----- Original Message -----
From: "Saggi Mizrahi" <smizrahi@redhat.com> To: "Nir Soffer" <nsoffer@redhat.com>, "Francesco Romani" <fromani@redhat.com> Cc: devel@ovirt.org, "Michal Skrivanek" <mskrivan@redhat.com>, "Federico Simoncelli" <fsimonce@redhat.com>, "Dan Kenigsberg" <danken@redhat.com> Sent: Sunday, July 13, 2014 1:03:20 PM Subject: Re: [ovirt-devel] [VDSM][sampling] thread pool status and handling of stuck calls
Hi, I will find time to go in depth in both patches but I still think that this is solving the problem in the wrong place.
You want to have the libvirt connection management code to be separate from the sampling.
It is - we did not suggest changing libvrit connections.
You want to have a libvirt connection pull that you can Queue requests and track their process. The connection pool would have, internally, a thread pool with the same amount of threads as libvirt has (read libvirt conf?). The amount of connections is unimportant if I understand libvirt's design correctly.
If we don't need multiple connections, then what the connection pool does? I guess that you don't mean *connection* pool.
By giving to the rest of VDSM an Async view of libvirt operations we gain few things:
A) All call to libvirt enjoy these features even if they are not sampling. B) We simplify the sampling code.
So lets assume we have an global libvirtConnectionPool
The api is something like: lcp.<nameOfOperation>(<args>) # returns a promise
for example: promise = lcp.listAllDomains()
if promise.wait(timeout): try: promise.cancel() except InProgress: # handle except AlreadyFninished: pass
So what you suggest is actually adding a timeout for libvirt calls, without heaving such timeout on the libvirt side. If we had this on the libvirt, side, we would not have to add anything, right? The current code that blocks on libvirt call will block on a condition variable, and the real call will block on libvirt in a worker thread. If the worker thread finish before the timeout, the calling code will get the result, and if it is stuck or finish after the timeout, the calling code will get a timeout. This is nice, but we are left with the worker thread stuck on libvirt, or if it finished after the timeout, we lost the result of the call. This design imply 2 threads - the calling thread, and the worker threads, so it does not scale and cannot be used for sampling, where we like to have only one thread triggering sampling calls, and the worker threads executing them. For the api calls, where we have a connection/request thread running libvirt code, this adds a timeout using small additional cost (few other worker threads), and a condition variable and lock for each worker thread. Michal, Francesco: do we want timeout on libvrit calls, leaving threads stuck on libvirt, or do we like to wait for libvirt call to return?
Sampling would be done with the scheduler and a separate thread pool would be used to handle the response or lack there of.
For sampling, we came to the conclusion that we don't want to abort a call after a timeout, but we want to stop sampling for this vm until the call return. So having a timeout is a non-goal.
def sample() p = lcp.doSampling() # setCallback() calls the callback immediately # if p is already done. p.setCallaback(onDoneSampling, args=(p,)) sched.schedule(timeout, onSamplingTimeout, args=(p,))
def onSamplingTimeout(p) status = p.getStatus() if status == InProgress: # In flight, so it's probably stuck # maybe one day we would be able to do something # other than log and weep log.warning("Sampling of vm X is stuck :(") sched.schedule(timeout, onDoneSampling=(p,)) elif status Finished: # Race, finished just in time, will be handled # in the callback shortly pass else: log.warning("libvirt under heavy load :(")
def onDoneSampling(p) threadpool.queue(doParsing(p.response())
That way the threadpool, sampling, and libvirt connection management are all separate. Also, we can hide how we optimize libvirt connections\threads and have it orthogonal to how the sampling works.
The current patches do not change libvirt connection management this is orthogonal issue. They are only about changing the way we do sampling.
We could have more operations free the threadpool thread they are using if they are waiting for libvirt ops.
- You have a worker thread stuck on libvirt api call. On the libvirt side, libvirt worker thread is blocked on qemu monitor, which is blocked in the kernel in uninterruptible wait. - You run code on another thread detecting that the first thread is stuck This is what Frencesco is suggesting - have a supervisor thread that handle stuck threads. Now what are you going to do about it?
If we ever get async APIs we could just have the libvirtConnectionManagement layer handle that for us and keep on using the same interface internally.
I think that what you suggest is orthogonal to the sampling problem that Francesco and me are trying to tackle.
