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From Josh Elser <josh.el...@gmail.com>
Subject Re: Synchronized Access to ZooCache Causing Threads to Block
Date Wed, 12 Feb 2014 15:59:31 GMT
Also, I forgot this part before:

The ZooCache instance that's used *typically* comes from the Instance 
object that your Connector was created from. In other words, if you 
create multiple Instances (ZooKeeperInstance, usually), you can get 
multiple ZooCaches which means that concurrent calls to methods off of 
those objects should not block one another (createScanner off of 
connector1 from instance1 should not block createScanner off of 
connector2 from instance2).

That should be something quick you can play with if you so desire.

On 2/12/14, 9:57 AM, Josh Elser wrote:
> Yep, you'll likely also block on BatchScanner, anything in
> TableOperations, and a host of other things.
>
> For scanners, there's likely a standing recommendation to amortize the
> use of those objects (if you want to look up 5 range, don't make 5
> scanners).
>
> Creating a cache per member in the work would likely require some kind
> of paxos implementation to provide consistency which is highly undesirable.
>
> One thing I'm curious about is the impact of removing ZooCache
> altogether from things like the client api and see what happens. I don't
> have a good way to measure that impact off the top of my head though.
>
> Anyways, is this causing you problems in your usage of the api? Could
> you elaborate a bit more on the specifics?
>
> On Feb 12, 2014 4:48 AM, "Ariel Valentin" <ariel@arielvalentin.com
> <mailto:ariel@arielvalentin.com>> wrote:
>
>     I have run into a problem related to ACCUMULO-1833, which appears to
>     have addressed the issue for MutliTableBatchWriter; however I am
>     seeing this issue on the scanner side also:
>
>     394750-"http-/192.168.220.196:8080-35" daemon prio=10
>     tid=0x00007f3108038000 nid=0x538a waiting for monitor entry
>     [0x00007f31287d1000]
>
>     394878:   java.lang.Thread.State: BLOCKED (on object monitor)
>
>     394933- at
>     org.apache.accumulo.fate.zookeeper.ZooCache.getInstance(ZooCache.java:301)
>
>     395012- - waiting to lock <0x00000000fa64f5b8> (a java.lang.Class
>     for org.apache.accumulo.fate.zookeeper.ZooCache)
>
>     395120- at
>     org.apache.accumulo.core.client.impl.Tables.getZooCache(Tables.java:40)
>
>     395196- at
>     org.apache.accumulo.core.client.impl.Tables.getMap(Tables.java:44)
>
>     395267- at
>     org.apache.accumulo.core.client.impl.Tables.getNameToIdMap(Tables.java:78)
>
>     395346- at
>     org.apache.accumulo.core.client.impl.Tables.getTableId(Tables.java:64)
>
>     395421- at
>     org.apache.accumulo.core.client.impl.ConnectorImpl.getTableId(ConnectorImpl.java:75)
>
>     395510- at
>     org.apache.accumulo.core.client.impl.ConnectorImpl.createScanner(ConnectorImpl.java:137)
>
>     I have not spent enough time reasoning about the code to understand
>     all of the nuances but I am interested in knowing if there are any
>     mitigating strategies for dealing with this thread contention e.g.
>     would creating a cache entry for each member of the Zookeeper
>     ensemble help relieve the strain? use multiple classloaders? or is
>     my only option to spawn multiple JVMs?
>
>     Thanks,
>
>     Ariel Valentin
>     e-mail: ariel@arielvalentin.com <mailto:ariel@arielvalentin.com>
>     website: http://blog.arielvalentin.com
>     skype: ariel.s.valentin
>     twitter: arielvalentin
>     linkedin: http://www.linkedin.com/profile/view?id=8996534
>     ---------------------------------------
>     *simplicity *communication
>     *feedback *courage *respect
>

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