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From Chris Jeris <cje...@brightcove.com>
Subject Re: InterProcessMutex acquire times out and then _succeeds_?
Date Thu, 17 Oct 2013 20:41:34 GMT
That's understood, and if the listener thread got the lock immediately
after the main thread released it, that would make sense.  But look at the
length of time that elapses:

MAIN THREAD: Acquired lock at 1382038570852, took 25
MAIN THREAD: Setting data to record loop-start-time 1382038570827
1;1382038570827: Starting lock section at 1382038571094
MAIN THREAD: Released lock at 1382038571287, total elapsed 460
# Note here, a case where, though the lock was released by MAIN THREAD,
# the already-started NodeCacheListener ran out its entire timeout before
# grabbing the lock.
1;1382038570827: Acquired lock at 1382038576106, took 5012
1;1382038570827: Released lock at 1382038576420, total elapsed 5326

The NodeCacheListener thread could have acquired the lock just after
1382038571287, when the main thread releases it (the 200ms sleep elapses
between when setData is called, triggering the listener, and when the main
thread releases the lock).  But that isn't what happens.  4800 more
milliseconds go by, and when the listener thread's lock timeout is just
about up (or already up?), _then_ it's able to get the lock.  If you set
the lock timeout in the example to 15 seconds instead of 5, then it waits
15 seconds.

I don't understand the internal mechanics, but it almost seems as if
whatever signal is supposed to be received by the listener thread's lock
watcher to let it know that the lock is available is instead trapped behind
the expiration of the timeout.  Sometimes.

thanks, Chris


On Thu, Oct 17, 2013 at 4:33 PM, Jordan Zimmerman <
jordan@jordanzimmerman.com> wrote:

> InterProcessMutex has the same semantics as a JVM lock. It's re-entrant
> for the same thread but it will block other threads that try to acquire it.
> In the example in the gist, the main thread locks and then calls setData.
> The NodeCacheListener, from a different thread, tries to acquire the same
> lock as the main thread has so it will have to wait until the main thread
> release that lock.
>
> -Jordan
>
> On Oct 17, 2013, at 12:43 PM, Chris Jeris <cjeris@brightcove.com> wrote:
>
> The issue seems to be caused by the fact that we have changes propagated
> by a NodeCache on a different node, and the NodeCacheListener's
> implementation attempts to lock the same lock.  Is this an instance of the
> "single threaded watcher" problem you're talking about?
>
> Here's an example that shows the problem:
> https://gist.github.com/ystael/7030926
>
> The sample output was generated by starting a single instance of the
> program.  You can see a couple of cases where, although the MAIN THREAD has
> released the lock, the NodeCacheListener thread which is trying to acquire
> the lock doesn't succeed until its timeout has elapsed all the way ("took
> 5012").  It doesn't happen every time, but it happens a substantial
> fraction of the time.
>
> If I change the implementation of listener to wrap the body of nodeChanged
> in a future, so that it executes in a separate thread, the problem goes
> away.  We had thought from examining the implementation of NodeCache that
> Listener bodies already execute in a separate background thread; is this
> not the case?
>
> thanks, Chris
>
>
> On Thu, Oct 17, 2013 at 11:08 AM, Jordan Zimmerman <
> jordan@jordanzimmerman.com> wrote:
>
>> This sounds suspiciously like a misuse of a watcher somewhere. Are you
>> doing other things with ZooKeeper? ZooKeeper watches are single threaded.
>> If you have a watcher that blocks, etc. it will prevent other ZooKeeper
>> recipes from functioning. See this Tech Note:
>> https://cwiki.apache.org/confluence/display/CURATOR/TN1
>>
>> Other than that, I haven't heard of this problem. If you can provide a
>> test case, that would help.
>>
>> -Jordan
>>
>> On Oct 17, 2013, at 7:47 AM, Chris Jeris <cjeris@brightcove.com> wrote:
>>
>> We have run into a knotty problem with InterProcessMutex, where calls to
>> .acquire will expend their full timeout and then _succeed_ in acquiring the
>> lock.  This generally happens when a different server process was the last
>> one to hold the lock, but it does not happen every time that is the case.
>>
>> The lock is a single InterProcessMutex object per server process (=
>> server machine), all on a single persistent Zookeeper node name (the object
>> being access controlled is a single piece of shared state, whose data is
>> not itself stored in ZK).  The problem arises in the context of a test
>> suite where requests to our server cluster are issued serially, so there is
>> basically no competing traffic on the lock, although there is traffic on
>> the ZK cluster from other applications.  The frequency of acquires on this
>> lock is not excessive (order 1 per second), and we are reasonably certain
>> our client code is not holding the lock longer than it should.
>>
>> The problem does not seem to be sensitive to the exact value of the
>> timeout.  If we set it to 15 seconds, we see lock acquires taking 15
>> seconds and then succeeding; if we set it to 60 seconds, we see them taking
>> 60 seconds and then succeeding.
>>
>> Right now we observe the problem with Curator 2.1.0 against both ZK 3.3.6
>> and 3.4.5.
>>
>> Is this a known or familiar issue?  Does it sound like we're doing
>> something wrong?
>>
>> thanks, Chris Jeris
>> --
>> Chris Jeris
>> cjeris@brightcove.com
>> freenode/twitter/github: ystael
>>
>>
>>
>
>
> --
> Chris Jeris
> cjeris@brightcove.com
> freenode/twitter/github: ystael
>
>
>


-- 
Chris Jeris
cjeris@brightcove.com (617) 686-3271
freenode/twitter/github: ystael

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