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

[Thomas Stüfe]

Hi Mikael,

On Thu, Oct 27, 2016 at 2:41 PM, Mikael Gerdin <mikael.gerdin at oracle.com>
wrote:

> 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 :)
>
>
:) That is good news, because I then can straighten the updates out, this
makes making the changes much simpler. Will probably do this in a separate
fix. Thank you!

Thomas


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