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From "Bryan Duxbury (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-512) Add configuration for global aggregate memcache size
Date Thu, 13 Mar 2008 16:48:25 GMT
Add configuration for global aggregate memcache size

                 Key: HBASE-512
                 URL: https://issues.apache.org/jira/browse/HBASE-512
             Project: Hadoop HBase
          Issue Type: Sub-task
            Reporter: Bryan Duxbury

Currently, we have a configuration parameter for the size a Memcache must reach before it
is flushed. This leads to pretty even sized mapfiles when flushes run, which is nice. However,
as noted in the parent issue, we can often get to a point where we run out of memory because
too much data is hanging around in Memcaches.

I think that we should add a new configuration parameter that governs the total amount of
memory that the region server should spend on Memcaches. This would have to be some number
less than the heap size - we'll have to discover the proper values through experimentation.
Then, when a put comes in, if the global aggregate size of all the Memcaches for all the stores
is at the threshold, then we should block the current and any subsequent put operations from
completing until forced flushes cause the memory usage to go back down to a safe level. The
existing strategy for triggering flushes will still be in play, just augmented with this blocking

This approach has the advantage of helping us avoid OOME situations by warning us well in
advance of overflow. Additionally, it becomes something of a performance tuning knob, allowing
you to allocate more memory to improve write performance. This is superior to the previously
suggested PhantomReference approach because that would possibly causes us to bump into further
OOMEs while we're trying to flush to avoid them. 

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