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

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Message-ID: <20091028222439.3e0be650@lxorguk.ukuu.org.uk>
Date:	Wed, 28 Oct 2009 22:24:39 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Igor Chudov <ichudov@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Hyperthreading on 4 core CPU DECREASES performance???

> This is Ubuntu Hardy, 2.6.24 kernel. I tried the same with 2.6.31,
> with the same effect.
> 
> This article from Intel:
> 
> http://software.intel.com/sites/oss/pdfs/mclinux.pdf
> 
> It talks about intelligent handling of multiple cores as a done deal,
> but in my experience that did not actually occur.
> 
> 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.

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.

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