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doc for HBASE-3303 

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1041696 13f79535-47bb-0310-9956-ffa450edef68
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Jean-Daniel Cryans 2010-12-03 05:34:32 +00:00
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@ -1009,6 +1009,33 @@ to ensure well-formedness of your document after an edit session.
with configuration such as this.
</para>
</section>
<section xml:id="hbase.regionserver.handler.count"><title><varname>hbase.regionserver.handler.count</varname></title>
<para>
This setting defines the number of threads that are kept open to answer
incoming requests to user tables. The default of 10 is rather low in order to
prevent users from killing their region servers when using large write buffers
with a high number of concurrent clients. The rule of thumb is to keep this
number low when the payload per request approaches the MB (big puts, scans using
a large cache) and high when the payload is small (gets, small puts, ICVs, deletes).
</para>
<para>
It is safe to set that number to the
maximum number of incoming clients if their payload is small, the typical example
being a cluster that serves a website since puts aren't typically buffered
and most of the operations are gets.
</para>
<para>
The reason why it is dangerous to keep this setting high is that the aggregate
size of all the puts that are currently happening in a region server may impose
too much pressure on its memory, or even trigger an OutOfMemoryError. A region server
running on low memory will trigger its JVM's garbage collector to run more frequently
up to a point where GC pauses become noticeable (the reason being that all the memory
used to keep all the requests' payloads cannot be trashed, no matter how hard the
garbage collector tries). After some time, the overall cluster
throughput is affected since every request that hits that region server will take longer,
which exacerbates the problem even more.
</para>
</section>
<section xml:id="big_memory">
<title>Configuration for large memory machines</title>
<para>