From torsakxx at gmail.com  Sun Feb 23 21:54:51 2020
From: torsakxx at gmail.com (=?UTF-8?Q?R=C3=A9mi_p?=)
Date: Sun, 23 Feb 2020 21:54:51 +0000
Subject: Troubeshooting scanned cards in remembered sets
Message-ID: <CACuzpt7Hp3UXEucNY6zGjVPyvFqjstqun_-iaAe2Eb4aUfs9dw@mail.gmail.com>

I am looking into reducing the young collection pause times in my Java 11
application, particularly in the Scan RS step. AFAIK the duration of this
phase is linear in the sum of sizes of the remembered sets of the regions
in the collection set (=old->young region references).

Is there a built-in or recommended way to troubleshoot where all the
old->young references are coming from? I was thinking about patching the
hotspot code to log the object types of randomly sampled old->young links
in order to give me some hints, but I am wondering if there is an
easier/less intrusive solution.

Thanks.
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From mohdshaban000123 at gmail.com  Mon Feb 24 07:45:07 2020
From: mohdshaban000123 at gmail.com (mohd shaban)
Date: Mon, 24 Feb 2020 13:15:07 +0530
Subject: How to do force full garbage collection when heap usage goes beyond a
 certain threshold
Message-ID: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>

Hi Team,

Is there a way to force GC when heap usage goes beyond a certain threshold?.
I am using G1GC Algorithm.

Thanks,
Mohd Shaban
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From thomas.schatzl at oracle.com  Mon Feb 24 08:56:46 2020
From: thomas.schatzl at oracle.com (Thomas Schatzl)
Date: Mon, 24 Feb 2020 09:56:46 +0100
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
Message-ID: <717860d1-dd8e-c1f7-9340-b3c24e4d6186@oracle.com>

Hi,

On 24.02.20 08:45, mohd shaban wrote:
> Hi Team,
> 
> Is there a way to force GC when heap usage goes beyond a certain threshold?.
> I am using G1GC Algorithm.
> 
> Thanks,
> Mohd Shaban
> 

   G1 fully supports the MemoryMXBean API [0] to monitor memory usage 
either via polling or notifications.

You can issue a System.gc() in response to this information. It forces 
some kind of GC.

Hth,
   Thomas

[0] 
https://docs.oracle.com/javase/10/docs/api/java/lang/management/MemoryMXBean.html

From pasthelod at gmail.com  Mon Feb 24 09:08:35 2020
From: pasthelod at gmail.com (Pas)
Date: Mon, 24 Feb 2020 10:08:35 +0100
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
Message-ID: <CAF1H-TDu8Mk99U3d2q-__yX29oQme9n1=FrVFsROdypSXk7xKA@mail.gmail.com>

Hello,

Are you familiar with how Initiating Heap Occupancy Percent works in G1? Do
you want something else?


https://docs.oracle.com/javase/9/gctuning/garbage-first-garbage-collector.htm#JSGCT-GUID-572C9203-AB27-46F1-9D33-42BA4F3C6BF3

https://docs.oracle.com/en/java/javase/13/gctuning/garbage-first-garbage-collector.html#GUID-AC383806-7FA7-4698-8B92-4FD092B9F368



Pas



On Mon, Feb 24, 2020, 08:45 mohd shaban <mohdshaban000123 at gmail.com> wrote:

> Hi Team,
>
> Is there a way to force GC when heap usage goes beyond a certain
> threshold?.
> I am using G1GC Algorithm.
>
> Thanks,
> Mohd Shaban
> _______________________________________________
> hotspot-gc-use mailing list
> hotspot-gc-use at openjdk.java.net
> https://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>
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From thomas.schatzl at oracle.com  Mon Feb 24 09:57:29 2020
From: thomas.schatzl at oracle.com (Thomas Schatzl)
Date: Mon, 24 Feb 2020 10:57:29 +0100
Subject: Troubeshooting scanned cards in remembered sets
In-Reply-To: <CACuzpt7Hp3UXEucNY6zGjVPyvFqjstqun_-iaAe2Eb4aUfs9dw@mail.gmail.com>
References: <CACuzpt7Hp3UXEucNY6zGjVPyvFqjstqun_-iaAe2Eb4aUfs9dw@mail.gmail.com>
Message-ID: <e46f4c2e-f757-2533-084d-6bb7ad071aea@oracle.com>

