linux-kernel - Re: [PATCH] [3/6] scheduler: Do devirtualization for sched

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Message-ID: <20071008114234.GC22199@elte.hu>
Date:	Mon, 8 Oct 2007 13:42:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...e.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [3/6] scheduler: Do devirtualization for sched_fair


* Andi Kleen <ak@...e.de> wrote:

> Some CPUs like K8 cannot predict indirect calls. A common optimization 
> in object oriented languages is to check for the most common call 
> target and then call it directly; otherwise do an indirect call. This 
> patch does this manually for sched_fair calls in sched.c
[...]

> I unfortunately wasn't able to measure a consistent difference in 
> lmbench3 lat_ctx -- the change seems to be below its (large) 
> instability.

> + * Simple devirtualization:
> + * Most tasks are fair tasks. Avoid the overhead of
> + * doing indirect calls for them by checking for the fair case and doing
> + * a direct call. Use only in real fast paths since it generates larger code.
> + */
> +#define __CLASS_CALL(class, name, args) \
> +	((class) == &fair_sched_class ? \
> +		name ## _fair args : (class)->name args)
> +#define CLASS_CALL(p, name, args) \
> +	__CLASS_CALL((p)->sched_class, name, args)

hm, i'm not convinced about this one. It increases the code size a bit 
and it's a sched.c local hack. If then this should be done on a generic 
infrastructure level - lots of other code (VFS, networking, etc.) could 
benefit from it i suspect - and then should be .configurable as well. 
Then the benefit might become measurable too.

	Ingo
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