linux-kernel - Re: [announce] CFS-devel, performance improvements

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Message-ID: <20070914081735.GC16159@elte.hu>
Date:	Fri, 14 Sep 2007 10:17:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	dimm <dmitry.adamushko@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [announce] CFS-devel, performance improvements


* dimm <dmitry.adamushko@...il.com> wrote:

> and here's something a bit more intrusive.
> 
> The initial idea was to completely get rid of 'se->fair_key'. It's 
> always equal to 'se->vruntime' for all runnable tasks but the 
> 'current'. The exact key within the tree for the 'current' has to be 
> known in order for __enqueue_entity() to work properly (if we just use 
> 'vruntime', we may go a wrong way down the tree while looking for the 
> correct position for a new element). Sure, it's possible to cache the 
> current's key in the 'cfs_rq' and add a few additional checks, but 
> that's not very nice... so what if we don't keep the 'current' within 
> the tree? :-)
> 
> The illustration is below. Some bits can be missed so far but a 
> patched kernel boots/works (haven't done real regression tests yet... 
> can say that the mail client is still working at this very moment :-).
> 
> There are 2 benefits:
> 
> (1) no more 'fair_key' ;
> (2) entity_tick() is simpler/more effective : 'update_curr()' now vs.
> 'dequeue_entity() + enqueue_entity()' before.

cool patch - i like it! It removes some code as well, besides shrinking 
struct task_struct with another 64-bit variable - so it's a nice 
speedup:

   text    data     bss     dec     hex filename
  34467    3466      24   37957    9445 sched.o.before
  34414    3466      24   37904    9410 sched.o.after

i've applied it to the end of the queue - it depends on whether 
->vruntime works out well.

	Ingo
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