Fwd: Feedback on G1GC

Fabian Lange fabian.lange at codecentric.de
Sun Dec 20 13:27:27 UTC 2015


Hi,
(originall posted on adoption-discuss)
since a while I have been recommending and using G1GC for JDK 8
applications.

This week I was looking at an application which should be the ideal
candidate.
It was given 4GB ram, has a steady memory usage of about 1-2GB and during
its work it generates only garbage. It reads data from sockets,
deserializes it, manipulates it, serializes it and writes it out to
sockets. It is processing 100k to 500k of such requests per second.

With the default G1 settings the machine was very loaded. The collection
times were pretty long. It even ran out of memory a few times because the
GC could not catch up.

When looking at the logs I was surprised to see extremely small eden/young
sizes. The old gen was really big (like 3.5GB, but mostly empty) while G1
was churning on 300MB young.

I raised the question on
https://groups.google.com/a/jclarity.com/d/msg/friends/hsZiz6HTm9M/MbuttBioCgAJ
where Charlie Hunt was so kind to explain the reasons behind the behaviour.
It either did not make sense to me, or I did not understand the
explanation.

What I did is what I always did regardless of the collector: I increased
young space, knowing it contains mostly garbage.
The overall behaviour of the JVM was much improved by that.

I found it irritating, that according to Charlie, the main reason for the
small eden is the Pause Time Limit. Because GC was not meeting its goal it
reduced eden. While I observed better results doing the opposite.

I also enabled -XX:+ParallelRefProcEnabled.

Logs are available from the above discussion, but I can send them in
separate mail if desired.

As far as I can tell the ergonomics are not working for me, and the changes
I need to do are counter intuitive. From other discussions I learned that
quite many people observed better overall performance with raising the
pause time restriction.

Is there public information to why the current defaults are as they are?
How would feedback on these defaults work?

Best regards,
Fabian
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