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Date: Tue, 5 Aug 2014 18:00:22 +0800
Message-ID: <CACVXFVMeu9N9OnTcfGV3EC5HOSBxaxrz2R_aodDG43gusS1dow@mail.gmail.com>
From: Ming Lei <ming.lei@canonical.com>
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Subject: Re: [Qemu-devel] [PATCH v1 00/17] dataplane: optimization and multi
 virtqueue support
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Fam Zheng <famz@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

On Tue, Aug 5, 2014 at 5:48 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 05.08.2014 um 05:33 hat Ming Lei geschrieben:
>> Hi,
>>
>> These patches bring up below 4 changes:
>>         - introduce object allocation pool and apply it to
>>         virtio-blk dataplane for improving its performance
>>
>>         - introduce selective coroutine bypass mechanism
>>         for improving performance of virtio-blk dataplane with
>>         raw format image
>
> Before applying any bypassing patches, I think we should understand in
> detail where we are losing performance with coroutines enabled.

>>From the below profiling data, CPU becomes slow to run instructions
with coroutine, and CPU dcache miss is increased so it is very
likely caused by switching stack frequently.

http://marc.info/?l=qemu-devel&m=140679721126306&w=2

http://pastebin.com/ae0vnQ6V

>
> I also think that the device emulation has no business in deciding
> whether the bypass is used (it depends solely on conditions outside of
> the device) and that leaking the fd number out of raw-posix is wrong.
> Both of them are layering violations that shouldn't be reintroduced.

Yes, that is right, and I have added comments that the bypass hint will
be moved to block layer completely in future.

Thanks,