Re: context switch vs. signal delivery [was: Re: Accelerating user mode linux]

From: Lincoln Dale (
Date: Mon Aug 05 2002 - 01:37:37 EST

At 05:35 AM 5/08/2002 +0000, Linus Torvalds wrote:
> >Linux got a lot slower in signal delivery when the SSE2 support was
> >added. That got this speed back.
>This will break _horribly_ when (if) glibc starts using SSE2 for things
>like memcpy() etc.
>I agree that it is really sad that we have to save/restore FP on
>signals, but I think it's unavoidable. Your hack may work for you, but
>it just gets really dangerous in general. having signals randomly
>subtly corrupt some SSE2 state just because the signal handler uses
>something like memcpy (without even realizing that that could lead to
>trouble) is bad, bad, bad.

how about putting the onus on userspace to tell the kernel if/when it uses
extensions that require FP state to be saved/restored?
if/when glibc starts using SSE2, it could then use these extensions.

could be as simple as user-space setting some bit somewhere.

>And yes, this signal handler thing is clearly visible on benchmarks.
>MUCH too clearly visible. I just didn't see any safe alternatives
>(and I still don't ;( )

it probably isn't worthwhile penalising all users of signal just for those
few userspace apps that actually do use SSE2.



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