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Date: Mon, 21 Jul 2014 17:15:40 +0200
From: =?iso-8859-1?Q?Beno=EEt?= Canet <benoit.canet@irqsave.net>
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Subject: Re: [Qemu-devel] is there a limit on the number of in-flight I/O
	operations?
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To: Chris Friesen <chris.friesen@windriver.com>
Cc: =?iso-8859-1?Q?Beno=EEt?= Canet <benoit.canet@irqsave.net>, Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org

The Monday 21 Jul 2014 =E0 08:59:45 (-0600), Chris Friesen wrote :
> On 07/19/2014 02:45 AM, Beno=EEt Canet wrote:
>=20
> >I think in the throttling case the number of in flight operation is li=
mited by
> >the emulated hardware queue. Else request would pile up and throttling=
 would be
> >inefective.
> >
> >So this number should be around: #define VIRTIO_PCI_QUEUE_MAX 64 or so=
mething like than that.
>=20
> Okay, that makes sense.  Do you know how much data can be written as pa=
rt of
> a single operation?  We're using 2MB hugepages for the guest memory, an=
d we
> saw the qemu RSS numbers jump from 25-30MB during normal operation up t=
o
> 120-180MB when running dbench.  I'd like to know what the worst-case wo=
uld
> be.

I think Linux as a limit of 512Kb for io size or something like that.
So the guest would have VIRTIO_PCI_QUEUE_MAX * 512Kb of in flight buffers=
 at max.

>=20
> Thanks,
> Chris