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

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Date:	Wed, 09 Sep 2009 03:36:08 +0200
From:	Felix Fietkau <nbd@...nwrt.org>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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

Ralf Baechle wrote:
>> 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.
> 
> Software refilled.  No misses ever for kernel space or low-mem; think of
> it as low-mem and kernel executable living in a 512MB page that is mapped
> by a mechanism outside the TLB.  Vmalloc ranges are TLB mapped.  Ioremap
> address ranges only if above physical address 512MB.
> 
> An emulated unaligned load/store is very expensive; one that is encoded
> properly by GCC for __attribute__((packed)) is only 1 cycle and 1
> instruction ( = 4 bytes) extra.
CFS definitely isn't causing any emulated unaligned load/stores on these
devices, we've tested that.

- Felix
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