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Re: FPU emulator unsafe for SMP?

To: Daniel Jacobowitz <>
Subject: Re: FPU emulator unsafe for SMP?
From: Jun Sun <>
Date: Wed, 20 Feb 2002 01:48:21 -0800
Cc: Greg Lindahl <>,
In-reply-to: <>; from on Tue, Feb 19, 2002 at 11:32:22PM -0500
References: <> <> <> <002b01c1b607$6afbd5c0$10eca8c0@grendel> <> <00af01c1b9a2$c0d6d5f0$10eca8c0@grendel> <> <> <> <>
User-agent: Mutt/1.2.5i
On Tue, Feb 19, 2002 at 11:32:22PM -0500, Daniel Jacobowitz wrote:
> > If you do use floating point, I think it is pretty common to have
> > only process that uses fpu and runs for very long.  In that case,
> > leaving FPU owned by the process also saves quite a bit.
> Not true.  For instance, on a processor with hardware FPU, setjmp()
> will save FPU registers.  That means most processes will actually end
> up taking the FPU at least once.

It is true that almost all process will take FPU once, but that 
does not affect my statement unless you have a lot of programs come in
and go away.

On other hand, I do agree with Greg that hand-waving does not mean
much here.  It would be nice to have some performance data on
a benchmark apps.  Any good candidate?  It should be easy to
do a comparison.

BTW, I just found out that almost all processes have their used_math
bit set - this is because init uses math at the beginning and
later all forked processes inherit that bit.  Interesting - that
also hides a couple of bugs related to if (!current->used) branch
in do_cpu(). 


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