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

[Paul Brook <paul@codesourcery.com>]

On Sunday 12 November 2006 11:49, 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?  The boot of a Linux kernel
> is quite heavy on various kinds of flushes and so is very
> different from "standard" applications.

MIPS is different because it has a relatively small software managed TLB. 
Other targets have a hardware managed TLB. On a hardware managed TLB the OS 
treats it as if it were infinite size, and invalidation only occurs when a OS 
changes the mappings. On a software managed TLB "flushes" are more likely to 
occur during normal operation as TLB slots are reused.

Paul