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Date: Tue, 9 Feb 2010 19:12:55 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: Slowdowns comparing qemu-kvm.git to qemu.git:
	vcpu/thread scheduling differences
Message-ID: <20100209191255.GA946@shareable.org>
References: <20100208134645.GA19130@amit-x200.redhat.com>
	<4B702641.5060907@codemonkey.ws>
	<20100208173519.GA19993@amit-x200.redhat.com>
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Amit Shah <amit.shah@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, KVM List <kvm@vger.kernel.org>

Anthony Liguori wrote:
> No, basically, the problem will boil down to, the IO thread is 
> select()'d waiting for an event to occur.  However, you've done 
> something in the VCPU thread that requires the IO thread to run it's 
> main loop.  You need to use qemu_notify_event() to force the IO thread 
> to break out of select().
> 
> Debugging these problems are very difficult and the complexity here is 
> the main reason the IO thread still hasn't been enabled by default in 
> qemu.git.

It is difficult.  One approach to debugging them, in general, is to
have a special debugging mode where the select() loop wakes up
repeatedly at high speed and checks all the conditions it's supposed
to to be waiting for that _should_ have triggered a wakeup, and if any
of them are asserted for two timed wakeups (or some sufficient duration,
checked by gettimeofday), emit a bug message because it probably is one.

I don't know if that could be applied in qemu's event loop.

-- Jamie