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Date: Mon, 21 Jul 2014 18:10:34 +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 09:35:29 (-0600), Chris Friesen wrote :
> On 07/21/2014 09:15 AM, Beno=EEt Canet wrote:
> >The Monday 21 Jul 2014 =E0 08:59:45 (-0600), Chris Friesen wrote :
> >>On 07/19/2014 02:45 AM, Beno=EEt Canet wrote:
> >>
> >>>I think in the throttling case the number of in flight operation is =
limited by
> >>>the emulated hardware queue. Else request would pile up and throttli=
ng would be
> >>>inefective.
> >>>
> >>>So this number should be around: #define VIRTIO_PCI_QUEUE_MAX 64 or =
something like than that.
> >>
> >>Okay, that makes sense.  Do you know how much data can be written as =
part of
> >>a single operation?  We're using 2MB hugepages for the guest memory, =
and we
> >>saw the qemu RSS numbers jump from 25-30MB during normal operation up=
 to
> >>120-180MB when running dbench.  I'd like to know what the worst-case =
would

Sorry I didn't understood this part at first read.

In the linux guest can you monitor:
benoit@Laure:~$ cat /sys/class/block/xyz/inflight ?

This would give us a faily precise number of the requests actually in fli=
ght between the guest and qemu.

Best regards

Beno=EEt

> >>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 buff=
ers at max.
>=20
> Those numbers don't line up with what we were seeing...we saw a 120MB+ =
jump
> when running dbench, which would map to more like 2MB per operation ass=
uming
> a limit of VIRTIO_PCI_QUEUE_MAX (i.e. 64).
>=20
> Unless there are other buffers involved that we haven't factored in...
>=20
> Chris
>=20
>=20