linux-kernel - Re: Volanomark slows by 80% under CFS

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Message-ID: <b647ffbd0707280536q4a7c5da3he3ec6e2a8a19fc00@mail.gmail.com>
Date:	Sat, 28 Jul 2007 14:36:54 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	tim.c.chen@...ux.intel.com
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: Volanomark slows by 80% under CFS

On 28/07/07, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> [ ... ]
> It may make sense to queue the
> yielding process a bit further behind in the queue.
> I made a slight change by zeroing out wait_runtime
> (i.e. have the process gives
> up cpu time due for it to run) for experimentation.

But that's wrong. The 'wait_runtime' might have been negative at this
point (i.e. a task is in the negative 'run-time' balance wrt the
'etalon' nice-0 task). Your change ends up helping such a task to
actually stay closer to the 'left most' element of the tree (or to be
it) and not "further behind in the queue" as your intention is.

I don't know Volanomark's details so refrain from speculating on why
this change "improves" benchmark results indeed (maybe some afected
tasks have
positive 'wait_runtime's on average for this setup).

If you want to make sure (just for a test) a yeilding task is not the
left-most (at least) for some short interval of time (likely to be <=
1 tick), take a look at yield_task_fair() in e.g. cfs-v15.

> Volanomark runs better
> and is only 40% (instead of 80%) down from old scheduler
> without CFS.

40 or 80 % is still a huge regression.

>
> Regards,
> Tim
>

-- 
Best regards,
Dmitry Adamushko
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