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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.

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