Hi,

On 23.02.20 22:54, R?mi p wrote:
> I am looking into reducing the young collection pause times in my Java 
> 11 application, particularly in the Scan RS step. AFAIK the duration of 
> this phase is linear in the sum of sizes of the remembered sets of the 
> regions in the collection set (=old->young region references).
> 
> Is there a built-in or recommended way to troubleshoot where all the 
> old->young references are coming from? I was thinking about patching the 
> hotspot code to log the object types of randomly sampled old->young 
> links in order to give me some hints, but I am wondering if there is an 
> easier/less intrusive solution.
> 

At this time there is no way official way to map remembered set entries 
to particular objects. You can get remembered set sizes for each region 
with gc+remset=trace and setting G1SummarizeRSetStatsPeriod to something 
 > 0; the reason for this convoluted process is that in jdk11 getting 
remembered set statistics is expensive (fixed in 14 [2]).

If you want to proceed in that direction, I recommend adding the 
sampling in the "rebuild remembered sets" concurrent phase [3]; hook 
into somewhere in the path starting with 
G1RebuildRemSetHeapRegionClosure::scan_for_references().

What you could try in 11 to decrease remembered set memory usage (and 
typically Scan RS time a bit) is increasing the use of sparse PRTs by 
manually increasing the thresholds when using them. I.e. set 
G1RSetSparseRegionEntries to something much higher than the defaults 
(4/8/12/16/20/24 entries for 1-32M regions respectively).

The JDK-8223162 change [0] in JDK13 does that for you, selecting better 
defaults.

Another option is to increase default region size, but I assume you 
already maxed it out (but you did not describe your previous attempts).

If you can try jdk14, we reimplemented the remembered set scan work 
distribution method [1], which can yield very significant pause time 
improvements. We've seen pause times reduced to 60% of baseline (jdk13 
to jdk14) on benchmarks with very large remembered sets.

Hth,
   Thomas

[0] https://bugs.openjdk.java.net/browse/JDK-8223162
[1] https://bugs.openjdk.java.net/browse/JDK-8213108
[2] https://bugs.openjdk.java.net/browse/JDK-8233919
[3] https://bugs.openjdk.java.net/browse/JDK-8180415

From leo.korinth at oracle.com  Mon Feb 24 13:42:02 2020
From: leo.korinth at oracle.com (Leo Korinth)
Date: Mon, 24 Feb 2020 14:42:02 +0100
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
Message-ID: <376bef8d-d711-df11-4bcd-8e67ef7b830a@oracle.com>

Hi Mohd,

You probably do not want to force a a full gc with g1. G1 is designed so 
that you should be abled to do young/mixed collections and never have to 
do a full one.

Your probably ought to be on running a full gc as few times as possible. 
Forcing premature full collections will probably add latency, reduce 
throughput and mess with valuable statistics for meeting deadlines. 
Also, before Java 10, full gc for g1 was single threaded.

If you still have a good use case for it after this warning, you might 
want to weaken the explicit gc by making it a concurrent one 
(-XX:+ExplicitGCInvokesConcurrent).

What underlying problem is it you are trying to solve? Maybe it can be 
solved by other means?

Thanks,
Leo


On 24/02/2020 08:45, mohd shaban wrote:
> Hi Team,
> 
> Is there a way to force GC when heap usage goes beyond a certain threshold?.
> I am using G1GC Algorithm.
> 
> Thanks,
> Mohd Shaban
> 
> _______________________________________________
> hotspot-gc-use mailing list
> hotspot-gc-use at openjdk.java.net
> https://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
> 

From mohdshaban000123 at gmail.com  Mon Feb 24 19:45:35 2020
From: mohdshaban000123 at gmail.com (mohd shaban)
Date: Tue, 25 Feb 2020 01:15:35 +0530
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CALKQDxVPfL8V9Bysjje9WJyk9mY=p6aJgyA1t+ZRHXowqP-fPg@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
 <376bef8d-d711-df11-4bcd-8e67ef7b830a@oracle.com>
 <CALKQDxVPfL8V9Bysjje9WJyk9mY=p6aJgyA1t+ZRHXowqP-fPg@mail.gmail.com>
Message-ID: <CALKQDxXh9LciPtYjTtsGYR9A_Putumhvqj0s1FuqZWAke-DoEA@mail.gmail.com>

