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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: InterProcessMutex acquire times out and then _succeeds_?
Date Thu, 17 Oct 2013 21:39:08 GMT
I only have a working knowledge of Clojure so it's hard for me to parse that code. I wrote
a quick test to make sure that acquire with a timeout is working correctly. Here it is: https://gist.github.com/Randgalt/7032632

I believe this shows the behavior that you expect, right?

-Jordan

On Oct 17, 2013, at 1:41 PM, Chris Jeris <cjeris@brightcove.com> wrote:

> 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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