linux-kernel - Re: HT (Hyper Threading) aware process scheduling doesn't work as it should

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Message-ID: <20111103081835.GA9330@elte.hu>
Date:	Thu, 3 Nov 2011 09:18:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Artem S. Tashkinov" <t.artem@...os.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>
Subject: Re: HT (Hyper Threading) aware process scheduling doesn't work as it
 should



( Sorry about the delay in the reply - folks are returning from and 
  recovering from the Kernel Summit ;-) I've extended the Cc: list.
  Please Cc: scheduler folks when reporting bugs, next time around. )

* Artem S. Tashkinov <t.artem@...os.com> wrote:

> Hello,
> 
> It's known that if you want to reach maximum performance on HT 
> enabled Intel CPUs you should distribute the load evenly between 
> physical cores, and when you have loaded all of them you should 
> then load the remaining virtual cores.
> 
> For example, if you have 4 physical cores and 8 virtual CPUs then 
> if you have just four tasks consuming 100% of CPU time you should 
> load four CPU pairs:
> 
> VCPUs: {1,2} - one task running
> 
> VCPUs: {3,4} - one task running
> 
> VCPUs: {5,6} - one task running
> 
> VCPUs: {7,8} - one task running
> 
> It's absolutely detrimental to performance to bind two tasks to 
> e.g. two physical cores {1,2} {3,4} and then the remaining two 
> tasks to e.g. the third core 5,6:
> 
> VCPUs: {1,2} - one task running
> 
> VCPUs: {3,4} - one task running
> 
> VCPUs: {5,6} - *two* task runnings
> 
> VCPUs: {7,8} - no tasks running
> 
> I've found out that even on Linux 3.0.8 the process scheduler 
> doesn't correctly distributes the load amongst virtual CPUs. E.g. 
> on a 4-core system (8 total virtual CPUs) the process scheduler 
> often run some instances of four different tasks on the same 
> physical CPU.
> 
> Maybe I shouldn't trust top/htop output on this matter but the same 
> test carried out on Microsoft XP OS shows that it indeed 
> distributes the load correctly, running tasks on different physical 
> cores whenever possible.
> 
> Any thoughts? comments? I think this is quite a serious problem.

If sched_mc is set to zero then this looks like a serious load 
balancing bug - you are perfectly right that we should balance 
between physical packages first and ending up with the kind of 
asymmetry you describe for any observable length is a bug.

You have not outlined your exact workload - do you run a simple CPU 
consuming loop with no sleeping done whatsoever, or something more 
complex?

Peter, Paul, Mike, any ideas?

Thanks,

	Ingo
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