Any new ideas on JDK-8187338 (Per anonymous class class loader data is costly)?

[Andrew Dinn]

On 23/11/17 11:56, Thomas Stüfe wrote:
> Hi Zhengyu and Andrew,
> 
> Have you had any new ideas on how to deal
> with https://bugs.openjdk.java.net/browse/JDK-8187338?

Zhengyu developed a prototype patch against jdk8u which extends the time
an anonymous loader is willing to hand out SpecializedChunks after the
initial one is exhausted (it will use up to 4 1K chunks before scaling
up to a 4K chunk). In his tests this brought the average waste down from
~60%$ to ~30%. This will be a help for lambdas but in the long term I
guess the real solution needs to be a solution like the one in M
Cimadamore's JEP (JDK-8158765).

I have already reviewed this patch privately and found it to be ok. I'll
leave Zhengyu to reply as to whether he thinks it is ready to push to
jdk10 and/or down to jdk8.

> My thoughts are
> . . .
> (1) could be quite easily implemented. What do you think?
Well, I guess option (1) might be useful for apps which have a lot of
normal (non-anon) classloaders that define only small numbers of
classes. However, the really bad waste %ges we saw with anon loaders
were because they had just one class and so always had one partial
chunk. That left a high potential for waste to be in the ~50% range
(well, actually, it was two partial chunks with CompressedClassPointers
-- one for the class object and one for the related non-class objects).
If you have enough classes to start biting into a 2nd chunk (i.e just
2-3) then your waste is going to be in the 0-50% range. 3 Chunks puts it
in the 0-34% range. So, 'small' loaders is not quite such an issue as
1-shot 'anon' loaders.

That said, I think (1) could still be helpful for some (unusual?) apps.
I don't think it is too difficult to implement or as you say to come up
with a workable default -- the hardest bit will probably be getting the
new product flag accepted. Zhengyu may have something to say here as well?

regards,


Andrew Dinn
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