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progo
n00b
Joined: 22 May 2006
Posts: 31
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 Posted: Wed May 28, 2008 3:26 pm Post subject: Threaded programs don't use several cores? |
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This problem (if it's such a thing) arose when I noticed mplayer can't use both of my dual cores of Athlon64 X2 3800+, even if I try setting the number via -lavdopts threads=2. Someone mentioned that there could be a kernel configuration issue, or a option that would enable this behaviour.
Otherwise my system scales well, multiple programs can use my cpu up to 100. But a lonely app just can't.
| Code: | Linux pihlaja 2.6.23-gentoo-r6 #2 SMP Sat Feb 9 16:29:00 EET 2008 i686 AMD Athlon(tm) 64 X2 Dual Core Processor 3800+ AuthenticAMD GNU/Linux
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Is there a threaded cpu stress program so I'd know for sure there's a problem? Portage's stressers all seem to be single-threaded. Thanks for replies! |
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anxt
Apprentice
Joined: 25 Feb 2003
Posts: 254
Location: Frozen Tundra, Canada
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| Posted: Thu May 29, 2008 7:59 am Post subject: |
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this is expected behaviour.
there is no reason to run mplayer across two cores on a modern chip. well threaded apps are tricky to write (from little i have tried). weird bugs manifest and it just isn't worth it.
where you see a huge perfomance boost is thinks like running parallel make jobs, a job goes to a core is finished and returned.
for example portage run parallel makes. i think transcode also does this well i haven't tried. there is a lot of overhead to have one thread share variables and data to another,
and a lot of room for error. |
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progo
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Joined: 22 May 2006
Posts: 31
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| Posted: Thu May 29, 2008 2:52 pm Post subject: |
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Uh-hu. 
Windows seems to cope well with those threaded apps (or is it just being inefficient?) Yes -- most apps that need performance split up with forks and such, but then there's some apps that don't. Umm, I've coped with it the last 8 months so I guess I can live with this.
Thank you for answering. |
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sundialsvc4
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Joined: 10 Nov 2005
Posts: 436
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| Posted: Fri May 30, 2008 1:11 am Post subject: |
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It's entirely up to the operating-system to determine what the cores and/or CPUs will be given to do. You cannot control that.
Devise your application as efficient and well-designed threads and let the operating-system decide how to execute it on the hardware. |
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Mad Merlin
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Hu
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Joined: 06 Mar 2007
Posts: 21635
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| Posted: Fri May 30, 2008 2:49 am Post subject: |
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| progo wrote: | Uh-hu.
Windows seems to cope well with those threaded apps (or is it just being inefficient?) Yes -- most apps that need performance split up with forks and such, but then there's some apps that don't. |
Windows has always been very slow to create Win32 processes, so Windows developers tend to prefer threads where Linux developers will fork and use a separate process. Also, the Windows API has a great many functions that do not provide a good way to multiplex waiting for disparate events, so it is common to see Windows programs use a thread just to sleep waiting for an event. I use event in the generic English sense, not in the sense of a Windows "event object". The corresponding Linux program might have all its work handled by a single thread using select, epoll, or a similar function to sleep waiting for any of several interesting events. |
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progo
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Joined: 22 May 2006
Posts: 31
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| Posted: Fri May 30, 2008 5:26 am Post subject: |
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So it's a matter of design.. I understand.
[quote=Mad Merlin]That should give you 2 processes using ~100% each.[/quote]Well that's going to be two processes.  |
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Mad Merlin
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Joined: 09 May 2005
Posts: 1155
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| Posted: Sat May 31, 2008 1:27 am Post subject: |
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| progo wrote: | So it's a matter of design.. I understand.
[quote=Mad Merlin]That should give you 2 processes using ~100% each. |
Well that's going to be two processes. [/quote]
Yes, of course, but that hardly matters. I thought the original issue was that you could only use 100% CPU time (ie, not 200% (ie, 100% of each core)). If you want to see ~200% CPU usage for a single process, try this:
| Code: |
cat <<_EOF_ >mthread.cpp
#include <pthread.h>
#include <stdio.h>
#define THREADS 16
void *busy(void *arg);
int main(void)
{
pthread_t t[THREADS];
for (int i = 0; i < THREADS; i++)
{
pthread_create(&t[i], NULL, busy, NULL);
}
for (int i = 0; i < THREADS; i++)
{
pthread_join(t[i], NULL);
}
printf("all done\n");
return 0;
}
void *busy(void *arg)
{
for (long long i = 0; i < 1000000000; i++);
printf("done\n");
}
_EOF_
g++ -lpthread -o mthread mthread.cpp
./mthread
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Make sure there's no leading whitespace before you paste that into a terminal (at the very least, _EOF_ cannot have leading whitespace). Also don't compile that with optimizations, GCC will optimize away the busy loop and it'll finish too quickly.
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Game! - Where the stick is mightier than the sword! |
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progo
n00b
Joined: 22 May 2006
Posts: 31
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| Posted: Sat May 31, 2008 5:30 am Post subject: |
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| Mad Merlin wrote: |
Yes, of course, but that hardly matters. I thought the original issue was that you could only use 100% CPU time (ie, not 200% (ie, 100% of each core)). If you want to see ~200% CPU usage for a single process, try this: |
Hey wow, that works and utilitizes both cores! This is getting very weird now... So threads are working like they should and kernel distributes them somewhat interestingly. |
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phsdv
Guru
Joined: 13 Mar 2005
Posts: 372
Location: Europe
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| Posted: Sat May 31, 2008 4:48 pm Post subject: Re: Threaded programs don't use several cores? |
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| progo wrote: | | This problem (if it's such a thing) arose when I noticed mplayer can't use both of my dual cores of Athlon64 X2 3800+, even if I try setting the number via -lavdopts threads=2. |
From man mplayer:
| Code: | threads=<1-8> (MPEG-1/2 and H.264 only)
number of threads to use for decoding (default: 1) |
What type of file did you play, or better how was this file encoded? |
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progo
n00b
Joined: 22 May 2006
Posts: 31
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| Posted: Sat May 31, 2008 4:52 pm Post subject: |
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| It seems to be my bad... I was told the file is x264 so I presumed it belongs to h264. midentify tells it is avc1. It's wrapped in matroska along with dts. So I think I've just presumed it wrong. |
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Kernel & Hardware
