>
>   3: 176 s
>
>   4: 165 s
>
> sum: 882 s
>
>
>
> 184 s (3 m : 4 s)
>
>
>
> So I divide the work load into 5 approximately even parts, each individual
> thread takes ~180 s, the sum of the individual 5 threads is 882 s and the
> entire routine took 184 s.
>
>
>
> So while running 5 thread gives a speed up over the sum, each individual
> thread now takes longer than when I do the entire calculation in 1 thread.
>
>
>
> I’m not sure what this all means except that more threads take longer.
> Does anyone have any more insight?
>
>
>
>
>
>
>
> *From:* Andrei Errapart <[email protected]>
> *Sent:* Monday, November 21, 2022 2:07 AM
> *To:* [email protected]
> *Subject:* Re: [Foxgui-users] multithreading
>
>
>
> Hi John,
>
>
>
> From the distance, your situation looks very much as if one thread were
> doing all the work and the others are either dead or doing nothing useful.
> You have definitely thought of that, starting up "htop" or "Task Manager"
> to check whether all the CPU cores are busy all the time as planned?
>
>
>
> Let me mention that the debugger performs a bunch of other operations at
> each step (including resuming and stopping all the threads), thus, it can
> easily disturb your experiment in finding out where the time goes.
>
>
>
> If I were you, I would measure where the CPU is spending his time,
> something along the following lines is sufficient for a start:
>
> const FXTime start_ns = FXDate::universalDate().getTime();
>
> ... some code
>
> const FXTime end_ns = FXDate::universalDate().getTime();
>
> const double time_seconds = 1e-9 * (end_ns - start_ns);
>
>
>
> In a multithreaded code one might be interested in:
> a) recording the start and stop times of each thread.
> b) having a separate log file for each thread - this helps to see whether
> the threads are performing similarly.
>
>
>
>
>
> best regards,
>
> Andrei
>
> Am 20.11.2022 um 20:57 schrieb John Selverian:
>
> It takes a very short time to fill the FXArray, pass the data and start
> the threads. When I step through the code with the debugger these steps
> combined take about 2 second per thread.
>
>
>
> I guess I’ll stick with 1 thread.
>
>
>
>
>
>
>
> *From:* JVZ <[email protected]> <[email protected]>
> *Sent:* Sunday, November 20, 2022 2:07 PM
> *To:* [email protected]; Enno Rehling
> <[email protected]> <[email protected]>
> *Subject:* Re: [Foxgui-users] multithreading
>
>
>
>
> ...amdahls law. Indeed. There is always a lot of serial stuff in any code.
> Whats worse is that the serial part has its 'working set' mostly
> cache-resident.
>
> Farming a chunk of work out to other threads first has to make up for the
> transfer of cached data to the other threads. This needs to happen before
> the other threads start to actually help, rather than impair, performance.
>
> Again the longer the cpujob is the less this transient behaviour hurts.
>
> Its why i would suggest vectorization first. It almost always helps..
>
>
>
>
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>
>
>
>
> _______________________________________________
>
> Foxgui-users mailing list
>
> [email protected]
>
> https://lists.sourceforge.net/lists/listinfo/foxgui-users
>
>
>
> _______________________________________________
> Foxgui-users mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/foxgui-users
>

_______________________________________________
Foxgui-users mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/foxgui-users