Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Chris Friesen]

On 07/21/2014 10:10 AM, Benoît Canet wrote:
> The Monday 21 Jul 2014 à 09:35:29 (-0600), Chris Friesen wrote :
> > On 07/21/2014 09:15 AM, Benoît Canet wrote:
> > > The Monday 21 Jul 2014 à 08:59:45 (-0600), Chris Friesen wrote :
> > > > On 07/19/2014 02:45 AM, Benoît 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 throttling 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:
> address@hidden:~$ cat /sys/class/block/xyz/inflight ?
>
> This would give us a faily precise number of the requests actually in flight 
> between the guest and qemu.


After a bit of a break I'm looking at this again.

While doing "dd if=/dev/zero of=testfile bs=1M count=700" in the guest, 
I got a max "inflight" value of 181.  This seems quite a bit higher than 
VIRTIO_PCI_QUEUE_MAX.

I've seen throughput as high as ~210 MB/sec, which also kicked the RSS 
numbers up above 200MB.

I tried dropping VIRTIO_PCI_QUEUE_MAX down to 32 (it didn't seem to work 
at all for values much less than that, though I didn't bother getting an 
exact value) and it didn't really make any difference, I saw inflight 
values as high as 177.

Chris