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Date: Tue, 1 Jul 2014 16:31:29 +0200
Message-ID: <CAJSP0QX70kWC87vSCObe56GCj6NKHnVSH3s7PuGrTWZneaOSkA@mail.gmail.com>
From: Stefan Hajnoczi <stefanha@gmail.com>
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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>
Cc: Kevin Wolf <kwolf@redhat.com>, 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, Jul 1, 2014 at 3:53 PM, Ming Lei <tom.leiming@gmail.com> wrote:
> On Mon, Jun 30, 2014 at 4:08 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>
>> Try:
>> $ perf record -e syscalls:* --tid <iothread-tid>
>> ^C
>> $ perf script # shows the trace log
>>
>> The difference between syscalls in QEMU 2.0 and qemu.git/master could
>> reveal the problem.
>
> The difference is that there are tons of write() and rt_sigprocmask()
> in qemu.git/master, I guess it is related coroutinue.
>
> For linux-aio, the coroutinue shouldn't be necessary because
> io_submit() won't block at most of times for O_DIRECT read/write.

You're forgetting about image formats and the other QEMU block layer
features like I/O throttling.  They do require coroutines.

Are you sure it's the extra syscall overhead?  Any ideas for avoiding them?

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().

Stefan