Re: virtio-blk performance regression and qemu-kvm

[Martin Mailand <martin@tuxadero.com>]

Hi,
I could reproduce it and I bisected it down to this commit.

12d4536f7d911b6d87a766ad7300482ea663cea2 is the first bad commit
commit 12d4536f7d911b6d87a766ad7300482ea663cea2
Author: Anthony Liguori <aliguori@us.ibm.com>
Date:   Mon Aug 22 08:24:58 2011 -0500


-martin


On 22.02.2012 20:53, Stefan Hajnoczi wrote:
> On Wed, Feb 22, 2012 at 4:48 PM, Dongsu Park
> <dongsu.park@profitbricks.com>  wrote:
>>> Try turning ioeventfd off for the virtio-blk device:
>>>
>>> -device virtio-blk-pci,ioeventfd=off,...
>>>
>>> You might see better performance since ramdisk I/O should be very
>>> low-latency.  The overhead of using ioeventfd might not make it
>>> worthwhile.  The ioeventfd feature was added post-0.14 IIRC.  Normally
>>> it helps avoid stealing vcpu time and also causing lock contention
>>> inside the guest - but if host I/O latency is extremely low it might
>>> be faster to issue I/O from the vcpu thread.
>> Thanks for the tip. I tried that too, but no success.
> My guesses have all been wrong.  Maybe it's time to git bisect this instead :).
>
> Stefan
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