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

[Marcelo Tosatti <mtosatti@redhat.com>]

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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?


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diff --git a/cpus.c b/cpus.c
index 9c50a34..2280db1 100644
--- a/cpus.c
+++ b/cpus.c
@@ -748,7 +748,7 @@ static void qemu_tcg_wait_io_event(void)
     CPUState *env;
 
     while (!any_cpu_has_work())
-        qemu_cond_timedwait(tcg_halt_cond, &qemu_global_mutex, 1000);
+        qemu_cond_timedwait(tcg_halt_cond, &qemu_global_mutex, qemu_calculate_timeout());
 
     qemu_mutex_unlock(&qemu_global_mutex);
 
diff --git a/vl.c b/vl.c
index 837be97..dbd81a1 100644
--- a/vl.c
+++ b/vl.c
@@ -1323,7 +1323,7 @@ void main_loop_wait(int nonblocking)
     if (nonblocking)
         timeout = 0;
     else {
-        timeout = qemu_calculate_timeout();
+        timeout = 1000;
         qemu_bh_update_timeout(&timeout);
     }