[Qemu-devel] Re: [PATCH 0/4] Improve -icount, fix it with iothread

[Paolo Bonzini]

On 02/23/2011 12:08 PM, Edgar E. Iglesias wrote:
> > >  No, this supersedes Marcelo's patch.  10-20% doesn't seem comparable to
> > >  "looks like it deadlocked" anyway.  Also, Jan has ideas on how to remove
> > >  the synchronization overhead in the main loop for TCG+iothread.
> I see. I tried booting two of my MIPS and CRIS linux guests with iothread
> and -icount 4. Without your patch, the boot crawls super slow. Your patch
> gives a huge improvement. This was the "deadlock" scenario which I
> mentioned in previous emails.
>
> Just to clarify the previous test where I saw slowdown with your patch:
> A CRIS setup that has a CRIS and basically only two peripherals,
> a timer block and a device (X) that computes stuff but delays the results
> with a virtual timer. The guest CPU is 99% of the time just
> busy-waiting for device X to get ready.
>
> This latter test runs in 3.7s with icount 4 and without iothread,
> with or without your patch.

Thanks for testing this.

> With icount 4 and iothread it runs in ~1m5s without your patch and
> ~1m20s with your patch. That was the 20% slowdown I mentioned earlier.

Ok, so it is in both cases with iothread.  We go from 16x slowdown to 
19x on one testcase :) and "huge improvement" on another.  (Also, the 
CRIS images on qemu.org simply hang for me without my patch and numeric 
icount---and the watchdog triggers---so that's another factor in favor 
of the patches).  I guess we can live with the slowdown for now, if 
somebody else finds the patch okay.

Do you have images for the slow test?

Paolo