Hi Paul,

sorry, I'm just back from a really busy week with two conferences. I will take a look tomorrow.


On 02/07/2018 11:17 PM, Hohensee, Paul wrote:
I’ve filed https://bugs.openjdk.java.net/browse/JDK-8196989 : Revamp G1 JMX 
MemoryPool and GarbageCollector MXBean definitions and the corresponding CSR 

Would you please comment on the CSR, and on the original CSR 



On 2/2/18, 1:20 PM, "hotspot-gc-dev on behalf of Hohensee, Paul" 
<hotspot-gc-dev-boun...@openjdk.java.net on behalf of hohen...@amazon.com> wrote:

     And, can a get a reviewer or reviewers for the CSR?
Thanks, Paul On 2/2/18, 1:14 PM, "Hohensee, Paul" <hohen...@amazon.com> wrote: +hotspot-gc-use. I’ve filed a CSR for the current patch, see https://bugs.openjdk.java.net/browse/JDK-8196719. Here’s the argument in favor. It’s possible that there are JDK8 users that rely on current G1 old gen CollectionUsage behavior, but imo it’s unlikely because it’s of little use. Perhaps Kirk and others with operational experience can weigh in. Let’s think about use cases. G1 full GC’s happen rarely and only under severe pressure, so when they do external reaction is pretty much limited to reducing load so the JVM can get back to a usable steady state, or just restarting the JVM. Old gen CollectionUsage is zero until a full GC occurs, after which its value includes both long-lived objects and any transient data that was in eden and the survivor space. That value doesn’t tell you anything about long term old gen occupancy or survivor size because it lumps them all together. So, it isn’t a useful metric, nor will it be after any subsequent full GCs. The only information it provides is on the first zero to non-zero transition, which just tells you that the JVM is/was in trouble. Further, the effect of the runup to a full GC is SLA violations, which are noticed before the full GC happens, so detecting the first full GC is confirmation, not prediction. Conclusion: G1 old gen CollectionUsage is unlikely to be in use in its current form, so changing its definition to something usable is low risk. “G1 Old Space” is fine, as is “G1 Archive Space”. Are you ok with the G1 archive space overlapping the G1 old space? Should we add an archive space to the other collectors? If so, how would it be defined and would having it overlap with the old generation as a live prefix be ok? "G1 Young Generation" is the currently young+mixed collector. You’re right, if one is iterating over all collectors, there will be redundancy if we keep the old ones. I’m usually leery of introducing a flag such as UseG1LegacyMXBeans (I changed the name, since all the interfaces are MXBeans, hope that’s ok) which must be either indefinitely maintained, or go through a deprecation cycle. I don’t see a way out of the ‘iterate over all collectors’ problem without it though. Paul On 1/31/18, 3:42 AM, "Erik Helin" <erik.he...@oracle.com> wrote: On 01/31/2018 02:30 AM, Hohensee, Paul wrote:
             > It’s true that my patch doesn’t completely solve the larger 
problem, but it fixes the most immediately important part of it, particularly for 
JDK8 where current expected behavior is entrenched.
Yes, your patch fixes part of the problem, but as I said, can
             potentially lead to more confusion. I'm not sure that doing this
             behavioral change for a public API in an JDK 8 update release is 
             right thing. There are likely users that rely on the memory pool 
"G1 Old
             Gen" only being updated by a full collection (even though that 
             is not correct), those uses will encounter a new behavior in an 
             release with your patch.
The good thing is that we have very experienced engineers participating
             in the CSR process that have much more experience than I have in
             evaluating the impact of behavioral changes such as this one. 
Would you
             please file a CSR request for your patch to get their opinion?
See https://wiki.openjdk.java.net/display/csr/Main for more details
             about CSR.
On 01/31/2018 02:30 AM, Hohensee, Paul wrote:
             If we’re going to fix the larger problem, imo we should file 
             bug/rfe to do it. I’d be happy to fix that one too, once we have a 
             > What do you think of my suggestions? To summarize:
             > - Keep the existing memory pools and add humongous and archive 
             > - Make the archive pool part of the old gen, and generalize it 
to all collectors.
