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

mandy chung mandy.chung at oracle.com
Mon Jan 29 18:52:06 UTC 2018

On 1/29/18 10:35 AM, mandy chung wrote:
> Thanks for the reply Paul.   Try to understand a little more on the 
> specific from gc-specific memory pool you depend on.
> On 1/29/18 8:27 AM, Hohensee, Paul wrote:
>> A name change would affect Amazon’s heap monitoring, and thus I 
>> expect it would affect other users as well.
>> As long as there are gc-specific memory pools, we’re going to need to 
>> be able to identify them, and right now that’s done via name.
> MemoryPoolMXBean::getType returns "heap" memory type for GC-specific 
> memory pools.  Are you using this method?  Do you use the name to 
> build in specific characteristic of a memory pool (e.g. eden vs old gen)?
>> All the mxbeans are identified by name, so that’s a general design 
>> principle. The only way I can think of to get rid of name dependency 
>> would be to figure out what abstract metrics users want to monitor 
>> and implement them for all collectors. HeapUsage (instantaneous 
>> occupancy) is one, CollectionUsage (long-lived occupancy) is another, 
>> both of these for the entire heap, not just particular memory pools.
> The sum of HeapUsage and CollectionUsage of all heap memory pools was 
> expected to give an incorrect approximation for the entire heap 
> usage.  Are you seeing issue/bug with the sum result?

typo: s/an incorrect approximation/an approximation.


> Mandy
>> That said, imo there will always be a demand for the ability to get 
>> collector and memory pool specific details, so I don’t see a way to 
>> get around providing named entities.
>> Paul
>> *From: *mandy chung <mandy.chung at oracle.com>
>> *Organization: *Oracle Corporation
>> *Date: *Friday, January 26, 2018 at 2:38 PM
>> *To: *"Hohensee, Paul" <hohensee at amazon.com>, Erik Helin 
>> <erik.helin at oracle.com>, David Holmes <david.holmes at oracle.com>
>> *Cc: *"serviceability-dev at openjdk.java.net" 
>> <serviceability-dev at openjdk.java.net>, 
>> "hotspot-gc-dev at openjdk.java.net" <hotspot-gc-dev at openjdk.java.net>
>> *Subject: *Re: RFR (S): 8195115: G1 Old Gen MemoryPool 
>> CollectionUsage.used values don't reflect mixed GC results
>> On 1/25/18 1:04 PM, Hohensee, Paul wrote:
>> > The JMX API spec doesn’t specify what the memory pool or garbage > 
>> collector names are, but the current names are de-facto part of the > 
>> API, so if we change the existing ones, imo a CSR should be filed.
>> The names are implementation details but I can see how an application
>> might be impacted if they depend on it.  CSR approval is not strictly
>> necessary while I think filing one to document the change would be
>> good.
>> Does the name change impact any application you know of?  I'm trying to
>> understand if any improvement to API is needed so that applications
>> don't need to depend on the names.
>> Mandy

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