RFR(s): 8170520: Make Metaspace ChunkManager counters non-atomic (original) (raw)

Thomas Stüfe thomas.stuefe at gmail.com
Wed Nov 30 07:43:35 UTC 2016

Hi all,

please take a look at this small fix. (This is one of the smaller fixes in preparation for a prototype for JDK-8166690, I try to move the smaller stuff out of the way first).

Issue: https://bugs.openjdk.java.net/browse/JDK-8170520

webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8170520-Make- Metaspace-ChunkManager-Counters-NonAtomic/webrev.00/webrev/

Small discussion here: http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-October/021709.html

Background: ChunkManager keeps counters (_free_chunks_count and free_chunks_total). These counters are updated using Atomics. Because this is expensive, code goes thru some lengths to accumulate updates, which in turn makes the code more complicated and error prone as necessary. But updating the counters atomically is not needed, because updates happen under lock protection anyway.

The proposed fix makes updates non-atomic and moved the call to inc_free_chunks_total() into ChunkManager::return_chunks() - close to where the chunk is actually returned to the freelist, so the time window where the counters are invalid is very small now.

It also fixes an assert and makes inc_free_chunks_total() private because now it can be private.

Note that I do not intend to push this into 9, so this is for the upcoming 10 repository. I would like to get some reviews nevertheless, so this small change can be pushed quickly once the 10 repo exists.

Thanks!

Best Regards, Thomas

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