             > - Split humongous regions off from the old gen pool into their 
own pool. The old gen and humongous pools are disjoint region sets.
             > - Keep the existing “G1 Young Generation” and “G1 Old 
Generation” collectors and their associated memory pools (net of this patch). We 
add the humongous pool to them.
             > - Add “G1 Full” as an alias of the existing “G1 Old Generation” 
             > - Add the “G1 Young”, “G1 Mixed” and “G1 Concurrent Cycle” 
             > - Set the G1 old gen memory pool max size to –Xmx, the archive 
space max size to whatever it is, and the rest of the G1 memory pool max sizes to 
-1 == undefined, as now.
             > The resulting memory pools:
             > “G1 Eden Space”
             > “G1 Survivor Space”
             > “G1 Old Gen”
             > “G1 Humongous Space”
             > “Archive Space”
The "Space" suffix is unfortunate, but acceptable. I'm least happy about
             the "Gen" suffix for the "G1 Old Gen", since G1's old regions 
             from a generation in the traditional sense as applied to e.g. 
             Parallel and CMS. I would be more happy to use a consistent naming
             scheme in the form of "G1 Old Space" (having only one pool ending 
             begs the question how it differs from the others ending in 
             Again, we could introduce a flag such as 
             for those that really wants the old names.
"Archive Space" should be named "G1 Archive Space" since it differs in
             implementation from the other collectors. It would be unfortunate 
             users thought they could change collector and the "Archive Space" 
             pool would keep the same behavior.
> The resulting collectors and their memory pools:
             > “G1 Young Generation” (the existing young/mixed collector), “G1 
Old Generation”/”G1 Full”, “G1 Mixed”
             > - “G1 Eden Space”
             > - “G1 Survivor Space”
             > - “G1 Old Gen”
             > - “G1 Humongous Space”
             > “G1 Young”
             > - “G1 Eden Space”
             > - “G1 Survivor Space”
             > - “G1 Humongous Space”
             > “G1 Concurrent Cycle”
             > - “G1 Old Gen”
             > - “G1 Humongous Space”
             > I’m not religious about the names, but I like my suggestions. :)
             I think it will be confusing for users to have both "G1 Old 
             and "G1 Full", particularly for tools iterating over all
             GarbageCollectorMXBeans. There is no way to indicate that a
             GarbageCollectorMXBeans is an alias of another 
GarbageCollectorMXBean (I
             thought about such a solution as well).
I guess I don't follow what the GarbageCollectorMXBean "G1 Young
             Generation" is meant to represent?
> The significant addition to my previous email, and an incompatible change, is splitting humongous regions off from the old gen pool. This means that apps that specifically monitor old gen occupancy will no longer see humongous regions. Monitoring apps that just add up info about all a collector’s pools won’t see a difference. I may be corrected (by Kirk, perhaps), but imo it’s not as bad a compatibility issue as one might think, because the type of app that uses a lot of humongous regions isn’t all that common. E.g., apps that cache data in the heap in the form of large compressed arrays, and apps with large hashmap bucket list arrays. The heaps such apps use are very often large enough to use 32mb regions, hence need really big objects to actually allocate humongous regions. So why not enable backwards compatibility by allowing a user to set the
             flag -XX:+UseG1LegacyPoolsAndBeans? It is not that cumbersome for 
us to
             maintain the current definition of memory pools and collectors. 
Such a
             flag allows us to start over and do this right and a user who 
relies on
             the current behavior can get that by just setting a flag. Doing 
such a
             change in a major release also allows us to clearly highlight the 
             in the release notes (users are more prepared for larger changes 
in a
             major release and that they might have to add flags to keep old 
It is not uncommon for memory pools to change in major releases. The
             perm gen pool was removed in JDK 8, the default pools changed when
             Parallel Old became default old collector way back in JDK 7 and 
             again when G1 became the default collector in JDK 9.
