Re: [Qemu-devel] [PATCH] Huge TLB performance improvement

[Thiemo Seufer]

Laurent Desnogues wrote:
> Daniel Jacobowitz a écrit :
> >
> >Straight qemu with my previously posted MIPS patches takes 6:13 to
> >start and reboot a MIPS userspace (through init, so lots of fork/exec).
> >
> >Thiemo's patch, which flushes the whole jump buffer, cuts it to 1:40.
> >
> >A patch which finds the entries which need to be flushed more
> >efficiently cuts it to 1:21.
> >
> >A patch which flushes up to 1/32nd of the jump buffer indiscriminately
> >cuts it to 1:11-1:13.
> 
> Warning:  I don't know anything about the Qemu MMU implementation
> so this question is perhaps stupid :)
> 
> Did you try to benchmark some user space applications with the
> various implementations you propose?

A "benchmark" I did was compiling lmbench, which became more than twice
as fast. I didn't bother to do real measurements.

> The boot of a Linux kernel
> is quite heavy on various kinds of flushes and so is very
> different from "standard" applications.

At least the MIPS kernel is indeed different in that it uses non-trivial
TLB mappings nearly excusively for modules and userland. IOW, the
kernel-side MMU overhead at boot time is neglectable. My patch made no
significant difference for that case.


Thiemo