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Re: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by de
From: |
Marcelo Tosatti |
Subject: |
Re: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default |
Date: |
Tue, 25 Jan 2011 11:34:53 -0200 |
User-agent: |
Mutt/1.5.20 (2009-08-17) |
On Tue, Jan 25, 2011 at 10:17:41AM +0100, Edgar E. Iglesias wrote:
> On Mon, Jan 24, 2011 at 04:28:48PM -0600, Anthony Liguori wrote:
> > On 01/24/2011 03:00 PM, Anthony Liguori wrote:
> > > Leave the disable options for now to help with testing but these will be
> > > removed
> > > once we're confident in the thread implementations.
> > >
> > > Disabled code bit rots. These have been in tree long enough that we need
> > > to
> > > either commit to making them work or just remove them entirely.
> > >
> >
> > I/O thread disables icount apparently.
> >
> > I'm not really sure why. Marcelo, do you know the reason
> > qemu_calculate_timeout returns a fixed value in the I/O thread
> > regardless of icount?
>
> Hi,
>
> The following commit hopefully fixed that issue.
>
> commit 225d02cd1a34d5d87e8acefbf8e244a5d12f5f8c
> Author: Edgar E. Iglesias <address@hidden>
> Date: Sun Jan 23 04:44:51 2011 +0100
>
> Avoid deadlock whith iothread and icount
>
> When using the iothread together with icount, make sure the
> qemu_icount counter makes forward progress when the vcpu is
> idle to avoid deadlocks.
>
> Signed-off-by: Edgar E. Iglesias <address@hidden>
>
> See http://lists.gnu.org/archive/html/qemu-devel/2011-01/msg01602.html
> for more info.
>
> One more thing I didn't mention on the email-thread or on IRC is
> that last time I checked, qemu with io-thread was performing
> significantly slower than non io-thread builds. That was with
> TCG emulation (not kvm). Somewhere between 5 - 10% slower, IIRC.
>
> Also, although -icount & iothread no longer deadlocks, icount
> still sometimes performs incredibly slow with the io-thread (compared
> to non-io-thread qemu). In particular when not using -icount auto but
> a fixed ticks per insn values. Sometimes it's so slow I thought it
> actually deadlocked, but no it was crawling :) I haven't had time
> to look at it any closer but I hope to do soon.
>
> These issues should be fixable though, so I'm not arguing against
> enabling it per default. Just mentioning what I've seen FYI..
Right, remember seeing 20% added overhead for network copy with TCG on
the initial iothread merge. One can argue its due to the added overhead
of locking/signalling between vcpu and iothread, where neither can run
in parallel (while in kvm mode they can). But its just handwaving until
detailed information is gathered on specific cases...
[Qemu-devel] [PATCH 3/7] Add support for glib based threading and convert qemu thread to use it, Anthony Liguori, 2011/01/24