> Thanks,
             > Paul
             > On 1/30/18, 5:51 AM, "Erik Helin" <erik.he...@oracle.com> wrote:
             >      On 01/30/2018 03:07 AM, Hohensee, Paul wrote:
             >      > That’s one reviewer who’s ok with a short term patch. 
Anyone else? And,
             >      > any reviewers for said short term patch? :)
             >      Well, the patch is not really complete as it is. The 
problem is the
             >      definitions of the MemoryPoolMXBeans and 
GarbageCollectorMXBeans, which,
             >      as I tried to hint at in my first email, is a mess for G1. 
The names and
             >      implementations of these MemoryPoolMXBeans and 
             >      for G1 are very old, G1 has changed a lot since those were 
             >      (hence my suggestion for finally fixing this).
             >      The issue with your patch is that the MemoryPoolMXBean named 
"G1 Old
             >      Gen" consists of both old and humongous regions (it will 
also include
             >      archive regions). Old regions can be collected by mixed, 
concurrent and
             >      full collections. Humongous regions can be collected by 
young, mixed or
             >      full collections and the concurrent cycle. Archive regions 
will never be
             >      collected. Your patch will update the pool in the case of a 
             >      collection collecting old regions or humongous regions, but 
misses the
             >      following cases:
             >      - a young collection collecting humongous regions
             >      - a concurrent cycle collecting humongous regions
             >      - a concurrent cycle collecting old regions
             >      Unfortunately I could not come up with a good way to solve 
the above
             >      without re-designing the pools. I'm not sure about 
accepting your patch
             >      as is, since it might cause even more confusion for users 
compared to
             >      the current (already confusing) situation.
             >      One idea we have discussed is to implement the re-design 
but also add a
             >      flag, -XX:+UseG1LegacyPoolsAndBeans (false by default), to 
allow for a
             >      smoother transition. Would that solution work for you?
             >      Thanks,
             >      Erik
             >      > Thanks,
             >      >
             >      > Paul
             >      >
             >      > *From: *mandy chung <mandy.ch...@oracle.com>
             >      > *Organization: *Oracle Corporation
             >      > *Date: *Monday, January 29, 2018 at 1:41 PM
             >      > *To: *"Hohensee, Paul" <hohen...@amazon.com>
             >      > *Cc: *"serviceability-...@openjdk.java.net"
             >      > <serviceability-...@openjdk.java.net>, 
             >      > <hotspot-gc-...@openjdk.java.net>
             >      > *Subject: *Re: RFR (S): 8195115: G1 Old Gen MemoryPool
             >      > CollectionUsage.used values don't reflect mixed GC results
             >      >
             >      > I created  JDK-8196362 to look into whether it makes 
sense to provide
             >      > some categorization to differentiate eden space vs the 
heap space for
             >      > long-lived objects.
             >      >
             >      > W.r.t. to JDK-8195115, I have to defer to GC team to 
comment on the
             >      > mixed collection update.  If they are okay, I have no 
objection to
             >      > implement a short-term fix and do the proper G1 memory 
pools as a
             >      > separate patch.
             >      >
             >      > Mandy
             >      >
             >      > On 1/29/18 1:02 PM, Hohensee, Paul wrote:
             >      >
             >      >     We don’t use getType, and you guessed correctly: we 
use the memory
             >      >     pool name as an indicator of the specific 
characteristics of a
             >      >     memory pool, in particular eden.
             >      >
             >      >     What we want is an indication of long term heap 
occupancy. We
             >      >     calculate it using CollectionUsage for non-eden heap 
memory pools,
             >      >     regardless of collector. We don’t use JMX 
notification, rather we
             >      >     periodically poll CollectionUsage for memory pools 
with names that
             >      >     contain “Old”, “Tenured”, or “Survivor”. We get the 
memory pools
             >      >     from the GarbageCollectorMXBeans (we don’t care what 
the collector
             >      >     names are). For the named memory pools, we sum 
             >      >     and divide by the sum of CollectionUsage.max to get a 
long term heap
             >      >     occupancy percentage. We don’t want to include eden 
because it’s
             >      >     really just an allocation buffer and not part of the 
storage for
             >      >     long-lived objects. I suppose we could use a negative 
test instead
             >      >     by using all memory pools with names that don’t 
include “Eden”.
