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Date: Thu, 31 Jul 2014 18:30:24 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 01/15] qemu coroutine: support bypass mode
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To: Ming Lei <ming.lei@canonical.com>
Cc: Kevin Wolf <kwolf@redhat.com>, 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>

Il 31/07/2014 18:13, Ming Lei ha scritto:
> Follows 'perf report' result on cycles event for with/without bypass
> coroutine:
> 
>     http://pastebin.com/ae0vnQ6V
> 
> From the profiling result, looks bdrv_co_do_preadv() is a bit slow
> without bypass coroutine.

Yeah, I can count at least 3.3% time spent here:

0.87%          bdrv_co_do_preadv
0.79%          bdrv_aligned_preadv
0.71%          qemu_coroutine_switch
0.52%          tracked_request_begin
0.45%          coroutine_swap

Another ~3% wasted in malloc, etc.

I suggest that we discuss it on the phone at next Tuesday's KVM call.
I'm not denying that bypassing coroutines is a useful thing to do.  But
*everyone* should be doing that if it is so useful, and it's hard to do
it without causing code duplication.

Paolo