Re: [BENCHMARK] Lmbench 2.5.54-mm2 (impressive improvements)

[Andrew Morton]

Andi Kleen wrote:
> 
> Andrew Morton <[e-mail]> writes:
> >
> > The teeny little microbenchmarks are telling us that the rmap overhead
> > hurts, that the uninlining of copy_*_user may have been a bad idea, that
> > the addition of AIO has cost a little and that the complexity which
> > yielded large improvements in readv(), writev() and SMP throughput were
> > not free.  All of this is already known.
> 
> If you mean the signal speed regressions they caused - I fixed
> that on x86-64 by inlining 1,2,4,8,10(used by signal fpu frame),16.
> But it should not use the stupud rep ; ..., of the old ersio but direct
> unrolled moves.

Yes, that would help a bit.  We should do that for ia32.  It's a little
worrisome that the return value from such a copy_*_user() implementation
will be incorrect - it is supposed to return the number of uncopied bytes.
Probably doesn't matter.

Most of the optimisation opportunities wrt signal delivery were soaked up
by replacing the copy_*_user() calls with put_user() and friends.

We could speed up signals heaps by re-lazying the fpu state storage in
some manner.

> x86-64 version in include/asm-x86_64/uaccess.h, could be ported
> to i386 given that movqs need to be replaced by two movls.
> 
> -Andi
> 
> P.S.: regarding recent lmbench slow downs: I'm a bit
> worried about the two wrmsrs which are in the i386 context switch
> in load_esp0 for sysenter now. Last time I benchmarked WRMSRs on
> Athlon they were really slow and knowing the P4 it is probably
> even slower there. Imho it would be better to undo that patch
> and use Linus' original trampoline stack.

hm.  How slow?  Any numbers on that?