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From Ken Hancock <ken.hanc...@schange.com>
Subject Re: Cassanda JVM GC defaults question
Date Thu, 24 Apr 2014 14:09:49 GMT
Wouldn't I want flush_largest_memtables_at larger than my
OccupyingFraction?  I want GC to kick in before I have to dump my memtables
not after.


On Wed, Apr 23, 2014 at 10:12 AM, Ruchir Jha <ruchir.jha@gmail.com> wrote:

> Lowering CMSInitiatingOccupancyFraction to less than 0.75 will lead to
> more GC interference and will impact write performance. If you're not
> sensitive to this impact, your expectation is correct, however make
> sure your flush_largest_memtables_at is always set to less than or
> equal to the occupancy fraction.
>
> On 4/23/14, Ken Hancock <ken.hancock@schange.com> wrote:
> > I'm in the process of trying to tune the GC and I'm far from an expert in
> > this area, so hoping someone can tell me I'm either out in left field or
> > on-track.
> >
> > Cassandra's default GC settings are (abbreviated):
> > +UseConcMarkSweepGC
> > +CMSInitiaitingOccupancyFraction=75
> > +UseCMSInitiatingOccupancyOnly
> >
> > Also in cassandra.yaml:
> > flush_largest_memtables_at: 0.75
> >
> > Since the new heap is relatively small, if I'm understanding this
> correctly
> > CMS will normally not kick in until it's at roughly 75% of the heap (75%
> of
> > size-new, new being relatively small compared to the overall heap).
>  These
> > two settings being very close would seem that both trigger at nearly the
> > same point which might be undesirable as the flushing would also create
> > more GC pressure (in addition to FlushWriter blocking if multiple tables
> > are queued for flushing because of this).
> >
> > Clearly more heap will give us more peak running room, but would also
> > lowering the CMSInitiatingOccupancyFraction help at the expense of some
> > added CPU for more frequent, smaller collections?
> >
> > Mikio Bruan's blog had some interesting tests in this area....
> > http://blog.mikiobraun.de/2010/08/cassandra-gc-tuning.html
> >
>



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