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Subject: Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit
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To: Ming Lei <tom.leiming@gmail.com>, Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

Il 01/07/2014 16:49, Ming Lei ha scritto:
> Let me provide some data when running randread(bs 4k, libaio)
> from VM for 10sec:
>
> 1), qemu.git/master
> - write(): 731K
> - rt_sigprocmask(): 417K
> - read(): 21K
> - ppoll(): 10K
> - io_submit(): 5K
> - io_getevents(): 4K
>
> 2), qemu 2.0
> - write(): 9K
> - read(): 28K
> - ppoll(): 16K
> - io_submit(): 12K
> - io_getevents(): 10K
>
>> > The sigprocmask can probably be optimized away since the thread's
>> > signal mask remains unchanged most of the time.
>> >
>> > I'm not sure what is causing the write().
> I am investigating it...

I would guess sigprocmask is getcontext (from qemu_coroutine_new) and 
write is aio_notify (from qemu_bh_schedule).

Both can be eliminated by introducing a fast path in 
bdrv_aio_{read,write}v, that bypasses coroutines in the common case of 
no I/O throttling, no copy-on-write, etc.

Paolo