Hi Leo ,

Actually we set up alerts for heap utilisation and we are getting to many
alerts.
So we have done the analysis and found more heap utilisation and once GC
happened it backs to normal but suppose my alert scheduler runs at 8:00 and
at the same heap usage reach to 90 then we get the alert, But when i
monitor the heap usage , after 8:02 GC happened and it back to normal usage
means 50%.

So that why we want if heap usage reaches to 80-85% we trigger the GC so
that we can't get the alerts.

Please suggest how can we achieve this?

Thanks,
Mohd Shaban

On Mon, Feb 24, 2020 at 11:09 PM mohd shaban <mohdshaban000123 at gmail.com>
wrote:

> Hi Leo ,
>
> Actually we set up alerts for heap utilisation and we are getting to many
> alerts.
> So we have done the analysis and found more heap utilisation and once GC
> happened it backs to normal but suppose my alert scheduler runs at 8:00 and
> at the same heap usage reach to 90 then we get the alert, But when i
> monitor the heap usage , after 8:02 GC happened and it back to normal usage
> means 50%.
>
> So that why we want if heap usage reaches to 80-85% we trigger the GC so
> that we can't get the alerts.
>
> Please suggest how can we achieve this?
>
> Thanks,
> Mohd Shaban
>
> On Mon, Feb 24, 2020 at 7:13 PM Leo Korinth <leo.korinth at oracle.com>
> wrote:
>
>> Hi Mohd,
>>
>> You probably do not want to force a a full gc with g1. G1 is designed so
>> that you should be abled to do young/mixed collections and never have to
>> do a full one.
>>
>> Your probably ought to be on running a full gc as few times as possible.
>> Forcing premature full collections will probably add latency, reduce
>> throughput and mess with valuable statistics for meeting deadlines.
>> Also, before Java 10, full gc for g1 was single threaded.
>>
>> If you still have a good use case for it after this warning, you might
>> want to weaken the explicit gc by making it a concurrent one
>> (-XX:+ExplicitGCInvokesConcurrent).
>>
>> What underlying problem is it you are trying to solve? Maybe it can be
>> solved by other means?
>>
>> Thanks,
>> Leo
>>
>>
>> On 24/02/2020 08:45, mohd shaban wrote:
>> > Hi Team,
>> >
>> > Is there a way to force GC when heap usage goes beyond a certain
>> threshold?.
>> > I am using G1GC Algorithm.
>> >
>> > Thanks,
>> > Mohd Shaban
>> >
>> > _______________________________________________
>> > hotspot-gc-use mailing list
>> > hotspot-gc-use at openjdk.java.net
>> > https://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>> >
>>
>
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From mohdshaban000123 at gmail.com  Mon Feb 24 19:46:07 2020
From: mohdshaban000123 at gmail.com (mohd shaban)
Date: Tue, 25 Feb 2020 01:16:07 +0530
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CAF1H-TDu8Mk99U3d2q-__yX29oQme9n1=FrVFsROdypSXk7xKA@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
 <CAF1H-TDu8Mk99U3d2q-__yX29oQme9n1=FrVFsROdypSXk7xKA@mail.gmail.com>
Message-ID: <CALKQDxWiP1vvpRrfKnCyGYfEvBSn5UtxjN95-Ba6MxvQ2ZF6Bg@mail.gmail.com>

Hi Pas ,

If we are not using "XX:InitiatingHeapOccupancyPercent" this JVM arguments
, then still default value of that parameter is 45% so when Heap usage
reaches to 45 it should trigger the GC but it is not triggering the GC. Or
do we need to set this parameter then it will work?.


And one more thing i want to understand , is it recommended to use
"UseStringDeduplication" jvm arguments. if yes then is there any
disadvantage if using this option?

