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Re: FPU emulator unsafe for SMP?

To: linux-mips@oss.sgi.com
Subject: Re: FPU emulator unsafe for SMP?
From: Greg Lindahl <lindahl@conservativecomputer.com>
Date: Tue, 19 Feb 2002 23:48:08 -0500
In-reply-to: <20020219202434.F25739@mvista.com>; from jsun@mvista.com on Tue, Feb 19, 2002 at 08:24:34PM -0800
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On Tue, Feb 19, 2002 at 08:24:34PM -0800, Jun Sun wrote:

> I think the comment might be an execuse. :-)  Never heard of gcc
> generating unnecessary floating point code.

I don't really remember, but I think the Alpha calling standards
encourages using some of the fp registers as scratch all the time. The
price is that you have to always save them, but you get more registers,
which helps you avoid spills, which speeds up all kinds of code.

> 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.

You're assuming, I guess, that there are a lot of interrupts. OK, so
how much CPU time is saved? You can't compare the cost if you don't
know the number.

> In this case, proc that uses fpu gets about 50% of one cpu, i.e.,
> 25% of total load, while the other two integer math proces split the
> rest 75%, which gives 37.5% each.  Not too bad in my opinion.

One man's "not too bad" can be another man's "oh my God, that's
horrible!"

-- greg




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