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Date: Tue, 09 Oct 2012 12:36:42 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re:
 [RFC PATCH 00/17] Support for multiple "AIO contexts""
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To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, Anthony Liguori <aliguori@us.ibm.com>, Ping Fan Liu <pingfank@linux.vnet.ibm.com>, qemu-devel@nongnu.org

Il 09/10/2012 11:26, Avi Kivity ha scritto:
> On 10/09/2012 11:08 AM, Stefan Hajnoczi wrote:
>> Here are the steps that have been mentioned:
>>
>> 1. aio fastpath - for raw-posix and other aio block drivers, can we reduce I/O
>>    request latency by skipping block layer coroutines?  
> 
> Is coroutine overhead noticable?

I'm thinking more about throughput than latency.  If the iothread
becomes CPU-bound, then everything is noticeable.

>> I'm also curious about virtqueue_pop()/virtqueue_push() outside the QEMU mutex
>> although that might be blocked by the current work around MMIO/PIO dispatch
>> outside the global mutex.
> 
> It is, yes.

It should only require unlocked memory map/unmap, not MMIO dispatch.
The MMIO/PIO bits are taken care of by ioeventfd.

Paolo