Thanks,
Mohd Shaban

On Mon, Feb 24, 2020 at 2:38 PM Pas <pasthelod at gmail.com> wrote:

> Hello,
>
> Are you familiar with how Initiating Heap Occupancy Percent works in G1?
> Do you want something else?
>
>
>
> https://docs.oracle.com/javase/9/gctuning/garbage-first-garbage-collector.htm#JSGCT-GUID-572C9203-AB27-46F1-9D33-42BA4F3C6BF3
>
>
> https://docs.oracle.com/en/java/javase/13/gctuning/garbage-first-garbage-collector.html#GUID-AC383806-7FA7-4698-8B92-4FD092B9F368
>
>
>
> Pas
>
>
>
> On Mon, Feb 24, 2020, 08:45 mohd shaban <mohdshaban000123 at gmail.com>
> wrote:
>
>> Hi Team,
>>
>> Is there a way to force GC when heap usage goes beyond a certain
>> threshold?.
>> I am using G1GC Algorithm.
>>
>> Thanks,
>> Mohd Shaban
>> _______________________________________________
>> hotspot-gc-use mailing list
>> hotspot-gc-use at openjdk.java.net
>> https://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>>
>
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From pasthelod at gmail.com  Mon Feb 24 19:49:39 2020
From: pasthelod at gmail.com (Pas)
Date: Mon, 24 Feb 2020 20:49:39 +0100
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CALKQDxX9wS0_S-rAYmqnG7a06yvkCYHH_sM42WeeK_BNw4j9hQ@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
 <CAF1H-TDu8Mk99U3d2q-__yX29oQme9n1=FrVFsROdypSXk7xKA@mail.gmail.com>
 <CALKQDxX9wS0_S-rAYmqnG7a06yvkCYHH_sM42WeeK_BNw4j9hQ@mail.gmail.com>
Message-ID: <CAF1H-TA3-Ua5JA6Lx4_=Z4MyzLsTGw6Yb0K0H1pPQ8UXhvqYjA@mail.gmail.com>

Hello,

I recommend CCing the list, as I'm just a regular JVM user, and it's
not exactly unlikely that my advice is sub par. :)

On Mon, 24 Feb 2020 at 18:49, mohd shaban <mohdshaban000123 at gmail.com> wrote:
>
> Hi Pas ,
>
> If we are not using "XX:InitiatingHeapOccupancyPercent" this JVM arguments , then still default value of that parameter is 45% so when Heap usage reaches to 45 it should trigger the GC but it is not triggering the GC. Or do we need to set this parameter then it will work?.

Which version are you using exactly? Also do you have GC logging
enabled? (-verbose:gc -XX:+PrintGCDetails) How big is the heap? (Do
you use Xms and Xmx? Do you set them to equal value?)

Furthermore, IHOP is defined in terms of "Old Gen", but there is also
the "Young Gen" ("Eden" and "Survivor" regions), and Metaspace too, so
45% of "Old Gen" requires quite a few GC cycles which gradually move
the surviving objects to the "Old Gen", and when "Old Gen" reaches 45%
then you should see the beginning of a Marking phase for the Old Gen.
(Or more likely a Mixed collection.)

>
>
> And one more thing i want to understand , is it recommended to use "UseStringDeduplication" jvm arguments. if yes then is there any disadvantage if using this option?
>

If your application has a lot of identical strings, then yes, it's
recommended. But it has a cost to scan all the strings (that survived
a few GCs already). Of course if you have good monitoring/telemetry,
then just flip it on, measure, and decide based on your measurement.

> Thanks,
> Mohd Shaban


Pas

>
> On Mon, Feb 24, 2020 at 2:38 PM Pas <pasthelod at gmail.com> wrote:
>>
>> Hello,
>>
>> Are you familiar with how Initiating Heap Occupancy Percent works in G1? Do you want something else?
>>
>>
>> https://docs.oracle.com/javase/9/gctuning/garbage-first-garbage-collector.htm#JSGCT-GUID-572C9203-AB27-46F1-9D33-42BA4F3C6BF3
>>
>> https://docs.oracle.com/en/java/javase/13/gctuning/garbage-first-garbage-collector.html#GUID-AC383806-7FA7-4698-8B92-4FD092B9F368
>>
>>
>>
>> Pas
>>
>>
>>
>> On Mon, Feb 24, 2020, 08:45 mohd shaban <mohdshaban000123 at gmail.com> wrote:
>>>
>>> Hi Team,
>>>
>>> Is there a way to force GC when heap usage goes beyond a certain threshold?.
>>> I am using G1GC Algorithm.
>>>
>>> Thanks,
>>> Mohd Shaban
>>> _______________________________________________
>>> hotspot-gc-use mailing list
>>> hotspot-gc-use at openjdk.java.net
>>> https://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use

