linux-kernel - Re: Hyperthreading on 4 core CPU DECREASES performance???

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Message-ID: <e86b7d350910281700x1383296i73cd1ee4694e9c8@mail.gmail.com>
Date:	Wed, 28 Oct 2009 19:00:10 -0500
From:	Igor Chudov <ichudov@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Hyperthreading on 4 core CPU DECREASES performance???

On Wed, Oct 28, 2009 at 5:24 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> This is Ubuntu Hardy, 2.6.24 kernel. I tried the same with 2.6.31,
>> with the same effect.

>> I would like to know how can I make the scheduler to prefer to spread
>> the tasks across physical cores as opposed to bundling two on one core
>> and leaving some cores idle.
>
> The scheduler will try and do balancing for packages and for HT. Make
> sure your distribution is built with the SCHED_SMT and SCHED_MC options
> enabled. Some of the non-enterprise distirbutions may well not have these
> enabled.

Yes, for the stock hardy -server and -generic kernel:

CONFIG_SCHED_MC=y
CONFIG_SCHED_SMT=y

>
> For hand laying out threads see: man pthread_setaffinity_np
>
> The win from HT depends a lot on the CPU and also the workload mix. In
> some workloads it will decrease performance.

OK, I think that I understand. Thanks.

So, what you are basically saying is that there is no option to
improve this behavior, right?

Thanks Alan.
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