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From Stack <st...@duboce.net>
Subject Re: Avoiding OutOfMemory Java heap space in region servers
Date Mon, 16 Aug 2010 05:49:45 GMT
On Sat, Aug 14, 2010 at 12:21 PM, Sean Bigdatafun
<sean.bigdatafun@gmail.com> wrote:
> On Sat, Aug 14, 2010 at 11:36 AM, Stack <stack@duboce.net> wrote:
>> You don't answer my questions above.
>>
> I ran 6GB now, but I really wonder why I should not run 12GB. (People said
> if I have too much heapsize, GC will kill me)
>

The bigger the heap, the longer it takes to sweep.

So, what have you experienced?  A long GC pause that caused your
RegionServers to shutdown?  You using default GC settings?  You played
with them at all?  Have you had a look at
http://wiki.apache.org/hadoop/PerformanceTuning or seen what others
have reported on this list lately the GC configs that work for their
hw/loading?  Are you swapping at all?  Whats your loading like?  All
writes?  A mix? Realtime?  Analytics? Whats your hardware like?

GC is a pain.  Its the bane of all java apps.  HBase seems to be
particularly taxing of GC algos.


> It would be really helpful if we can get a detail suggestion on the heapsize
> configuration. Theoretically, a) the more heap size I configure, the more
> data I can hold in memory; b) but does GC pause insanely long on a large
> heap size JVM?

Sorry.  We (or should I say java) doesn't have a one answer that fits
all deploys/hardward/loadings.  Yeah, a.) is true above -- it means
you can cache more so you'll get better read performance.  b.) it can
happen, for sure.  Thats the JVM for you.  See the cited, if a little
stale, link above and what others have posted mitigating b.).  Ongoing
there is HBASE-2902 wherein we hope to improve our default shipping GC
config. and a few of the lads are going beyond deep diving on this
stuff.  We'll report back if they find any pearls.

Yours,
St.Ack

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