Re: [libvirt] [PATCH] daemon: Dynamically create worker threads when some get stuck

On Thu, Jun 16, 2011 at 04:03:36PM -0400, Dave Allan wrote: > On Thu, Jun 16, 2011 at 06:29:09PM +0100, Daniel P. Berrange wrote: > > On Thu, Jun 16, 2011 at 04:29:55PM +0200, Michal Privoznik wrote: > > > Up to now, we've created new worker threads only during new connection. > > > This patch monitors worker threads for liveness and dynamically create > > > new one if all are stuck, waiting for hypervisor to reply. This > > > situation can happen. All one need to do is send STOP signal to > > qemu. > > We will also need to consider the case of qemu processes in > uninterruptible sleep, e.g., in the case of a failed NFS mount. That is no different as far as libvirt is concerned. The end result is always simply that libvirt sends a monitor command & does not get a response in an appropriate timeframe.

> > > The amount of time when we evaluate thread as stuck is defined in > > > WORKER_TIMEOUT macro. > > > > > > With this approach we don't need to create new worker thread on incoming > > > connection. However, as number of active worker threads grows, it might > > > happen we need to size up the pool of worker threads and hence exceed > > > the max_worker configuration value. > > > > This is really not desirable. The max_workers limit is in the > > configuration as a static limit, to prevent client applications > > from making libvirtd spawn an unlimited number of threads. We > > must *always* respect the max_workers limit. > > > > I don't think automatically spawning workers is the right way > > to deal with the QEMU issue anyway. As mentioned before, we need > > to improve the QEMU monitor driver so that we can safely allow > > monitor commands to time out > > Dan, can you suggest some possible strategies here? I don't have a > strong opinion on the implementation, although I agree with your > concern about spawning unlimited numbers of threads. As I mentioned, we need to make the QEMU monitor timeout after some period of time waiting, and ensure that the monitor for that VM cannot be used thereafter. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|