----- Original Message -----
From: "Nir Soffer" <nsoffer@redhat.com> To: "Francesco Romani" <fromani@redhat.com> Cc: devel@ovirt.org, "Michal Skrivanek" <mskrivan@redhat.com>, "Federico Simoncelli" <fsimonce@redhat.com>, "Saggi Mizrahi" <smizrahi@redhat.com>, "Dan Kenigsberg" <danken@redhat.com> Sent: Saturday, July 12, 2014 8:07:27 PM Subject: Re: [ovirt-devel] [VDSM][sampling] thread pool status and handling of stuck calls
----- Original Message -----
From: "Francesco Romani" <fromani@redhat.com> To: devel@ovirt.org Cc: "Nir Soffer" <nsoffer@redhat.com>, "Michal Skrivanek" <mskrivan@redhat.com>, "Federico Simoncelli" <fsimonce@redhat.com>, "Saggi Mizrahi" <smizrahi@redhat.com>, "Dan Kenigsberg" <danken@redhat.com> Sent: Saturday, July 12, 2014 1:59:22 PM Subject: Re: [ovirt-devel] [VDSM][sampling] thread pool status and handling of stuck calls
(I'm continuing from here but this probably deserves a new thread. However.)
----- Original Message -----
From: "Federico Simoncelli" <fsimonce@redhat.com> To: devel@ovirt.org Cc: "Nir Soffer" <nsoffer@redhat.com>, "Michal Skrivanek" <mskrivan@redhat.com>, "Adam Litke" <alitke@redhat.com>, "Francesco Romani" <fromani@redhat.com> Sent: Wednesday, July 9, 2014 4:57:53 PM Subject: Re: [ovirt-devel] [VDSM][sampling] thread pool status and handling of stuck calls
So basically the current threading model is the behavior we want?
If some call get stuck, stop sampling this vm. Continue when the call returns.
Michal? Federico?
Yep - but with less threads, and surely with a constant number of them. Your schedule library (review in my queue at very high priority) is indeed a nice step in this direcation.
Waiting for Federico's ack.
That looks good. Now I would like to summarize few things.
We know that when a request gets stuck on a vm also the subsequent ones will get stuck (at least until their timeout is up, except for the first one that could stay there forever).
We want a limited number of threads polling the statistics (trying to match the number of threads that libvirt has).
Given those two assumptions we want a thread pool of workers that are picking up jobs *per-vm*. The jobs should be smart enough to:
- understand what samples they have to take in that cycle (cpu? network? etc.) - resubmit themselves in the queue
Now this will ensure that in the queue there's only one job per-vm and if it gets stuck it is not re-submitted (no other worker will get stuck).
In the last few days I was thinking really hard and long about our last discussions, feedback and proposals and how to properly fit all the pieces together. Michal and me also had a chat about this topics on Friday, and eventually I come up with this new draft
http://gerrit.ovirt.org/#/c/29977
(yes, that's it, just this) which builds on Nir's Schedule, the existing Threadpool hidden inside vdsm/storage, and which I believe provides a much, much better ground for further development or discussion . Driving forces behind this new draft: - minimize bloat. - minimize changes. - separate nicely concerns (Scheduler schedules, threadpool executes, Sampling cares about the actual sampling only). - leverage as much as possible existing infrastracture; avoid to introduce new fancy stuff unless absolutely needed.
And here it is. Almost all the concepts and requirements we discussed are there. The thing which is lacking here is strong isolation about VMs/samplings.
This new concept does nothing to recover stuck worker threads: if the pool is exausted, everything eventually stops, after a few sampling intervals. Stuck jobs are detected and the corresponding VMs are marked unresponsive (leveraging existing infrastructure). When (if?) stuck jobs eventually restart working, everything else restarts as well.
The changes are minimal, and there is still room for refactoring and cleanup, but I believe the design is nicer and cleaner.
Further steps: * replace existing thread pool with a fancier one which can replace stuck threads, or dinamically resize himself, to achieve better isolation among VMs or jobs? * Split the new VmStatsCollector class in smaller components? * Stale data detection. Planned but not yet there, I just need to get how to properly fit it into the AdvancedStatsFunction windowing sample. Should nt be a big deal, however.
I also have already quite few cleanup patches for the existing threadpool and for the sampling code in the queue, some are on gerrit, some are not.
I think most of them can wait once we agree on the overall design.
Nir also provided further suggestions (thanks for that!) and possible design alternatives which I'm now evaluating carefully.
I agree with Federico and you - I think this is the way we should explore.
But I don't understand the way you are implementing this using the scheduler in http://gerrit.ovirt.org/29977, and it seems that this does not ensure that every vm has only one sampling task running at the same time.
I started to work on a prototype during the last week and I think that this is the right way to implement. Please check this patch: http://gerrit.ovirt.org/29980
This use the current storage thread pool, but I don't think it is good enough. I think we should continue with http://gerrit.ovirt.org/29191 so we can handle stuck worker thread without decreasing the work force of the thread pool.
Nir