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Date: Mon, 08 Feb 2010 08:57:05 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: [Qemu-devel] Re: Slowdowns comparing qemu-kvm.git to qemu.git:
 vcpu/thread scheduling differences
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To: Amit Shah <amit.shah@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, KVM List <kvm@vger.kernel.org>

On 02/08/2010 07:46 AM, Amit Shah wrote:
> Hello,
>
> In my testing of virtio-console, I found qemu-kvm.git introduces a lot
> of overhead in thread scheduling compared to qemu.git.
>
> My test sends a 260M file from the host to a guest via a virtio-console
> port and then computes the sha1sum of the file on the host as well as on
> the guest, compares the checksum and declares the result based on the
> checksum match. The test passes in all the scenarios listed below,
> indicating there's no unsafe data transfer.
>
>
> Repo               Time taken
> -----              ----------
> qemu.git<  1 m (typically 30s)
> qemu-kvm.git>  16m
> qemu-iothread        ~ 5m
>    

That very likely suggests that there are missing qemu_notify_events() in 
qemu-kvm.git and you're getting blocked waiting for the next timer event 
to fire.

IOW, I assume that during the qemu-kvm.git run, the CPU isn't pegged at 
100% whereas it is in qemu.git.

Regards,

Anthony Liguori