[8u] RFR backport of JDK-8198423: Improve metaspace chunk allocation

[Thomas Stüfe]

Hi Denghui,

This is quite a large change and will be a bit of work getting into jdk8.
But offhand I think JDK-8198423 should work in 8.

However, it is not the best technical solution. As you mentioned there is
currently work going on for https://openjdk.java.net/jeps/8221173, which is
a complete rewrite of the metaspace allocator and is quite a bit better
than what we have now in 11+. There exists a prototype in the jdk-sandbox
repository: http://hg.openjdk.java.net/jdk/sandbox/ , branch
"stuefe-new-metaspace-branch". It is stable already and shows quite a bit
of promise; in particular it is way more elastic than the current
allocator. I plan to bring the new allocator down to 11 when it is done but
I cannot say when that will be. My hope is to get it in for jdk15 (14 is
probably too late).

So if the matter is not pressing and you can wait a bit, you may skip
downporting JDK-8198423 and downport a superior solution,
https://openjdk.java.net/jeps/8221173.

(BTW, I am not sure whether 8221173 will come in the form of a JEP, there
is currently a lot of confusion about what needs a JEP and what doesnt.
JDK-8198423 was supposed to be a JEP too but then I just did it as a normal
RFE.)

Cheers, Thomas



> Hi all,
>   I'd like to request a backport of JDK-8198423.
>   I found some long-time running application will oom caused by metaspace
if
> the application loads / unloads classes (by different classloaders)
frequently,
> even if there are many free chunks in freelist.
>   The root cause is the algorithm of metaspace allocation in 8u didn't
support chunk
> split and merge, and if there is no chunk of the same size(except
humongous chunk)
> as the request on the free list, the algorithm will commit more memory.
>   So i plan to backport JDK-8198423, this patch imporve metaspace chunk
allocation,
> the chunks on freelist can be split and merged.
>   By the way, i also found https://openjdk.java.net/jeps/8221173, but it
still be
> a draft.
> Original bug: https://bugs.openjdk.java.net/browse/JDK-8198423
> Original patch:http://hg.openjdk.java.net/jdk/jdk/rev/825f006619e5
>   This patch doesn't apply cleanly to 8u-dev, if you think this backport
is reasonable,
> i will provide my webrev.
>   What's your comments?
>
> Thanks
> Denghui Dong