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

Erik Helin erik.helin at oracle.com
Mon Feb 26 16:46:28 UTC 2018

Hi Paul,

a couple of comments on the patch:

- memoryService.hpp:
   + 150                   bool countCollection,
   + 151                   bool allMemoryPoolsAffected = true);

   There is no need to use a default value for the parameter
   allMemoryPoolsAffected here. Skipping the default value also allows
   you to put allMemoryPoolsAffected to TraceMemoryManager::initialize
   in the same relative position as for the constructor parameter (this
   will make the code more uniform and easier to follow).

- memoryManager.cpp

   Instead of adding a default parameter, maybe add a new method?
   Something like GCMemoryManager::add_not_always_affected_pool()
   (I couldn't come up with a shorter name at the moment).

- TestMixedOldGenCollectionUsage.java

   The test is too strict about how and when collections should
   occur. Tests written this way often become very brittle, they might
   e.g. fail to finish a concurrent mark on time on a very slow, single
   core, machine. It is better to either force collections by using the
   WhiteBox API or make the test more lenient.


On 02/22/2018 09:54 PM, Hohensee, Paul wrote:
> Ping for a review please.
> Thanks,
> Paul
> 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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