8196989: Revamp G1 JMX MemoryPool and GarbageCollector MXBean definitions

JC Beyler jcbeyler at google.com
Fri Oct 19 02:45:59 UTC 2018


Hi Paul,

Looks much better to me. The other question I have is about the legacy
mode. I understand why, from a tool's perspective, having a legacy mode is
practical. By doing it this way, we are ensuring we don't break any tools
(or at least they can use a flag to be "unbroken") and give time to
migrate. This also provides an easier means to backport this fix to older
JDKs because now the legacy mode can be used to not break anything and yet
provide a means to migrate to a more sane vision of G1 collector
definitions.

Should the flag perhaps be automatically put in deprecation and then we can
mark it as obsolete for JDK13? That would give a limited time for a flag
but again I'm not sure this is really done?

Or is the plan to keep the flag for a given number of versions, try out
these new pools and ensure they provide what we need?

Thanks!
Jc

On Thu, Oct 18, 2018 at 3:18 PM Hohensee, Paul <hohensee at amazon.com> wrote:

> Thanks for your review, JC.  New webrev:
> http://cr.openjdk.java.net/~phh/8196989/webrev.03/
>
>
>
> I updated the copyright date in memoryService.hpp because I forgot to do
> so in the patch for https://bugs.openjdk.java.net/browse/JDK-8195115.
> Thomas asked me to fix in it a separate CR, so I’ve reverted it. Ditto the
> #include fixes in g1FullGCOopClosures.inline.hpp and g1HeapVerifier.cpp. At
> one point during development, clang complained about the latter, but no
> longer does.
>
>
>
> The ‘break’ on the same line as the ‘}’ was in the original version, but
> I’ve moved it. :)
>
>
>
> The comment is indeed a bit opaque. I changed it to:
>
>
>
>         // Only check heap pools that support a usage threshold.
>
>         // This is typically only the old generation space
>
>         // since the other spaces are expected to get filled up.
>
>         if (p.getType() == MemoryType.HEAP &&
>
>            p.isUsageThresholdSupported()) {
>
>                // In all collectors except G1, only the old generation
> supports a
>
>                 // usage threshold. The G1 legacy mode "G1 Old Gen" also
> does. In
>
>                 // G1 default mode, both the old space ("G1 Old Space":
> it's not
>
>                 // really a generation in the non-G1 collector sense) and
> the
>
>                 // humongous space ("G1 Humongous Space"), support a usage
> threshold.
>
>                 // So, the following condition is true for all non-G1 old
> generations,
>
>                 // for the G1 legacy old gen, and for the G1 default
> humongous space.
>
>                // It is not true for the G1 default old gen.
>
>                 //
>
>                 // We're allocating humongous objects in this test, so the
> G1 default
>
>                 // mode "G1 Old Space" occupancy doesn't change, because
> humongous
>
>                 // objects are allocated in the "G1 Humongous Space". If
> we allowed
>
>                 // the G1 default mode "G1 Old Space", notification would
> never
>
>                 // happen because no objects are allocated there.
>
>                if (!p.getName().equals("G1 Old Space")) {
>
>
>
> Finally, the G1MonitoringScope constructor now does a better job of
> selecting a memory manager.
>
>
>
> Paul
>
>
>
> *From: *JC Beyler <jcbeyler at google.com>
> *Date: *Wednesday, October 17, 2018 at 4:47 PM
> *To: *"Hohensee, Paul" <hohensee at amazon.com>
> *Cc: *"hotspot-gc-dev at openjdk.java.net" <hotspot-gc-dev at openjdk.java.net>,
> "serviceability-dev at openjdk.java.net" <serviceability-dev at openjdk.java.net
> >
> *Subject: *Re: 8196989: Revamp G1 JMX MemoryPool and GarbageCollector
> MXBean definitions
>
>
>
> Hi Paul,
>
>
>
> I looked at this but it took time for me to "digest" it and I haven't
> entirely gone through the real GC changes :)
>
>
>
> My few remarks on the webrev itself are:
>
>    -
> http://cr.openjdk.java.net/~phh/8196989/webrev.02/src/hotspot/share/services/memoryService.hpp.udiff.html
>
>       - There is no need to change the copyright, right?
>
>
>
>   -
> http://cr.openjdk.java.net/~phh/8196989/webrev.02/test/hotspot/jtreg/gc/TestMemoryMXBeansAndPoolsPresence.java.udiff.html
>
>      - the break seems to be on the wrong line, no?
>
>
>
> +                }                break;
>
>
>
>
>
>     -
> http://cr.openjdk.java.net/~phh/8196989/webrev.02/test/jdk/java/lang/management/MemoryMXBean/LowMemoryTest.java.udiff.html
>
>     and
>
>
> http://cr.openjdk.java.net/~phh/8196989/webrev.02/test/jdk/java/lang/management/MemoryMXBean/MemoryManagement.java.udiff.html
>
>
>
> +                        // In G1, humongous objects are tracked in the
> old space only in
>
> +                        // legacy monitoring mode. In default mode, G1
> tracks humongous
>
> +                        // objects in the humongous space, which latter
