Date: Fri, 11 Apr 2003 02:00:01 -0400 (EDT) From: Daniel Eischen <eischen@pcnet1.pcnet.com> To: Peter Wemm <peter@wemm.org> Cc: freebsd-threads@freebsd.org Subject: Re: patch for %gs saving Message-ID: <Pine.GSO.4.10.10304110158570.23482-100000@pcnet1.pcnet.com> In-Reply-To: <20030411053722.782152A7EA@canning.wemm.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thu, 10 Apr 2003, Peter Wemm wrote: > "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. So this is akin to lazy FPU switching? -- Dan Eischen
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.GSO.4.10.10304110158570.23482-100000>