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>
index | next in thread | previous in thread | raw e-mail
----- Original Message ----- 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 > "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 ? > > 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. > > 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. > > Bruce Evans understands this better than I do, I would suggest not making > this change without talking about it with him first. > 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 = 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/B5help
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?012401c2ffef$e50657e0$f001a8c0>