Re: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default

[Edgar E. Iglesias]

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..

Cheers