max heap size for compressed oops

[Matthias Wahl]

Hello,

i was delving into performance considerations that some java based
projects suggested. One is restricting the maximum heap size on 64 bit
systems due to the limitation on compressed Oops. The docs e.g.
https://wiki.openjdk.java.net/display/HotSpot/CompressedOops mention an
upper limit of 32GB.

Howerer the elasticsearch documentation recommends to not increase the
heap size beyond 30.5GB :
https://www.elastic.co/guide/en/elasticsearch/guide/current/heap-sizing.html#compressed_oops

it's the only source i know using this recommendation.
They recently changed it due to an advice from oracle, they say. but no
reason why is given.

Do you know any reason for limiting the heap size to 30.5 GB instead of
32 GB?

thanks in advance!

Matthias Wahl

this is kind of a cross-post as it was already asked on IRC. But I'll
repeat it here, so any answer may persist for others having the same
question.