linux-kernel - Re: BFS vs. mainline scheduler benchmarks and measurements

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Message-ID: <4AA6579C.2080607@openwrt.org>
Date:	Tue, 08 Sep 2009 15:09:48 +0200
From:	Felix Fietkau <nbd@...nwrt.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Ingo Molnar <mingo@...e.hu>, Michael Buesch <mb@...sch.de>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements

Benjamin Herrenschmidt wrote:
> On Tue, 2009-09-08 at 09:48 +0200, Ingo Molnar wrote:
>> So either your MIPS system has some unexpected dependency on the 
>> scheduler, or there's something weird going on.
>> 
>> Mind poking on this one to figure out whether it's all repeatable 
>> and why that slowdown happens? Multiple attempts to reproduce it 
>> failed here for me.
> 
> Could it be the scheduler using constructs that don't do well on MIPS ? 
> 
> I remember at some stage we spotted an expensive multiply in there,
> maybe there's something similar, or some unaligned or non-cache friendly
> vs. the MIPS cache line size data structure, that sort of thing ...
> 
> Is this a SW loaded TLB ? Does it misses on kernel space ? That could
> also be some differences in how many pages are touched by each scheduler
> causing more TLB pressure. This will be mostly invisible on x86.
The TLB is SW loaded, yes. However it should not do any misses on kernel
space, since the whole segment is in a wired TLB entry.

- Felix
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