> also supports a
>
> +                        // usage threshold. Since we're allocating
> humongous objects in
>
> +                        // this test, in default mode the old space
> doesn't change. For
>
> +                        // this test, we use the old space in legacy mode
> (it's called
>
> +                        // "G1 Old Gen" and the humongous space in
> default mode. If we
>
> +                        // used "G1 Old Space" in default mode,
> notification would never
>
> +                        // happen.
>
>
>
> -> latter seems ot be the wrong word or something is missing in that
> sentence
>
> -> the parenthesis is never closed (it's called.... is missing a )
> somewhere
>
>
>
> Thanks,
>
> Jc
>
>
>
> On Wed, Oct 17, 2018 at 1:18 PM Hohensee, Paul <hohensee at amazon.com>
> wrote:
>
> Ping.
>
>
>
> *From: *serviceability-dev <serviceability-dev-bounces at openjdk.java.net>
> on behalf of "Hohensee, Paul" <hohensee at amazon.com>
> *Date: *Thursday, October 11, 2018 at 6:46 PM
> *To: *"hotspot-gc-dev at openjdk.java.net" <hotspot-gc-dev at openjdk.java.net>,
> "serviceability-dev at openjdk.java.net" <serviceability-dev at openjdk.java.net
> >
> *Subject: *Re: 8196989: Revamp G1 JMX MemoryPool and GarbageCollector
> MXBean definitions
>
>
>
> Any takers? :)
>
>
>
> *From: *serviceability-dev <serviceability-dev-bounces at openjdk.java.net>
> on behalf of "Hohensee, Paul" <hohensee at amazon.com>
> *Date: *Monday, October 8, 2018 at 7:50 PM
> *To: *"hotspot-gc-dev at openjdk.java.net" <hotspot-gc-dev at openjdk.java.net>,
> "serviceability-dev at openjdk.java.net" <serviceability-dev at openjdk.java.net
> >
> *Subject: *RFR: 8196989: Revamp G1 JMX MemoryPool and GarbageCollector
> MXBean definitions
>
>
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8196989
>
> CSR: https://bugs.openjdk.java.net/browse/JDK-8196991
>
> Webrev: http://cr.openjdk.java.net/~phh/8196989/webrev.02/
>
>
>
> As requested, I split the jstat counter update off from the MXBean update.
> This is the MXBean update. The jstat counter RFE is
> https://bugs.openjdk.java.net/browse/JDK-8210965 and its CSR is
> https://bugs.openjdk.java.net/browse/JDK-8210966.
>
>
>
> The MXBean CSR is in draft state, I’d greatly appreciate review and
> sign-off.
>
>
>
> It’s been suggested that we add another pool to represent the free region
> set, but doing so would be incompatible with existing MXBean use invariants
> for all GCs. These are:
>
>
>
>    1. The sum of the pools’ MemoryUsage.max properties is the total
>    reserved heap size.
>    2. The sum of the pools’ MemoryUsage.committed properties is the total
>    committed size.
>    3. The sum of the pools’ MemoryUsage.used properties is the total size
>    of the memory containing objects, live and dead-and-yet-to-be-collected, as
>    the case might be, plus intentional gaps between them.
>    4. The total free space is (sum of the max properties – sum of the
>    used properties).
>    5. The total uncommitted space is (sum of the max properties – sum of
>    the committed properties).
>    6. The total committed free space is (2) – (3).
>
>
>
> To keep invariants 1, 2 and 3, the free region pool’s “max” property
> should be “undefined” (i.e., -1). The intuitive, to me, “used” property
> value would be the total free space, but that would violate invariant 4
> above. Defining the “committed” property as the total committed free space
> would violate invariants 2 and 6.
>
>
>
> The patch passes the submit repo, hotspot tier1, and, separately, the
> serviceability, jfr, and gc jtreg tests. I’m uncertain how to construct a
> test that checks for valid MXBean content: the existing tests don’t. Any
> such test will be fragile due to possible future Hotspot changes that
> affect the values, and to run-to-run variability. I’ve done by-hand
> comparisons between the old and new MXBean content using the SwingSet2
> demo, including using App CDS, and the numbers look reasonable.
>
>
>
> The guts of the change are in G1MonitoringSupport::recalculate_sizes,
> initialize_serviceability, memory_managers, memory_pools, and
> G1MonitoringScope. I also defined TraceConcMemoryManagerStats to track the
> concurrent cycle in a way analogous to TraceCMSMemoryManagerStats. The
> changes to the includes in g1FullGCOopClosures.inline.hpp and
> g1HeapVerifier.cpp are to satisfy compiler complaints. I changed the 3rd
> argument to the G1MonitoringScope constructor to a mixed_gc flag, and use
> accessor methods instead of direct field accesses when accessor methods
> exist. I believe I’ve minimized the latter. I updated the copyright date to
> 2018 in memoryService.hpp because I neglected to do so in my previous G1
> MXBean patch.
>
>
>
> Thanks,
>
>
>
> Paul
>
>
>
>
>
>
>
>
> --
>
>
>
> Thanks,
>
> Jc
>


-- 

Thanks,
Jc
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