if I had to guess I would say it was spending time handling tombstones. If you see it happen again, and are interested, turn the logging up to DEBUG and look for messages from something starting with "Slice"
Minor (automatic) compaction will, over time, purge the tombstones. Until then reads must read discard the data deleted by the tombstones. If you perform a big (i.e. 100k's ) delete this can reduce performance until compaction does it's thing. My second guess would be read repair (or the simple consistency checks on read) kicking in. That would show up in the "ReadRepairStage" in TPSTATS it may have been neither of those two things, just guesses. If you have more issues let us know and provide some more info. Cheers ----------------- Aaron Morton Freelance Cassandra Developer @aaronmorton http://www.thelastpickle.com On 29/09/2011, at 6:35 AM, Daning wrote: > I have an app polling a few CFs (select first N * from CF), there were data > in CFs but later were deleted so CFs were empty for a long time. I found > Cassandra CPU usage was getting high to 80%, normally it uses less than 30%. > I issued the select query manually and feel the response is slow. I have > tried nodetool compact/repair for those CFs but that does not work. later, I > issue 'truncate' for all the CFs and CPU usage gets down to 1%. > > Can somebody explain to me why I need to truncate an empty CF? and what else > I could do to bring the CPU usage down? > > I am running 0.8.6. > > Thanks, > > Daning >
