Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

From: Ingo Molnar
Date: Sun Feb 22 2015 - 03:22:40 EST

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> - Do you have enough RAM that there's essentially no IO
> in the system worth speaking of? Do you have enough RAM
> to copy a whole kernel tree to /tmp/linux/ and do the
> measurement there, on ramfs?

Doing that will also pin down the page cache: kernel build
times are very sensitive to the page cache layout, and once
a page cache layout is established on systems with lots of
RAM it does not tend to be flushed out. But the next bootup
will generate another random page cache layout - which
makes inter-kernel kernel build times comparisons much
noiser than the run-to-run numbers suggest.

So to get more precise measurements a 'pinned' page cache
layout and the dynamic debug switch you implemented is very
helpful and --repeat stddev will be pretty representative
of the true noise of the measurement.


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