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 <[email protected]> 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 <[email protected]> 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 > > > -- *Ken Hancock *| System Architect, Advanced Advertising SeaChange International 50 Nagog Park Acton, Massachusetts 01720 [email protected] | www.schange.com | NASDAQ:SEAC<http://www.schange.com/en-US/Company/InvestorRelations.aspx> Office: +1 (978) 889-3329 | [image: Google Talk:] [email protected] | [image: Skype:]hancockks | [image: Yahoo IM:]hancockks [image: LinkedIn]<http://www.linkedin.com/in/kenhancock> [image: SeaChange International] <http://www.schange.com/>This e-mail and any attachments may contain information which is SeaChange International confidential. The information enclosed is intended only for the addressees herein and may not be copied or forwarded without permission from SeaChange International.
