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From Oleksandr Shulgin <>
Subject Re: Java GC pauses, reality check
Date Sat, 26 Nov 2016 19:17:05 GMT
On Nov 26, 2016 20:04, "Graham Sanderson" <> wrote:

Not AFAIK; is marked
as resolved in 3.4, though we are not running it so I can’t say much about

But I was referring to
which removed it again in 3.10 and 3.0.10.


It looks like Zing is no longer tied price wise per core which was a show
stopper for us, but it is now priced per server which may affect others

Note in fact ironically, running 2.1.x with off heap memtables, we had some
of our JVMs running for over a year which made us hit when we restarted
some nodes for other reasons.

On Nov 26, 2016, at 12:07 AM, Oleksandr Shulgin <> wrote:

On Nov 25, 2016 23:47, "Graham Sanderson" <> wrote:

If you are seeing 25-30 second GC pauses then (unless you are so badly
configured) seeing full GC under CMS (though G1 may have similar problems).

With CMS eventual fragmentation causing promotion failure is inevitable
(unless you cycle your nodes before it happens). Either your heap has way
too big an old gen, or too small a young gen (but then you need pretty
hefty boxes to be able to run with a large young gen - of the say 4-8G
range) without young collections taking too long.

Depending on your C* version I would highly recommend off heap men-tables.
With those we were able to considerably reduce our heap sizes, despite
having large throughput on a smallish number of nodes.

Aren't offheap memtables discontinued in the most recent releases of 3.0
and 3.x for a good reason? I thought using them could lead to segfaults?


I recommend reading this if you use CMS
011/10/java-cg-hotspots-cms-and-heap.html, and also not that if you see a
lot of objects of size 131074 in promotion failures then memtables are the
problem - you can try and flush them sooner, but moving them off heap works
better I think.

On Nov 25, 2016, at 4:38 PM, Kant Kodali <> wrote:

+1 Chris Lohfink response

I would also restate the following sentence "java GC pauses are pretty much
a fact of life" to "Any GC based system pauses are pretty much a fact of

I would be more than happy to see if someone can counter prove.

On Fri, Nov 25, 2016 at 1:41 PM, Chris Lohfink <> wrote:

> No tuning will eliminate gcs.
> 20-30 seconds is horrific and out of the ordinary. Most likely
> implementing antipatterns and/or poorly configured. Sub 1s is realistic but
> with some workloads still may require some tuning to maintain. Some
> workloads are very unfriendly to GCs though (ie heavy tombstones, very wide
> partitions).
> Chris
> On Fri, Nov 25, 2016 at 3:25 PM, S Ahmed <> wrote:
>> Hello!
>> From what I understand java GC pauses are pretty much a fact of life, but
>> you can tune the jvm to reduce the likelihood of the frequency and length
>> of GC pauses.
>> When using Cassandra, how frequent or long have these pauses known to
>> be?  Even with tuning, is it safe to assume they cannot be eliminated?
>> Would a 20-30 second pause be something out of the ordinary?
>> Thanks.

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