LCS is IO ontensive but CPU is also relevant. On slower disks compaction may not be cpu bound.
If you aren't seeing more than one compaction thread at a time, I suspect your system is not compaction bound. all the best, Sebastián On Jan 15, 2016 7:20 PM, "Kai Wang" <dep...@gmail.com> wrote: > Sebastian, > > Because I have this impression that LCS is IO intensive and it's > recommended only on SSDs. So I am curious to see how far it can stress > those SSDs. But it turns out the most expensive part about LCS is not IO > bound but CUP bound, or more precisely single core speed bound. This is a > little surprising. > > Of course LCS is still superior in other aspects. > On Jan 15, 2016 6:34 PM, "Sebastian Estevez" < > sebastian.este...@datastax.com> wrote: > >> Correct. >> >> Why are you concerned with the raw throughput, are you accumulating >> pending compactions? Are you seeing high sstables per read statistics? >> >> all the best, >> >> Sebastián >> On Jan 15, 2016 6:18 PM, "Kai Wang" <dep...@gmail.com> wrote: >> >>> Jeff & Sebastian, >>> >>> Thanks for the reply. There are 12 cores but in my case C* only uses one >>> core most of the time. *nodetool compactionstats* shows there's only >>> one compactor running. I can see C* process only uses one core. So I guess >>> I should've asked the question more clearly: >>> >>> 1. Is ~25 M/s a reasonable compaction throughput for one core? >>> 2. Is there any configuration that affects single core compaction >>> throughput? >>> 3. Is concurrent_compactors the only option to parallelize compaction? >>> If so, I guess it's the compaction strategy itself that decides when to >>> parallelize and when to block on one core. Then there's not much we can do >>> here. >>> >>> Thanks. >>> >>> On Fri, Jan 15, 2016 at 5:23 PM, Jeff Jirsa <jeff.ji...@crowdstrike.com> >>> wrote: >>> >>>> With SSDs, the typical recommendation is up to 0.8-1 compactor per core >>>> (depending on other load). How many CPU cores do you have? >>>> >>>> >>>> From: Kai Wang >>>> Reply-To: "user@cassandra.apache.org" >>>> Date: Friday, January 15, 2016 at 12:53 PM >>>> To: "user@cassandra.apache.org" >>>> Subject: compaction throughput >>>> >>>> Hi, >>>> >>>> I am trying to figure out the bottleneck of compaction on my node. The >>>> node is CentOS 7 and has SSDs installed. The table is configured to use >>>> LCS. Here is my compaction related configs in cassandra.yaml: >>>> >>>> compaction_throughput_mb_per_sec: 160 >>>> concurrent_compactors: 4 >>>> >>>> I insert about 10G of data and start observing compaction. >>>> >>>> *nodetool compaction* shows most of time there is one compaction. >>>> Sometimes there are 3-4 (I suppose this is controlled by >>>> concurrent_compactors). During the compaction, I see one CPU core is 100%. >>>> At that point, disk IO is about 20-25 M/s write which is much lower than >>>> the disk is capable of. Even when there are 4 compactions running, I see >>>> CPU go to +400% but disk IO is still at 20-25M/s write. I use *nodetool >>>> setcompactionthroughput 0* to disable the compaction throttle but >>>> don't see any difference. >>>> >>>> Does this mean compaction is CPU bound? If so 20M/s is kinda low. Is >>>> there anyway to improve the throughput? >>>> >>>> Thanks. >>>> >>> >>>