From ecki at zusammenkunft.net  Mon Feb 24 21:03:28 2020
From: ecki at zusammenkunft.net (Bernd Eckenfels)
Date: Mon, 24 Feb 2020 21:03:28 +0000
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <CALKQDxXh9LciPtYjTtsGYR9A_Putumhvqj0s1FuqZWAke-DoEA@mail.gmail.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
 <376bef8d-d711-df11-4bcd-8e67ef7b830a@oracle.com>
 <CALKQDxVPfL8V9Bysjje9WJyk9mY=p6aJgyA1t+ZRHXowqP-fPg@mail.gmail.com>,
 <CALKQDxXh9LciPtYjTtsGYR9A_Putumhvqj0s1FuqZWAke-DoEA@mail.gmail.com>
Message-ID: <AM6PR03MB43897EF2E45D57FBA4667700FFEC0@AM6PR03MB4389.eurprd03.prod.outlook.com>

Hello Mohd,

Would be good if you can upload sample GC logs which show the issue. If a mixed collection does not clean up your heap but a FullGC does you might need to look into soft references or direct buffers.

If you have generally smallish heap one option would be to go to ParallelOld GC, it is less sensitive to fragmentation.

Gruss
Bernd
--
http://bernd.eckenfels.net
________________________________
Von: hotspot-gc-use <hotspot-gc-use-bounces at openjdk.java.net> im Auftrag von mohd shaban <mohdshaban000123 at gmail.com>
Gesendet: Monday, February 24, 2020 8:45:35 PM
An: Leo Korinth <leo.korinth at oracle.com>; hotspot-gc-use at openjdk.java.net <hotspot-gc-use at openjdk.java.net>
Betreff: Re: How to do force full garbage collection when heap usage goes beyond a certain threshold

Hi Leo ,

Actually we set up alerts for heap utilisation and we are getting to many alerts.
So we have done the analysis and found more heap utilisation and once GC happened it backs to normal but suppose my alert scheduler runs at 8:00 and at the same heap usage reach to 90 then we get the alert, But when i monitor the heap usage , after 8:02 GC happened and it back to normal usage means 50%.

So that why we want if heap usage reaches to 80-85% we trigger the GC so that we can't get the alerts.

Please suggest how can we achieve this?

Thanks,
Mohd Shaban

On Mon, Feb 24, 2020 at 11:09 PM mohd shaban <mohdshaban000123 at gmail.com<mailto:mohdshaban000123 at gmail.com>> wrote:
Hi Leo ,

Actually we set up alerts for heap utilisation and we are getting to many alerts.
So we have done the analysis and found more heap utilisation and once GC happened it backs to normal but suppose my alert scheduler runs at 8:00 and at the same heap usage reach to 90 then we get the alert, But when i monitor the heap usage , after 8:02 GC happened and it back to normal usage means 50%.

So that why we want if heap usage reaches to 80-85% we trigger the GC so that we can't get the alerts.

Please suggest how can we achieve this?

Thanks,
Mohd Shaban

On Mon, Feb 24, 2020 at 7:13 PM Leo Korinth <leo.korinth at oracle.com<mailto:leo.korinth at oracle.com>> wrote:
Hi Mohd,

You probably do not want to force a a full gc with g1. G1 is designed so
that you should be abled to do young/mixed collections and never have to
do a full one.

Your probably ought to be on running a full gc as few times as possible.
Forcing premature full collections will probably add latency, reduce
throughput and mess with valuable statistics for meeting deadlines.
Also, before Java 10, full gc for g1 was single threaded.

