Re: [Qemu-devel] [virtio-dev] Re: [PATCH v5 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Wang, Wei W]

On Monday, March 26, 2018 11:04 PM, Daniel P. Berrangé wrote:
> On Mon, Mar 26, 2018 at 02:54:45PM +0000, Wang, Wei W wrote:
> > On Monday, March 26, 2018 7:09 PM, Daniel P. Berrangé wrote:
> > >
> > > As far as libvirt is concerned there are three sets of threads it
> > > provides control over
> > >
> > >  - vCPUs - each VCPU in KVM has a thread. Libvirt provides per-thread
> > >    tunable control
> > >
> > >  - IOThreads - each named I/O thread can be associated with one or more
> > >    devices. Libvirt provides per-thread tunable control.
> > >
> > >  - Emulator - any other QEMU thread which isn't an vCPU thread or IO
> thread
> > >    gets called an emulator thread by libvirt. There is no-per thread
> > >    tunable control - we can set tunables for entire set of emulator 
> > > threads
> > >    at once.
> > >
> >
> >
> > Hi Daniel,
> > Thanks for sharing the details, they are very helpful. I still have a 
> > question:
> >
> > There is no fundamental difference between iothread and our
> > optimization thread (it is similar to the migration thread, which is
> > created when migration begins and terminated when migration is done) -
> > both of them are pthreads and each has a name. Could we also add the
> > similar per-thread tunable control in libvirt for such threads?
> >
> > For example, in QEMU we can add a new migration qmp command,
> > migrate_enable_free_page_optimization (just like other commands
> > migrate_set_speed 10G), this command will create the optimization thread.
> > In this way, creation of the thread is in libvirt's control, and
> > libvirt can then support tuning the thread (e.g. pin it to any pCPU), right?
> 
> Anything is possible from a technical level, but from a design POV we would
> rather not start exposing tunables for arbitrary device type specific threads
> as they are inherantly non-portable and may not even exist in QEMU long
> term as it is just an artifact of the current QEMU implementation.
> 

OK. My actual concern with iothread is that it exists during the whole QEMU 
lifecycle. Block device using it is fine since block device access happens 
frequently during the QEMU lifecycle. Usages like live migration, if they 
happen once per day, running this iothread would be a waste of CPU cycles.

Best,
Wei