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From Edward Capriolo <>
Subject Re: Cassandra memtable and GC
Date Mon, 22 Nov 2010 16:54:06 GMT
On Mon, Nov 22, 2010 at 8:28 AM, Shotaro Kamio <> wrote:
> Hi Peter,
> I've tested again with recording LiveSSTableCount and MemtableDataSize
> via jmx. I guess this result supports my suspect on memtable
> performance because I cannot find Full GC this time.
> This is a result in smaller data size (160million records on
> cassandra) on different disk configuration from my previous post. But
> the general picture doesn't change.
> The attached files:
> - graph-read-throughput-diskT.png:  read throughput on my client program.
> - graph-diskT-stat-with-jmx.png: graph of cpu load, LiveSSTableCount
> and logarithm of MemtableDataSize.
> - log-gc.20101122-12:41.160M.log.gz: GC log with -XX:+PrintGC
> -XX:+PrintGCDetails -XX:+PrintGCTimeStamps
> As you can see from the second graph, logarithm of MemtableDataSize
> and cpu load has a clear correlation. When a memtable is flushed and a
> new SSTable is created (LiveSSTableCount is incremented), read
> performance will be recovered. But it degrades soon.
> I couldn't find Full GC in GC log in this test. So, I guess that this
> performance is not a result of GC activity.
> Regards,
> Shotaro
> On Sat, Nov 20, 2010 at 6:37 PM, Peter Schuller
> <> wrote:
>>> After a memtable flush, you see minimum cpu and maximum read
>>> throughput both in term of disk and cassandra records read.
>>> As memtable increase in size, cpu goes up and read drops.
>>> If this is because of memtable or GC performance issue, this is the
>>> big question.
>>> As each memtable is just 128MB when flushed, I don't really expect GC
>>> problem or caching issues.
>> A memtable is basically just a ConcurrentSkipListMap. Unless you are
>> somehow triggering some kind of degenerate casein the CSLM itself,
>> which seems unlikely, the only common circumstance where filling the
>> memtable should be resulting in a very significant performance drop
>> should be if you're running really close to heap size and causing
>> additional GC asymptotally as you're growing the memtable.
>> But that doesn't seem to be the case. I don't know, maybe I missed
>> something in your original post, but I'm not sure what to suggest that
>> I haven't already without further information/hands-on
>> experimentation/observation.
>> But running with verbose GC as I mentioned should at least be a good
>> start (-Xloggc:path/to/gclog
>> -XX:+PrintGC -XX:+PrintGCDetails -XX:+PrintGCTimestamps).
>> --
>> / Peter Schuller
> --
> Shotaro Kamio

"As you can see from the second graph, logarithm of MemtableDataSize
and cpu load has a clear correlation."

This makes sense.

"You'll see the disk read throughput is periodically going down and up.
At 17:45:00, it shows zero disk read/sec. " --> This must mean that
your load is being completely served from cache. If you have a very
high cache hit rate CPU/Memory are the ONLY factor. If CPU and
memtables are the only factor then larger memtables will start to
perform slower then smaller memtables.

Possibly with SSD the conventional thinking on Larger SSTables does
not apply (at least for your active set)

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