Con Kolivas
<[email protected] To: Marcel Zalmanovici/Haifa/IBM@IBMIL
rg> cc: [email protected]
Subject: Re: Inconsistent timing results of multithreaded program on an SMP
20/11/2005 12:28 machine.
On Sun, 20 Nov 2005 21:18, Marcel Zalmanovici wrote:
> Hi Con,
>
> Thanks for the quick reply.
No problems.
Please reply below what you are replying to so we can keep track of email
threads.
> <[email protected] To: Marcel
> Zalmanovici/Haifa/IBM@IBMIL rg> cc:
> [email protected] Subject: Re: Inconsistent timing results of
> multithreaded program on an SMP 20/11/2005 11:54 machine.
>
> On Sun, 20 Nov 2005 20:27, Marcel Zalmanovici wrote:
> > Hi,
> >
> > I am trying, as part of my thesis, to make some improvement to the
linux
> > scheduler.
> > Therefore I've written a small multithreaded application and tested how
> > long on average it takes for it to complete.
> >
> > The results were very surprising. I expected to see the completion time
> > vary 1 to 2 seconds at most.
> > Instead what I've got was an oscillation where the maximum time was
twice
> > and more than the minimum!! For a short test results ranged ~7sec to
~16
> > sec, and the same happened to longer tests where the min was ~1min and
>
> the
>
> > max ~2:30min.
> >
> > Does anyone have any clue as to what might happen ?
> > Is there anything I can do to get stable results ?
> >
> > Here is a small test case program:
> >
> > (See attached file: sched_test.c)
> >
> > The test was always done on a pretty much empty machine. I've tried
both
> > kernel 2.6.4-52 and 2.6.13.4 but the results were the same.
> >
> > I'm working on a Xeon Intel machine, dual processor, hyperthreaded.
>
> ^^^^^^^^^^^^^^
>
> I suspect what you're seeing is the random nature of threads to bind
either
> to
> a hyperthread sibling or a real core. If all your threads land on only
one
> physical cpu and both hyperthread siblings it will run much slower than
if
> half land on one physical cpu and half on the other physical cpu. To
> confirm
> this, try setting cpu affinity just for one logical core of each phyiscal
> cpu
> (like affinity for cpu 1 and 3 only for example) or disabling hyperthread
> in
> the bios.
Ok I've had a look at the actual program now ;) Are you timing the time it
takes to completion of everything?
Yes. I'm timing the whole program.
This part of your program:
for (i= 0; i<8; i++)
pthread_join(tid[i], NULL);
Cares about the order the threads finish. Do you think this might be
affecting
your results?
I highly doubt that. The worst case scenario would have thread 0 finish
last right ? but then, all other queries would simply return without
waiting.
Cheers,
Con
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