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Re: [ Re: [RFC] FPU context switch]

To: <>, "Jun Sun" <>
Subject: Re: [ Re: [RFC] FPU context switch]
From: "Kevin D. Kissell" <>
Date: Wed, 18 Sep 2002 02:10:17 +0200
Original-recipient: rfc822;
References: <>
> ----- Forwarded message from Jun Sun <> -----
Sep 17, 2002 at 03:58:54PM -0700, Greg Lindahl wrote:
> > 
> > The only good test is Linux with and without lazy saves. Throwing in a
> > new OS complicates matters. It sounds like Jun already has working
> > code for (1) and (3), so he can do a good test.
> >
> I actually have 2) and 3).  1) is easy to do, though.  
> Anyone can recommand some test programs to try?
> A while back, I tried lmbench which is not very telling.
> I think the reason is that most of the tests are not using
> FPU at all.

"Not very telling?"  Sounds to me as if it confirms the
hypothesis that the benefits of these optimizations are
maginal.  ;-)
> However I might try it again anyway.  It might tell the
> difference between 1) and 2)&3) easily.

If I wanted to see the effect at its strongest, I'd whip
up an FP-intensive, low-I/O program along the lines
of the old fashioned Whetstone benchmark that runs
for at least a few seconds, then time a script that 
forks off N of them in parallel with M instances of
a program that does no FP.  You can then play with 
M and N to see where a hack becomes advantageous.  
If all runnable programs are using the FPU, there's 
clearly no benefit from the optimization.

Are you able to test this stuff on a proper SMP
system, by the way?  The efficiency of the code 
that manipulates interprocessor control variables 
can reasonably be expected to drop off a bit
in a system with MP cache invalidations blasting
left and right. 


            Kevin K.

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