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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: LeveledCompaction and/or SnappyCompressor causing memory pressure during repair
Date Sun, 11 Mar 2012 18:32:36 GMT
One thing you may want to look at is the meanRowSize from nodetool
cfstats and your compression block size. In our case the mean
compacted size is 560 bytes and 64KB block size caused CPU tickets and
a lot of short lived memory. I have brought by block size down to 16K.
The result tables are not noticeably larger and less memory pressure
on the young gen. I might try going down to 4 K next.

On Sat, Mar 10, 2012 at 5:38 PM, Edward Capriolo <edlinuxguru@gmail.com> wrote:
> The only downside of compression is it does cause more memory
> pressure. I can imagine something like repair could confound this.
> Since it would seem like building the merkle tree would involve
> decompressing every block on disk.
>
> I have been attempting to determine if the block size being larger or
> smaller has any effect on memory pressure.
>
> On Sat, Mar 10, 2012 at 4:50 PM, Peter Schuller
> <peter.schuller@infidyne.com> wrote:
>>> However, when I run a repair my CMS usage graph no longer shows sudden drops
>>> but rather gradual slopes and only manages to clear around 300MB each GC.
>>> This seems to occur on 2 other nodes in my cluster around the same time, I
>>> assume this is because they're the replicas (we use 3 replicas). Parnew
>>> collections look about the same on my graphs with or without repair running
>>> so no trouble there so far as I can tell.
>>
>> I don't know why leveled/snappy would affect it, but disregarding
>> that, I would have been suggesting that you are seeing additional heap
>> usage because of long-running repairs retaining sstables and delaying
>> their unload/removal (index sampling/bloom filters filling your heap).
>> If it really only happens for leveled/snappy however, I don't know
>> what that might be caused by.
>>
>> --
>> / Peter Schuller (@scode, http://worldmodscode.wordpress.com)

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