             >      >
             >      >     The bug is that the “G1 Old Gen” memory pool isn’t 
being updated
             >      >     when the “G1 Young Generation” collector runs a mixed 
collection. As
             >      >     far as JMX is concerned, that collector only knows 
about eden and
             >      >     the survivor space. The patch adds the old gen to the 
memory pools
             >      >     it knows about and has mixed collections update the 
old gen’s
             >      >     CollectionUsage.
             >      >
             >      >     I managed to get a submit repo run to succeed last 
week and it found
             >      >     a problem. I’ve uploaded a new webrev that fixes the 
failure of the
             >      >     jtreg test TestMemoryMXBeansAndPoolsPresence.java due 
to the young
             >      >     gen collector being expected to know only about eden 
and the
             >      >     survivor space.
             >      >
             >      >     http://cr.openjdk.java.net/~phh/8195115/webrev.hs.01/
             >      >     
             >      >
             >      >     Waiting on the submit repo to come back with a result 
on it.
             >      >
             >      >     Thanks,
             >      >
             >      >     Paul
             >      >
             >      >     *From: *mandy chung <mandy.ch...@oracle.com>
             >      >     <mailto:mandy.ch...@oracle.com>
             >      >     *Organization: *Oracle Corporation
             >      >     *Date: *Monday, January 29, 2018 at 10:52 AM
             >      >     *To: *"Hohensee, Paul" <hohen...@amazon.com>
             >      >     <mailto:hohen...@amazon.com>, Erik Helin 
             >      >     <mailto:erik.he...@oracle.com>, David Holmes
             >      >     <david.hol...@oracle.com> 
             >      >     *Cc: *"serviceability-...@openjdk.java.net"
             >      >     <mailto:serviceability-...@openjdk.java.net>
             >      >     <serviceability-...@openjdk.java.net>
             >      >     <mailto:serviceability-...@openjdk.java.net>,
             >      >     "hotspot-gc-...@openjdk.java.net"
             >      >     <mailto:hotspot-gc-...@openjdk.java.net>
             >      >     <hotspot-gc-...@openjdk.java.net>
             >      >     <mailto:hotspot-gc-...@openjdk.java.net>
             >      >     *Subject: *Re: RFR (S): 8195115: G1 Old Gen MemoryPool
             >      >     CollectionUsage.used values don't reflect mixed GC 
             >      >
             >      >     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 
             >      >
             >      >             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, 
             >      >             (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
             >      >
             >      >
             >      >
             >      >         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 
             >      >
             >      >             Paul
             >      >
             >      >             *From: *mandy chung <mandy.ch...@oracle.com>
             >      >             <mailto:mandy.ch...@oracle.com>
             >      >             *Organization: *Oracle Corporation
             >      >             *Date: *Friday, January 26, 2018 at 2:38 PM
             >      >             *To: *"Hohensee, Paul" <hohen...@amazon.com>
             >      >             <mailto:hohen...@amazon.com>, Erik Helin
             >      >             <erik.he...@oracle.com> 
             >      >             David Holmes <david.hol...@oracle.com>
             >      >             <mailto:david.hol...@oracle.com>
             >      >             *Cc: *"serviceability-...@openjdk.java.net"
             >      >             <mailto:serviceability-...@openjdk.java.net>
             >      >             <serviceability-...@openjdk.java.net>
             >      >             <mailto:serviceability-...@openjdk.java.net>,
             >      >             "hotspot-gc-...@openjdk.java.net"
             >      >             <mailto:hotspot-gc-...@openjdk.java.net>
             >      >             <hotspot-gc-...@openjdk.java.net>
             >      >             <mailto:hotspot-gc-...@openjdk.java.net>
             >      >             *Subject: *Re: RFR (S): 8195115: G1 Old Gen 
             >      >             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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