CMS Promotion Failures

Y. Srinivas Ramakrishna y.s.ramakrishna at
Mon Nov 15 19:08:05 PST 2010

On 11/15/2010 4:36 PM, Brian Williams wrote:
> Thanks Ramki.  If you can entertain a few followup questions:
> 1.  If there is anything that could explain, beyond application usage, getting the promotion failures closer and closer together.

I have not seen that behaviour before. The only cases where i can think of that occurring is if
the heap occupancy is also montonically increasing so that the "free space" available keeps
getting smaller. But I am grasping at straws here.

> 2.  And as a follow on question.  If calling System.gc() leaves the heap in a better state than a promotion failure?  (This will help us to answer whether we want to push for a server restart or a scheduled GC).

If you are not using +ExplicitGCInvokesConcurrent, then both would leave the heap
in an identical state, because they both cause a single-threaded (alas, still) compacting
collection of the entire heap. So, yes, scheduling explicit gc's to compact down
the heap at an opportune time would definitely be worthwhile, if possible.

> 3.  Would using fewer parallel GC threads help reduce the fragmentation by having fewer PLABs?

Yes, this is usually the case. More PLAB's (in the form of cached free lists with the
individual GC worker threads) does translate to potentially more fragmentation, although
i have generally found that our autonomic per-block inventory control usually results
in keeping such fragmentation in check (unless the threads are "far too many" and the
free space "too little").

-- ramki

> Brian

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