Taming Metaspace: a look at the machinery, and a proposal for a better one
Channel: FOSDEM
Date: 08/08/2020

by Thomas Stüfe

At: FOSDEM 2020
https://video.fosdem.org/2020/H.1302/metaspace.webm

When examining memory footprint of a JVM process, the delta between Java
heap usage and actual working set size can be surprisingly large. The JVM
uses off-heap memory for a lot of things: thread stacks, compiler arenas,
code heap, byte buffers, GC control... however, one of the largest
consumers of off-heap memory can be class metadata. Class metadata are
stored in Metaspace, which includes the Compressed Class Space.


The talk will explore what Metaspace actually is and what is stored there;
describe the architecture of the Metaspace allocator and the Compressed
Class Space; how it interacts with the GC; how it is sized. We will
highlight waste areas and demonstrate how to use jcmd to examine Metaspace
internals.


The current implementation of the Metaspace allocator suffers from a number
of shortcomings. They can manifest in excessive waste and a certain
"clinginess" - an unwillingness to let go of unused memory.


At SAP, we developed an improved version which is more frugal with memory
and provides a much improved elasticity. So the second part of this talk
will concentrate on our new implementation. We will highlight the
differences to the old architecture, demonstrate advantages and examine how
it works.

Room: H.1302 (Depage)
Scheduled start: 2020-02-01 16:20:00