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From Yonik Seeley <>
Subject Re: JVM GC Issue
Date Sun, 03 Dec 2017 16:10:01 GMT
On Sat, Dec 2, 2017 at 8:59 PM, S G <> wrote:
> I am a bit curious on the docValues implementation.
> I understand that docValues do not use JVM memory and
> they make use of OS cache - that is why they are more performant.
> But to return any response from the docValues, the values in the
> docValues' column-oriented-structures would need to be brought
> into the JVM's memory. And that will then increase the pressure
> on the JVM's memory anyways. So how do docValues actually
> help from memory perspective?

docValues are not more performant than the on-heap fieldCache once a
fieldCache entry has been populated.
docValues do help with memory in a number of ways:
1) off-heap memory use degrades much more gracefully... heap memory
will just cause an OOM exception when heap size is exceeded
2) off-heap memory (memory used by OS cache) can be dynamically shared
with other processes running on the system (the OS allocates as
3) easier to configure (as opposed to max-heap size) since the OS will
just automatically use all free memory to cache relevant parts of
memory mapped disk files
4) off-heap memory does not need to be garbage collected (helps with
people that have huge GC pauses)


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