Bug#634149: kvm extremely __slow__ under 2.6.39-2-amd64 compared to 2.6.32-5-amd64

[Jonathan Nieder <jrnieder@gmail.com>]

severity 634149 wishlist
quit

Hi,

Michael Tokarev wrote:

> So, the problem is that with this kernel commit:
>
> commit 7972995b0c346de76fe260ce0fd6bcc8ffab724a
> Author: Gleb Natapov <gleb@redhat.com>
> Date:   Thu Mar 18 15:20:24 2010 +0200
>
>     KVM: x86 emulator: Move string pio emulation into emulator.c
[...]
> which went into mainline with 2.6.35, there has been
> quite some changes in PIO emulation handling in kvm
> which resulted in correct but slow (as opposed by
> fast but incorrect) emulation.  This slowed down
> guests that use PIO to access disks. Among these
> are WinXP (it switches from DMA to PIO after some
> I/O errors), WinNT (ditto) and - apparently - Hurd.

Thanks for a nice explanation.

[...]
> I'd mark this as "notabug" (not possible with BTS)
> or "wontfix", but it's definitely possible to
> optimize the new code further to speed things up.
> If it's worth the effort is another question.

That means "wishlist", and probably not worth tracking unless we want
to document it somewhere or someone is interested in working on it,
right?

Svante, if I were in your shoes, I'd come up with some words of
warning for a qemu-performance.txt file and send them (ideally as a
patch against the qemu or qemu-kvm package) to kvm@vger.kernel.org,
cc-ing this bug.  And if you are looking for a low-level Hurd coding
project, perhaps look into teaching the Hurd some performance tricks
--- it would help on raw hardware, too. :)  Investigating how to
improve kvm's PIO emulation might also be interesting.

Hope that helps,
Jonathan