If you still have a good use case for it after this warning, you might
want to weaken the explicit gc by making it a concurrent one
(-XX:+ExplicitGCInvokesConcurrent).

What underlying problem is it you are trying to solve? Maybe it can be
solved by other means?

Thanks,
Leo


On 24/02/2020 08:45, mohd shaban wrote:
> Hi Team,
>
> Is there a way to force GC when heap usage goes beyond a certain threshold?.
> I am using G1GC Algorithm.
>
> Thanks,
> Mohd Shaban
>
> _______________________________________________
> hotspot-gc-use mailing list
> hotspot-gc-use at openjdk.java.net<mailto:hotspot-gc-use at openjdk.java.net>
> https://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>
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From thomas.schatzl at oracle.com  Tue Feb 25 09:54:51 2020
From: thomas.schatzl at oracle.com (Thomas Schatzl)
Date: Tue, 25 Feb 2020 10:54:51 +0100
Subject: How to do force full garbage collection when heap usage goes
 beyond a certain threshold
In-Reply-To: <AM6PR03MB43897EF2E45D57FBA4667700FFEC0@AM6PR03MB4389.eurprd03.prod.outlook.com>
References: <CALKQDxWq5Rsfu7JzMqfXBKWAnzB4_35d77JajYuLyFM3O_0V+Q@mail.gmail.com>
 <376bef8d-d711-df11-4bcd-8e67ef7b830a@oracle.com>
 <CALKQDxVPfL8V9Bysjje9WJyk9mY=p6aJgyA1t+ZRHXowqP-fPg@mail.gmail.com>
 <CALKQDxXh9LciPtYjTtsGYR9A_Putumhvqj0s1FuqZWAke-DoEA@mail.gmail.com>
 <AM6PR03MB43897EF2E45D57FBA4667700FFEC0@AM6PR03MB4389.eurprd03.prod.outlook.com>
Message-ID: <28b80aee-8d44-61aa-8e71-31788e6f40cf@oracle.com>

Hi,

On 24.02.20 22:03, Bernd Eckenfels wrote:
> Hello Mohd,
> 
> Would be good if you can upload sample GC logs which show the issue. If 
> a mixed collection does not clean up your heap but a FullGC does you 
> might need to look into soft references or direct buffers.
> 
> If you have generally smallish heap one option would be to go to 
> ParallelOld GC, it is less sensitive to fragmentation.
> 
> Gruss
> Bernd
> -- 
> http://bernd.eckenfels.net
> ------------------------------------------------------------------------
> *Von:* hotspot-gc-use <hotspot-gc-use-bounces at openjdk.java.net> im 
> Auftrag von mohd shaban <mohdshaban000123 at gmail.com>
> *Gesendet:* Monday, February 24, 2020 8:45:35 PM
> *An:* Leo Korinth <leo.korinth at oracle.com>; 
> hotspot-gc-use at openjdk.java.net <hotspot-gc-use at openjdk.java.net>
> *Betreff:* Re: How to do force full garbage collection when heap usage 
> goes beyond a certain threshold
> Hi Leo ,
> 
> Actually we set up alerts for heap utilisation and we are getting to 
> many alerts.
> So we have done the analysis and found more heap utilisation and once GC 
> happened it backs to normal but suppose my alert scheduler runs at 8:00 
> and at the same heap usage reach to 90 then we get the alert, But when i 
> monitor the heap usage , after 8:02 GC happened and it back to normal 
> usage means 50%.
> 
> So that why we want if heap usage reaches to 80-85% we trigger the GC so 
> that we can't get the alerts.
> 
> Please suggest how can we achieve this?

   by default, in jdk9+ targets 90% heap occupancy (100 - 
G1ReservePercent) to be the threshold at which old generation should be 
reclaimed. I.e. G1 will automatically start liveness analysis in the old 
gen so that at (roughly) 90% of heap usage it will start cleaning out 
old gen.

So unless you get full gcs (or evacuation failures, i.e. "to-space 
exhausted") this behavior is normal with default options, and could 
likely be fixed by increasing the G1ReservePercent value (default 10). 
GC logs, options used and JDK version used would help a lot to further 
diagnose the issue.

Thanks,
   Thomas