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From Sebastian Estevez <sebastian.este...@datastax.com>
Subject Re: compaction throughput
Date Thu, 21 Jan 2016 15:11:49 GMT
@penguin There have been steady improvements in the different compaction
strategies recently but not major re-writes.

All the best,


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Sebastián Estévez

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On Thu, Jan 21, 2016 at 9:12 AM, Kai Wang <depend@gmail.com> wrote:

> 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,
>>>
>>> Sebastián
>>> 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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>>>> email has been sent from a virus-free computer protected by Avast.
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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.
>>>>>
>>>>> Thanks.
>>>>>
>>>>> 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
>>>>>>
>>>>>> 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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