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From "Ariel Weisberg (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-10766) OOM from 22000 memtables
Date Wed, 02 Dec 2015 20:50:11 GMT


Ariel Weisberg commented on CASSANDRA-10766:

[~amorton] can you come up with instructions to reproduce this? Is this something that has
only happened once, infrequently, repeatedly?

> OOM from 22000 memtables 
> -------------------------
>                 Key: CASSANDRA-10766
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>         Environment: * 32 cores, physical machines
> * 8 SSD drives in raid 0
> * hotspot java 8.60 
> * Linux version 3.8.13-98.el6uek.x86_64 (mockbuild@x86-ol6-builder-04) (gcc version 4.4.7
20120313 (Red Hat 4.4.7-11)
> * G1 heap with 26 GB heap, 24 concurrent and 24 parallel threads, 500 ms pause target
>            Reporter: amorton
>            Assignee: Ariel Weisberg
>         Attachments: MemtableFlushWriter.txt, SlabPoolCleaner.txt, lots-of-memtables.png,
thread-dump-flush.txt, thread-dump-full.txt
> Hi folks, we observed this on a new cassandra 2.1.11 cluster. 
> The node ran out of memory, and when it did it had 22,000 + memtables in memory with
a corresponding number of pending MemtableFlushWriter and MemtablePostFlushWriter tasks. We
have seen other nodes going OOM, but have not looked at them as deeply as this one.
> We also noticed that while there was 8 flush writers configured, there were times when
only one (or maybe two) were making progress. And that there was times when the SlabPoolCleaner
was flushing memtables 50 times a second. 
> I'll try to put as much info below that makes sense. I have the full system logs and
a heap dump and can go through them with someone if needed. Please ping me on IRC or the usual
email if I am not on. 
> Below is part of the Heap Dump showing the huge amount of memtables in ram. 
> I have also attached copied of the same thread dump:
> * thread-dump-full.txt is the full dump, node that is includes approximately 300 shared
worker threads. 
> * thread-dump-flush.txt is the MemtableFlushWriter and MemtablePostFlush threads. 
> Note that all the MemtableFlushWriter threads are trying to update the DataTracker, and
that the MemtablePostFlush is waiting on the countdown latch to be set. The PostFlush object
in the MemtablePostFlush thread is associated with the MemtableFlushWriter:744 thread. 
> Looking at the MemtableFlushWriter threads they are trying to flush the following tables:
> {noformat}
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:750" 266120 120
> system.hints
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:744" 223864 120
> system.sstable_activity
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:752" 220848 120
> system.sstable_activity
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:754" 219736 120
> system.sstable_activity
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:749" 214336 120
> system.sstable_activity
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:753" 211632 120
> system.sstable_activity
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:748" 149616 120
> system.sstable_activity
> java.lang.Thread [Thread, Stack Local] "MemtableFlushWriter:751" 143480 120
> system.sstable_activity
> {noformat}
> When I saw that these were all on the same table I was wondering if the CAS in {{DataTracker.replaceFlushed()}}
was starving some threads. 
> The attached SlabPoolCleaner.txt file is the lines from the SlabPoolCleaner thread from
a couple of system.log files. Notes:
> * The machine went OOM at 08:38:25,463 server time. 
> * The server was flushing user tables between 100 and 200 MB until approx 2015-11-22
07:40:39,669 server time.
> * After that the server was flushing files that were 10's of bytes. 
> The attached MemtableFlushWriter.txt includes log lines from the MemtableFlushWriter
threads. Notes:
> * from approx 2015-11-22 07:41:18 it often looks like only one MemtableFlushWriter thread
progresses at a time. 
> A few other notes:
> * This machine has logged a few dozen LEAK DETECTED messages. 
> * This has logged messages about corrupted SSTables. 
> At this point I am unsure if the SlabPoolCleaner was reacting to low memory and correctly
flushing, or it was incorrectly flushing a lot which overloaded the flush system. We are also
unsure what role the CAS in {{DataTracker.replaceFlushed()}} played, all the MemtableFlushWriter
threads with the same progress seems odd.
> As a test for the CAS theory we ran the node with {{memtable_flush_writers}} set to 2,
it has been running for approx 12 hours under load and has not OOM'd yet. This is a good sign.

> Any thoughts on what could be happening ? 

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