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From Otto Fowler <ottobackwa...@gmail.com>
Subject Re: ForkJoinPool.commonPool() in Nifi
Date Wed, 28 Mar 2018 13:47:25 GMT
I would think NiFi should have it’s own thread pool.


On March 28, 2018 at 09:29:31, Oleksi Derkatch (oderkatch@vitalimages.com)
wrote:

Anyone have any thoughts on this? Should I make a JIRA ticket?

________________________________
From: Oleksi Derkatch <oderkatch@vitalimages.com>
Sent: Thursday, March 8, 2018 4:36:51 PM
To: dev@nifi.apache.org
Subject: ForkJoinPool.commonPool() in Nifi

Hi,


A few weeks ago we encountered a problem with one of our custom processors
which seemed to deadlock all processing in Nifi under load. We believe the
issue is that our processors were relying on ForkJoinPool.commonPool, but
so was the Nifi engine during it's scheduling (both via CompletableFuture).
As such, when we did a thread dump, we saw something like this:


"ForkJoinPool.commonPool-worker-6" #381 daemon prio=5 os_prio=0
tid=0x00007f300d934000 nid=0x4be4 waiting on condition [0x00007f2fd53e7000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000006c8b00568> (a
java.util.concurrent.CompletableFuture$Signaller)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at
java.util.concurrent.CompletableFuture$Signaller.block(CompletableFuture.java:1693)

at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3313)
at
java.util.concurrent.CompletableFuture.waitingGet(CompletableFuture.java:1729)

at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
at customcode(customcode.java:83)
at customcode.lambda$null$23(customcode.java:320)
at customcode$$Lambda$261/442205945.call(Unknown Source)
at
com.google.common.cache.LocalCache$LocalManualCache$1.load(LocalCache.java:4724)

at
com.google.common.cache.LocalCache$LoadingValueReference.loadFuture(LocalCache.java:3522)

at
com.google.common.cache.LocalCache$Segment.loadSync(LocalCache.java:2315)
at
com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2278)

- locked <0x00000006c8b007f0> (a
com.google.common.cache.LocalCache$StrongWriteEntry)
at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2193)
at com.google.common.cache.LocalCache.get(LocalCache.java:3932)
at
com.google.common.cache.LocalCache$LocalManualCache.get(LocalCache.java:4721)

at customcode.lambda$customethod$24(customcode.java:309)
at customcode$$Lambda$258/1540137328.get(Unknown Source)
at
java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)

at
java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)

at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at
java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at
java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)



I think what happened here is that since we were both using
ForkJoinPool.commonPool() like this, we actually ran out of threads and
deadlocked. We were waiting (in the nifi processor) on a future that was
also submitted to the same commonPool at the time of the deadlock. The
solution was for us to use a dedicated thread pool instead of a shared one.


It might be worth considering changing this in Nifi for the future, in case
other custom processors use this pattern as well.


This also brings up another question. By default, the size of this thread
pool is (# of CPUs - 1). How does that affect processing when we set the
maximum number of threads in the Nifi UI to be much higher than that?
Shouldn't this thread pool be configured for the same size? This is tunable
with the -Djava.util.concurrent.ForkJoinPool.common.parallelism java flag
(which we also adjusted in troubleshooting this).










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