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From "Jonathan Shook (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-10403) Consider reverting to CMS GC on 3.0
Date Wed, 30 Sep 2015 15:54:04 GMT


Jonathan Shook commented on CASSANDRA-10403:

This statement carries certain assumptions about the whole system, which may not be fair across
the board. For example, buffer cache is a critical consideration, but to a varying degree
depending on how cache-friendly the workload is. Further, the storage subsystem determines
a very large part of how much of a cache-miss penalty there is. So, prioritizing the cache
at the expense of the heap is not a sure win. Often it is not the right balance.

With system that have high concurrency, it is possible to scale up the performance on the
node as long as you can provide reasonable tunings to effectively take advantage of available
resources without critically bottle-necking on one. For example, with systems that have higher
effective IO concurrency and IO bandwidth across many devices, you actually need higher GC
throughput in order to match the overall IO capacity of the system, from storage subsystem
all the way to the network stack.

This rationale has been evidenced in the field when we have made tuning improvements with
G1 in certain systems as an opportunistic test. My explanation above is a probably a gross
oversimplification, but it reflects experience addressing GC throughput (and pauses, and phi,
and hints, and load shifting ... etc) issues.

> Consider reverting to CMS GC on 3.0
> -----------------------------------
>                 Key: CASSANDRA-10403
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Config
>            Reporter: Joshua McKenzie
>            Assignee: Paulo Motta
>             Fix For: 3.0.0 rc2
> Reference discussion on CASSANDRA-7486.
> For smaller heap sizes G1 appears to have some throughput/latency issues when compared
to CMS. With our default max heap size at 8G on 3.0, there's a strong argument to be made
for having CMS as the default for the 3.0 release.

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