linux-kernel - Re: How to measure enable_kernel

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Message-ID: <20110605015023.9860.qmail@science.horizon.com>
Date:	4 Jun 2011 21:50:23 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	andi@...stfloor.org, linux@...izon.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: How to measure enable_kernel_fpu overhead?

> Different CPU models. This years CPUs does this and next years that.

Er, yes, exactly.  That's why we have multiple code paths, and
run-time selection of the best.  The basic principle is used rather
a lot in the Linux kernel, e.g. the alternative() feature.

To me the closest equivalent is the RAID6 code, which has no less than 12
different versions (5 integer, 4 AltiVec, 1 MMX, 1 SSE1, and 1 SSE2),
and on x86 it benchmarks the 8 different versions at boot time and
chooses the best.

What confuses me is:
> I would suggest KISS.

This suggestion is not quite clear to me; I'm not quite sure what you
think is "simple".  Do you mean always use the integer code and ignore
the SSE registers?  Or something else?

Could you expand on that remark a little, please?

Thank you!
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