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From "B. Todd Burruss" <bburr...@real.com>
Subject Re: Key Caching
Date Mon, 26 Jul 2010 23:33:27 GMT
i run cassandra with a 30gb heap on machines with 48gb total with good
results.  i don't use more just because i want to leave some for the OS
to cache disk pages, etc.  i did have the problem a couple of times with
GC doing a full stop on the JVM because it couldn't keep up.  my
understanding of the CMS GC is that it kicks in when a certain
percentage of the JVM heap is used.  by tweaking
XX:CMSInitiatingOccupancyFraction you can make this kick in sooner (or
later) and this fixed it for me.

my JVM opts differ just slightly from the latest cassandra changes in
0.6

JVM_OPTS=" \
        -ea \
        -Xms30G \
        -Xmx30G \
        -XX:SurvivorRatio=128 \
        -XX:MaxTenuringThreshold=0 \
        -XX:TargetSurvivorRatio=90 \
        -XX:+AggressiveOpts \
        -XX:+UseParNewGC \
        -XX:+UseConcMarkSweepGC \
        -XX:+CMSParallelRemarkEnabled \
        -XX:CMSInitiatingOccupancyFraction=88 \
        -XX:+HeapDumpOnOutOfMemoryError \
        -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -verbose:gc \
        -Dnetworkaddress.cache.ttl=60 \
        -Dcom.sun.management.jmxremote.port=6786 \
        -Dcom.sun.management.jmxremote.ssl=false \
        -Dcom.sun.management.jmxremote.authenticate=false \
"






On Mon, 2010-07-26 at 14:04 -0700, Peter Schuller wrote:
> > If the cache is stored in the heap, how big can the heap be made
> > realistically on a 24gb ram machine? I am a java newbie but I have read
> > concerns with going over 8gb for the heap as the GC can be too painful/take
> > too long. I already have seen timeout issues (node is dead errors) under
> > load during GC or compaction. Can/should the heap be set to 16gb with 24gb
> > ram?
> 
> I have never run Cassandra in production with such a large heap, so
> I'll let others comment on practical experience with that.
> 
> In general however, with the JVM and the CMS garbage collector (which
> is enabled by default with Cassandra), having a large heap is not
> necessarily a problem depending on the application's workload.
> 
> In terms of GC:s taking too long - with the default throughput
> collector used by the JVM you will tend to see the longest pause times
> scale roughly linearly with heap size. Most pauses would still be
> short (these are what is known as young generation collections), but
> periodically a so-called full collection is done. WIth the throughput
> collector, this implies stopping all Java threads while the *entire*
> Java heap is garbage collected.
> 
> WIth the CMS (Concurrent Mark/Sweep) collector the intent is that the
> periodic scans of the entire Java heap are done concurrently with the
> application without pausing it. Fallback to full stop-the-world
> garbage collections can still happen if CMS fails to complete such
> work fast enough, in which case tweaking of garbage collection
> settings may be required.
> 
> One thing to consider in any case is how much memory you actually
> need; the more you give to the JVM, the less there is left for the OS
> to cache file contents. If for example your true working set in
> cassandra is, to grab a random number, 3 GB and you set the heap
> sizeto 15 GB - now you're wasting a lot of memory by allowing the JVM
> to postpone GC until it starts approaching the 15 GB mark. This is
> actually good (normally) for overall GC throughput, but not
> necessarily good overall for something like cassandra where there is a
> direct trade-off with cache eviction in the operating system possibly
> causing additional I/O.
> 
> Personally I'd be very interested in hearing any stories about running
> cassandra nodes with 10+ gig heap sizes, and how well it has worked.
> My gut feeling is that it should work reasonable well, but I have no
> evidence of that and I may very well be wrong. Anyone?
> 
> (On a related noted, my limited testing with the G1 collector with
> Cassandra has indicated it works pretty well. Though I'm concerned
> with the weak ref finalization based cleanup of compacted sstables
> since the G1 collector will be much less deterministic in when a
> particular object may be collected. Has anyone deployed Cassandra with
> G1 on very large heaps under real load?)
> 



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