metaspace.cpp: why are counters in ChunkManager updated atomically?

[Mikael Gerdin]

Hi Thomas,

On 2016-10-27 14:08, Thomas Stüfe wrote:
> Hi all,
>
> I am currently working on a prototype for
> https://bugs.openjdk.java.net/browse/JDK-8166690 and have a question about
> the ChunkManager class in metaspace.cpp.
>
> ChunkManager has the _free_chunks_total, _free_chunks_count counters. It
> seems the coding goes some lengths to avoid updating them often, so instead
> of updating them when a chunk is freed it attemps to delay and accumulate
> updates. This makes changing the MetaChunk allocation quite complicated,
> because there are large windows during which the counters are invalid and
> do not reflect reality.
>
> I see that the counters are updated atomically, so I assume the reason for
> delaying the updates is that atomics are expensive. But I could not find a
> good reason why the counters are updated atomically. To me, all
> modifications seem happen under lock protection
> (SpaceManager::expand_lock()). What am I overlooking?

I don't think you are overlooking anything. The fact that these are 
updated with atomics is something that I've noticed as well at some 
point but I don't think I ever got around to fixing that.


I'm not sure I understand where in the code the delayed and accumulated 
updates take place but if you think that's the case then it's probably 
true. I suspect that at this point you are one of the handful of people 
who are familiar with the chunk allocation code :)

Regards
/Mikael

>
> Thanks a lot,
>
> Kind Regards, Thomas
>