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

[Marcelo Tosatti]

On Mon, Feb 07, 2011 at 08:12:55AM -0200, Marcelo Tosatti wrote:
> > > 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.
> 
> Can you recall what was the test ?
> 
> > > 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.

Edgar, please give the attached patch a try with fixed icount value. The
calculation for next event makes no sense for iothread timeout, only for
vcpu context.

> > > 
> > > 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.
> 
> This is not the case anymore, network transfer speed is comparable.
> Probably due to SIG_IPI delivery being reliable, which was fixed later.

Is there any other issue that prevents turning CONFIG_IOTHREAD on by
default?

qemu-fix-icount-timeout-iothread.patch
Description: Text document