RFR (S): 8195115: G1 Old Gen MemoryPool CollectionUsage.used values don't reflect mixed GC results

Hohensee, Paul hohensee at amazon.com
Thu Feb 22 20:54:47 UTC 2018

Ping for a review please.



On 2/16/18, 12:26 PM, "serviceability-dev on behalf of Hohensee, Paul" <serviceability-dev-bounces at openjdk.java.net on behalf of hohensee at amazon.com> wrote:

    The CSR https://bugs.openjdk.java.net/browse/JDK-8196719 for the original fix has been approved, so I’m back to requesting a code review, please.
    Bug: https://bugs.openjdk.java.net/browse/JDK-8195115
    Webrev: http://cr.openjdk.java.net/~phh/8195115/webrev.hs.01/
    Passed a submit repo run, passes its jtreg test, and a JDK8 version is in production use at Amazon.
    From the original RR:
        > The bug is that from the JMX point of view, G1’s incremental collector
        > (misnamed as the “G1 Young Generation” collector) only affects G1’s
        > survivor and eden spaces. In fact, mixed collections run by this
        > collector also affect the G1 old generation.
        > This proposed fix is to record, for each of a JMX garbage collector's
        > memory pools, whether that memory pool is affected by all collections
        > using that collector. And, for each collection, record whether or not
        > all the collector's memory pools are affected. After each collection,
        > for each memory pool, if either all the collector's memory pools were
        > affected or the memory pool is affected for all collections, record
        > CollectionUsage for that pool.
        > For collectors other than G1 Young Generation, all pools are recorded as
        > affected by all collections and every collection is recorded as
        > affecting all the collector’s memory pools. For the G1 Young Generation
        > collector, the G1 Old Gen pool is recorded as not being affected by all
        > collections, and non-mixed collections are recorded as not affecting all
        > memory pools. The result is that for non-mixed collections,
        > CollectionUsage is recorded after a collection only the G1 Eden Space
        > and G1 Survivor Space pools, while for mixed collections CollectionUsage
        > is recorded for G1 Old Gen as well.
        > Other than the effect of the fix on G1 Old Gen MemoryPool.
        > CollectionUsage, the only external behavior change is that
        > GarbageCollectorMXBean.getMemoryPoolNames will now return 3 pool names
        > rather than 2.
        > With this fix, a collector’s memory pools can be divided into two
        > disjoint subsets, one that participates in all collections and one that
        > doesn’t. This is a bit more general than the minimum necessary to fix
        > G1, but not by much. Because I expect it to apply to other incremental
        > region-based collectors, I went with the more general solution. I
        > minimized the amount of code I had to touch by using default parameters
        > for GCMemoryManager::add_pool and the TraceMemoryManagerStats constructors.

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