I am using 2.2.4 and have seen multiple compactors running on the same table. The number of compactors seems to be controlled by concurrent_compactors. As of type of compactions, I've seen normal compaction, tombstone compaction. Validation and Anticompaction seem to always be single threaded.

On Thu, Jan 21, 2016 at 8:28 AM, PenguinWhispererThe . <th3penguinwhisperer@gmail.com> wrote:
Thanks for that clarification Sebastian! That's really good to know! I never took increasing this value in consideration because of my previous experience.

In my case I had a table that was compacting over and over... and only one CPU was used. So that made me believe it was not multithreaded (I actually believe I asked this on IRC however it's been a few months ago so I might be wrong).

Have there been behavioral changes on this lately? (I was using 2.0.9 or 2.0.11 I believe).

2016-01-21 14:15 GMT+01:00 Sebastian Estevez <sebastian.estevez@datastax.com>:

>So compaction of one table will NOT spread over different cores.

This is not exactly true. You actually can have multiple compactions running at the same time on the same table, it just doesn't happen all that often. You essentially would have to have two sets of sstables that are both eligible for compactions at the same time.

all the best,


On Jan 21, 2016 7:41 AM, "PenguinWhispererThe ." <th3penguinwhisperer@gmail.com> wrote:
After having some issues myself with compaction I think it's noteworthy to explicitly state that compaction of a table can only run on one CPU. So compaction of one table will NOT spread over different cores.
To really have use of concurrent_compactors you need to have multiple table compactions initiated at the same time. If those are small they'll finish way earlier resulting in only one core using 100% as compaction is generally CPU bound (unless your disks can't keep up).
I believe it's better to be CPU(core) bound on one core(or at least not all) for compaction than disk IO bound as this would result in writes and reads, ... having performance impact.
Compaction is a maintenance task so it shouldn't be eating all your resources.

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2016-01-16 0:18 GMT+01:00 Kai Wang <depend@gmail.com>:
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.


On Fri, Jan 15, 2016 at 5:23 PM, Jeff Jirsa <jeff.jirsa@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


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?