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

[Benoît Canet]

The Monday 21 Jul 2014 à 15:12:31 (-0600), Chris Friesen wrote :
> On 07/21/2014 01:47 PM, 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
> >>>>be.
> >
> >At first start QEMU start only a bunch of IO threads.
> >When IOs activity kick some other IO threads will be started and can show up 
> >in
> >memory measurement because the same physical memory will be referenced by 
> >multiple threads.
> >
> >Are you sure this is not the case you are seeing ?
> >
> >The H option of ps on the host could help.
> 
> I'm pretty sure that this wouldn't change the RES (aka RSS) value, since all
> those threads are sharing the same address space.
> 
> If we create IO threads on activity though, we could end up causing some
> overhead due to the per-thread stack (8MB by default).  Do we have a limit
> on how many IO threads could get created?
> 
> Chris
> 

thread-pool.c:
pool->max_threads = 64;