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From Jiri Horky <>
Subject Re: High GC activity on node with 4TB on data
Date Thu, 12 Feb 2015 07:36:15 GMT
Hi Chris,

On 02/09/2015 04:22 PM, Chris Lohfink wrote:
>  - number of tombstones - how can I reliably find it out?
> If not getting much compression it may be worth trying to disable it,
> it may contribute but its very unlikely that its the cause of the gc
> pressure itself.
> 7000 sstables but STCS? Sounds like compactions couldn't keep up.  Do
> you have a lot of pending compactions (nodetool)?  You may want to
> increase your compaction throughput (nodetool) to see if you can catch
> up a little, it would cause a lot of heap overhead to do reads with
> that many.  May even need to take more drastic measures if it cant
> catch back up.
I am sorry, I was wrong. We actually do use LCS (the switch was done
recently). There are almost none pending compaction. We have increased
the size sstable to 768M, so it should help as as well.

> May also be good to check `nodetool cfstats` for very wide partitions.  
There are basically none, this is fine.

It seems that the problem really comes from having so much data in so
many sstables, so classes
consumes more memory than 0.75*HEAP_SIZE, which triggers the CMS over
and over.

We have turned off the compression and so far, the situation seems to be

Jirka H.

> Theres a good chance if under load and you have over 8gb heap your GCs
> could use tuning.  The bigger the nodes the more manual tweaking it
> will require to get the most out of
> them also has
> some ideas.
> Chris
> On Mon, Feb 9, 2015 at 2:00 AM, Jiri Horky <
> <>> wrote:
>     Hi all,
>     thank you all for the info.
>     To answer the questions:
>      - we have 2 DCs with 5 nodes in each, each node has 256G of
>     memory, 24x1T drives, 2x Xeon CPU - there are multiple cassandra
>     instances running for different project. The node itself is
>     powerful enough.
>      - there 2 keyspaces, one with 3 replicas per DC, one with 1
>     replica per DC (because of amount of data and because it serves
>     more or less like a cache)
>      - there are about 4k/s Request-response, 3k/s Read and 2k/s
>     Mutation requests  - numbers are sum of all nodes
>      - we us STCS (LCS would be quite IO have for this amount of data)
>      - number of tombstones - how can I reliably find it out?
>      - the biggest CF (3.6T per node) has 7000 sstables
>     Now, I understand that the best practice for Cassandra is to run
>     "with the minimum size of heap which is enough" which for this
>     case we thought is about 12G - there is always 8G consumbed by the
>     SSTable readers. Also, I though that high number of tombstones
>     create pressure in the new space (which can then cause pressure in
>     old space as well), but this is not what we are seeing. We see
>     continuous GC activity in Old generation only.
>     Also, I noticed that the biggest CF has Compression factor of 0.99
>     which basically means that the data come compressed already. Do
>     you think that turning off the compression should help with memory
>     consumption?
>     Also, I think that tuning CMSInitiatingOccupancyFraction=75 might
>     help here, as it seems that 8G is something that Cassandra needs
>     for bookkeeping this amount of data and that this was sligtly
>     above the 75% limit which triggered the CMS again and again.
>     I will definitely have a look at the presentation.
>     Regards
>     Jiri Horky
>     On 02/08/2015 10:32 PM, Mark Reddy wrote:
>>     Hey Jiri, 
>>     While I don't have any experience running 4TB nodes (yet), I
>>     would recommend taking a look at a presentation by Arron Morton
>>     on large
>>     nodes:
>>     to see if you can glean anything from that.
>>     I would note that at the start of his talk he mentions that in
>>     version 1.2 we can now talk about nodes around 1 - 3 TB in size,
>>     so if you are storing anything more than that you are getting
>>     into very specialised use cases.
>>     If you could provide us with some more information about your
>>     cluster setup (No. of CFs, read/write patterns, do you delete /
>>     update often, etc.) that may help in getting you to a better place.
>>     Regards,
>>     Mark
>>     On 8 February 2015 at 21:10, Kevin Burton <
>>     <>> wrote:
>>         Do you have a lot of individual tables?  Or lots of small
>>         compactions?
>>         I think the general consensus is that (at least for
>>         Cassandra), 8GB heaps are ideal.  
>>         If you have lots of small tables it’s a known anti-pattern (I
>>         believe) because the Cassandra internals could do a better
>>         job on handling the in memory metadata representation.
>>         I think this has been improved in 2.0 and 2.1 though so the
>>         fact that you’re on 1.2.18 could exasperate the issue.  You
>>         might want to consider an upgrade (though that has its own
>>         issues as well).
>>         On Sun, Feb 8, 2015 at 12:44 PM, Jiri Horky <
>>         <>> wrote:
>>             Hi all,
>>             we are seeing quite high GC pressure (in old space by CMS
>>             GC Algorithm)
>>             on a node with 4TB of data. It runs C* 1.2.18 with 12G of
>>             heap memory
>>             (2G for new space). The node runs fine for couple of days
>>             when the GC
>>             activity starts to raise and reaches about 15% of the C*
>>             activity which
>>             causes dropped messages and other problems.
>>             Taking a look at heap dump, there is about 8G used by
>>             SSTableReader
>>             classes in
>>             Is this something expected and we have just reached the
>>             limit of how
>>             many data a single Cassandra instance can handle or it is
>>             possible to
>>             tune it better?
>>             Regards
>>             Jiri Horky
>>         -- 
>>         Founder/CEO <>
>>         Location: *San Francisco, CA*
>>         blog:* *
>>         … or check out my Google+ profile
>>         <>
>>         <>

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