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From "PenguinWhispererThe ." <>
Subject Re: compaction throughput
Date Thu, 21 Jan 2016 12:41:47 GMT
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

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2016-01-16 0:18 GMT+01:00 Kai Wang <>:

> 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 <>
> 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: ""
>> Date: Friday, January 15, 2016 at 12:53 PM
>> To: ""
>> 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.

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