Thanks, very much appreciated.
Paul
On 2/12/18, 7:42 AM, "Erik Helin" <erik.he...@oracle.com> wrote:
Hi Paul,
sorry, I'm just back from a really busy week with two conferences. I
will take a look tomorrow.
Thanks,
Erik
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 https://bugs.openjdk.java.net/browse/JDK-8196991.
>
> Would you please comment on the CSR, and on the original CSR
https://bugs.openjdk.java.net/browse/JDK-8196719?
>
> Thanks,
>
> Paul
>
> 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 the
> 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 behavior
> is not correct), those uses will encounter a new behavior in
an update
> 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
another
> bug/rfe to do it. I’d be happy to fix that one too, once we
have a spec.
> >
> > What do you think of my suggestions? To summarize:
> >
> > - Keep the existing memory pools and add humongous and
archive pools.
> > - 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” collector.
> > - Add the “G1 Young”, “G1 Mixed” and “G1 Concurrent Cycle”
collectors.
> > - 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 differ
> from a generation in the traditional sense as applied to
e.g. Serial,
> 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 "Gen"
> begs the question how it differs from the others ending in
"Space").
> Again, we could introduce a flag such as
-XX:+UseG1LegacyPoolsAndBeans
> 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 if
> users thought they could change collector and the "Archive
Space" memory
> 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
Generation"
> 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 change
> 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 behavior).
>
> 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 changed
> again when G1 became the default collector in JDK 9.
>
> Thanks,
> Erik
>
> > 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
GarbageCollectionMXBeans
> > for G1 are very old, G1 has changed a lot since those
were implemented
> > (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 mixed
> > 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-dev@openjdk.java.net"
> > > <serviceability-dev@openjdk.java.net>,
"hotspot-gc-...@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
CollectionUsage.used
> > > 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/
> > >
<http://cr.openjdk.java.net/%7Ephh/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
<erik.he...@oracle.com>
> > > <mailto:erik.he...@oracle.com>, David Holmes
> > > <david.hol...@oracle.com>
<mailto:david.hol...@oracle.com>
> > > *Cc: *"serviceability-dev@openjdk.java.net"
> > > <mailto:serviceability-dev@openjdk.java.net>
> > > <serviceability-dev@openjdk.java.net>
> > > <mailto:serviceability-dev@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 results
> > >
> > > 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
> > >
> > >
> > >
> > > 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.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>
<mailto:erik.he...@oracle.com>,
> > > David Holmes <david.hol...@oracle.com>
> > > <mailto:david.hol...@oracle.com>
> > > *Cc:
*"serviceability-dev@openjdk.java.net"
> > >
<mailto:serviceability-dev@openjdk.java.net>
> > > <serviceability-dev@openjdk.java.net>
> > >
<mailto:serviceability-dev@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 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
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
>
>
>
>
>
>
