Date: Thu, 20 Mar 2003 19:42:30 -0500 (EST) From: Andrew Gallatin <gallatin@cs.duke.edu> To: Peter Grehan <grehan@freebsd.org> Cc: Jake Burkholder <jake@locore.ca>, src-committers@freebsd.org, cvs-src@freebsd.org, cvs-all@freebsd.org Subject: Re: cvs commit: src/sys/powerpc/powerpc fpu.c genassym.c swtch.S trap.c Message-ID: <15994.24566.411903.248919@grasshopper.cs.duke.edu> In-Reply-To: <3E7A43E1.DA0D75F0@freebsd.org> References: <200303201028.h2KASKU9090258@repoman.freebsd.org> <20030320110240.A3985@grasshopper.cs.duke.edu> <20030320144838.O76446@locore.ca> <15994.7655.668902.294563@grasshopper.cs.duke.edu> <3E7A43E1.DA0D75F0@freebsd.org>
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Peter Grehan writes:
> > Yes. At least on the alpha, gcc uses the floating point
> > registers/instructions extensively. So even a process which isn't
> > intentionally using floating point will likely take a FEN trap.
>
Using a very simplistic approach of always saving/restoring the fp
regs on each context switch, it turns out not to be not really worth
it for UP alpha. It shaves 2-3 seconds of a make -j4 kernel build
(781sec vs 778sec), but bloats the context switch time by a much more
measurable amount in most cases:
-------------------------------------------------------------------------
Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw
--------- ------------- ------ ------ ------ ------ ------ ------- -------
thunder.s FreeBSD 5.0-C 5.8900 7.1300 17.0 14.0 63.2 21.3 84.2
thunder.s FreeBSD 5.0-C 4.5200 6.9400 19.3 14.0 49.9 20.8 65.4
(first entry is non-lazy, second entry is lazy)
> Although I've not seen it, this does appear to be the case with PPC as well,
> judging by a recent NetBSD thread:
>
> http://mail-index.netbsd.org/port-powerpc/2003/03/17/0001.html
>
Wow, it would be great to fix gcc to not be so friendly with the fp
registers. That seems to be the best idea.
Drew
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