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

Erik Helin erik.helin at oracle.com
Thu Jan 25 13:20:02 UTC 2018

Hi Paul,

thanks for your interest in this area and for your patch! The 
GarbageCollectorMXBean and MemoryPoolMXBean support for G1 is in need of 
some updates, so thanks for working on this.

Looking at your patch, I'm not sure that this is the direction we want 
to go in. I discussed this a bit with Thomas and Stefan J, and our 
current line of thinking is the following:

- Memory pools (MemoryMXBean):
   - "G1 Eden Regions"
   - "G1 Survivor Regions"
   - "G1 Old Regions"
   - "G1 Humongous Regions"
   - "G1 Archive Regions" (if CDS and/or AppCDS is used)

`init` for these pools would be 0, `used` would be total size of the 
"live" objects in the used regions of that type, `committed` the total 
size of the used regions of that that type and `max` would be 
MaxHeapSize. Note that "live" here means live from the GCs point of 
view, i.e. an object might be dead in an old region but the GC will 
consider that object live until a concurrent cycle has marked through 
the heap and deemed it dead.

- Collectors (GarbageCollectorMXBean):
   - "G1 Young Collector" with the pools
     - "G1 Eden Regions"
     - "G1 Survivor Regions"
     - "G1 Humongous Regions" (due to early reclamation)
   - "G1 Mixed Collector" with the pools
     - "G1 Eden Regions"
     - "G1 Survivor Regions"
     - "G1 Old Regions"
     - "G1 Humongous Regions" (due to early reclamation)
   - "G1 Full Collector" with the pools
     - "G1 Eden Regions"
     - "G1 Survivor Regions"
     - "G1 Old Regions"
     - "G1 Humongous Regions" (can collect empty humongous regions)
   - "G1 Concurrent Cycle" with the pools
     - "G1 Old Regions" (can collect empty old regions)
     - "G1 Humongous Regions" (can collect empty humongous regions)

Note that with this design, the 
GarbageCollectorMXBean::getCollectionTime() method for "G1 Concurrent 
Cycle" would be the wall clock time from start of the first initial mark 
to end of the last cleanup (also including the time of any eventual 
young collection during the concurrent cycle). So 
GarbageCollectorMXBean::getCollectionTime() would be a mix of concurrent 
and STW time for the GarbageCollectorMXBean with name "G1 Concurrent Cycle".

Also note that the MemoryPoolMXBean with name "G1 Archive Regions" will 
not be attached to any GarbageCollectorMXBean, since those regions will 
never be collected.

What do you think about this design, would it work for your use case?

If we want to go ahead with this design, then I think we might have to 
file a CSR. David (who is the HotSpot CSR representative), do we have to 
file a CSR for changing the names of MemoryPoolMXBeans and 


On 01/20/2018 12:40 AM, Hohensee, Paul wrote:
> I’d appreciate a review please.
> Bug: https://bugs.openjdk.java.net/browse/JDK-8195115
> Webrev: http://cr.openjdk.java.net/~phh/8195115/webrev.00/
> 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.
> Tested by running the new jtreg test included in the webrev. I tried to 
> use the submit repo, but it was out of order earlier today, so I’d be 
> much obliged if someone could run it through mach5 and sponsor an 
> eventual push. I successfully ran a JDK8 version of the patch through 
> all the JDK8 jtreg tests as well as the JDK8 TCK.
> Thanks,
> Paul

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