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Re: [Qemu-devel] [Qemu-block] [PATCH v4 8/8] linux-aio: share one LinuxA

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v4 8/8] linux-aio: share one LinuxAioState within an AioContext
Date: Tue, 10 May 2016 12:32:27 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 10/05/2016 11:40, Kevin Wolf wrote:
> > Regarding performance, I'm thinking about a guest with 8 disks (queue
> > depth 32).  The worst case is when the guest submits 32 requests at once
> > but the Linux AIO event limit has already been reached.  Then the disk
> > is starved until other disks' requests complete.
> Sounds like a valid concern.

Oh, so you're concerned about the non-dataplane case.  My suspicion is
that, with such a number of outstanding I/O, you probably have bad
performance unless you use dataplane.

Also, aio=threads has had a 64 I/O limit for years and we've never heard
about problems with stuck I/O.  But I agree that it's one area to keep
an eye on.


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