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From "Pierre Laporte (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8150) Revaluate Default JVM tuning parameters
Date Wed, 03 Dec 2014 17:17:15 GMT


Pierre Laporte commented on CASSANDRA-8150:

[~mstump] By any chance, have you collected Cassandra gc logs against various scenarios? 
That would be really valuable to find the right values.

I ran a test of java-driver against a C* instance on GCE n1-standard-1 server (1 vCPU, 3,75
GB RAM).  The young generation size was 100 MB (80MB for Eden, 10MB for each survivor)  and
the old generation size was 2,4GB.

I had the following:
* Average allocation rate: 352MB/s (outliers above 600MB/s)
* 4.5 DefNew cycles per second
* 1 CMS cycle every 10 minutes

Therefore, during the test, Cassandra was promoting objects at a rate of 3,8MB/s.

I think the size of Eden could be determined mostly by the allocation rate and the DefNew/ParNew
frequency we want to achieve.  Here, for instance, I would rather have had a bigger young
generation to have ~1 DefNew cycle/s.

I did not enable {{-XX:+PrintTenuringDistribution}} so I do not know whether the objects were
prematurely promoted.  That would have given pointers on survivors sizing as well.

Do you have any gc logs with such flag ?

> Revaluate Default JVM tuning parameters
> ---------------------------------------
>                 Key: CASSANDRA-8150
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Config
>            Reporter: Matt Stump
>            Assignee: Brandon Williams
>         Attachments: upload.png
> It's been found that the old twitter recommendations of 100m per core up to 800m is harmful
and should no longer be used.
> Instead the formula used should be 1/3 or 1/4 max heap with a max of 2G. 1/3 or 1/4 is
debatable and I'm open to suggestions. If I were to hazard a guess 1/3 is probably better
for releases greater than 2.1.

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