RFC: (How to?) Replace use of mark-word in leak-profiler (Lilliput)

[Markus Gronlund]

Hi Roman,

Providing another hashtable works fine to solve this problem, but I am a little bit concerned that the overhead might not fully warrant it. The default hashtable has 1009 buckets IIRC, but the number of leak candidates (samples) found during heap traversal might only be a relatively small number (default queue size is 256).

I was trying to come up with something more lightweight, albeit the hashtable solution might be more straightforward to understand.

This solution still uses space available in the markword, since we have already provisioned to use this as a "scratch area" as part of setup - but it is now limited to accommodate Lilliput (i.e. restricted to use only the lower 32-bits only).

I hope this will enable you and the Lilliput team to make progress.

https://github.com/openjdk/jdk/pull/5918

Thanks
Markus

-----Original Message-----
From: Roman Kennke <rkennke at redhat.com> 
Sent: den 12 oktober 2021 15:20
To: Markus Gronlund <markus.gronlund at oracle.com>; hotspot-jfr-dev <hotspot-jfr-dev at openjdk.java.net>; lilliput-dev at openjdk.java.net
Subject: Re: RE: RFC: (How to?) Replace use of mark-word in leak-profiler (Lilliput)

Thank you, Markus!

In the meantime I implemented proposal #1, map oop->Edge* using a hashtable:

https://urldefense.com/v3/__https://github.com/openjdk/lilliput/pull/18__;!!ACWV5N9M2RV99hQ!ZmQY9j8-lGVFEdImbsZ59XMHW7lZ3Z-y_m468GDj-fk7Diiu714wMSWlaQFJqekrscIc$ 

If you find a better way to do that, I would appreciate it! Meanwhile, I will leave that PR#18 open.

Thanks,
Roman


> Hi Roman,
> 
> Thank you for bringing this up.
> 
> I am exploring a few ideas, and I will get back to you soon.
> 
> Cheers
> Markus
> 
> -----Original Message-----
> From: hotspot-jfr-dev <hotspot-jfr-dev-retn at openjdk.java.net> On 
> Behalf Of Roman Kennke
> Sent: den 8 oktober 2021 17:44
> To: hotspot-jfr-dev <hotspot-jfr-dev at openjdk.java.net>; 
> lilliput-dev at openjdk.java.net
> Subject: RFC: (How to?) Replace use of mark-word in leak-profiler 
> (Lilliput)
> 
> Hi there,
> 
> I'm currently thinking about an issue that came up when I tested JFR/Leakprofiler with Lilliput.
> 
> First come context: In Lilliput I am storing a (compressed) Klass* in the object header, and want to phase out all uses of the dedicated Klass*-word and then remove that. This means that we need to be a little more careful when accessing the object header and/or the object's klass.
> 
> In the leak-profiler, we (temporarily) store an Edge* into the object header, and preserve the actual mark in a table. However, that means that the heap traversal would not work anymore because the object doesn't have a valid Klass* anymore.
> 
> Therefore I'm thinking about alternative ways to associate objects with an Edge*:
> 
> 1. Use a (hash-)table for it (oop->Edge*), in EdgeStore.
> 2. Compress the Edge* to 32bits and store that only in the lower 32bits of the header (the Klass* is in the upper 32bits, currently). Not quite sure how to do this, though, it means we have to control allocation of the Edge instances in a contiguous space.
> 3. Store the object Klass* somewhere as long as we have it (before 
> overriding it in EdgeStore::associate_leak_content_with_candidate()),
> and use that for iterating the object (in BFSClosure::iterate()). But where? Maybe in the Edge to the object? I.o.w. each Edge would also store the Klass* of its pointee?
> 
> I wanted to check with you if any of those approaches make sense to you, or maybe you have even better ideas?
> 
> Thanks for helping,
> Roman
>