Date: Fri, 11 Apr 2003 14:02:12 +0800 From: "David Xu" <davidxu@freebsd.org> To: "Peter Wemm" <peter@wemm.org> Cc: freebsd-threads@freebsd.org Subject: Re: patch for %gs saving Message-ID: <012401c2ffef$e50657e0$f001a8c0@davidw2k> References: <20030411053722.782152A7EA@canning.wemm.org>
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----- Original Message -----=20 From: "Peter Wemm" <peter@wemm.org> To: "David Xu" <davidxu@freebsd.org> Cc: <freebsd-threads@freebsd.org> Sent: Friday, April 11, 2003 1:37 PM Subject: Re: patch for %gs saving=20 > "David Xu" wrote: > > Here is the patch for kernel to save %gs, > > it works well on my machine. > > http://people.freebsd.org/~davidxu/i386_gs.diff > > Daniel, is this the reason in your libpthread > > patch that doesn't use getcontext syscall ? >=20 > To put some background on the issue, there is a reason why we did not > do this. %gs is not used by the kernel, so it does not normally need = to > be saved and restored on every trap into the kernel. Setting a = segment > register is Really Slow - measured in hundreds of clock cycles. >=20 > So, we normally only touch %gs when we context switch to a different = process > that may have a different %gs. Or when one of the context syscalls = wants > it changed. We cannot avoid touching %fs because we use it for kernel > private data. But if it wasn't for that, we wouldn't be touching > %fs for regular traps/syscalls/etc either. >=20 > Bruce Evans understands this better than I do, I would suggest not = making > this change without talking about it with him first. >=20 Yes, I know loading a descriptor is slow, but in real world, such = optimization will be lost in real work noise. And setcontext syscall is broken by = this optimization, userland will fail to set his context in atomic operation, set pcb->gs =3D userland_gs does not work, so the cost is obviously, I = can optimize my patch to only save and restore %gs at trap and interrupt time, when = entering kernel, I can always zero %gs because kernel does not use it, I think = this can reduce clock cycles at context switch, this might be better than current = code which loading %gs at every context switch. > Cheers, > -Peter > -- > Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com > "All of this is for nothing if we don't go to the stars" - JMS/B5
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