G1 - Mixed and Full GC's

Thu Dec 15 17:52:28 UTC 2016

Hi Jenny / Wolfgang

Thanks for your comments

@ Jenny  - Please find the adaptive sizing policy outputs are requested , it is not picking any old regions. It doesn't look like Pause time goal has reached as well, as we are not having very aggressive pause time goal.

45349.894: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 735623, predicted base time: 79.06 ms, remaining time: 420.94 ms, target pause time: 500.00 ms]

45349.894: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 155 regions, survivors: 2 regions, predicted young region time: 26.61 ms]

45349.894: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 155 regions, survivors: 2 regions, old: 0 regions, predicted pause time: 105.67 ms, target pause time: 500.00 ms]

2016-12-14T12:59:09.082-0500: 45349.968: [SoftReference, 0 refs, 0.0003840 secs]2016-12-14T12:59:09.083-0500: 45349.969: [WeakReference, 13 refs, 0.0002010 secs]2016-12-14T12:59:09.083-0500: 45349.969: [FinalReference, 137 refs, 0.000255

0 secs]2016-12-14T12:59:09.083-0500: 45349.969: [PhantomReference, 0 refs, 0.0001880 secs]2016-12-14T12:59:09.083-0500: 45349.969: [JNI Weak Reference, 0.0000050 secs] 45349.970: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason

: recent GC overhead higher than threshold after GC, recent GC overhead: 30.14 %, threshold: 10.00 %, uncommitted: 0 bytes, calculated expansion amount: 0 bytes (20.00 %)]

45349.970: [G1Ergonomics (Concurrent Cycles) do not request concurrent cycle initiation, reason: still doing mixed collections, occupancy: 6926893056 bytes, allocation request: 0 bytes, threshold: 4509715620 bytes (60.00 %), source: end

of GC]

I will try making the changes suggested by you and update you all with the results. Thanks again for your help.

Regards

Prasanna

-----Original Message-----
From: yu.zhang at oracle.com [mailto:yu.zhang at oracle.com]
Sent: 15 December 2016 17:28
To: Wolfgang Pedot <wolfgang.pedot at finkzeit.at>; Gopal, Prasanna CWK <prasanna.gopal at blackrock.com>; hotspot-gc-use at openjdk.java.net
Subject: Re: G1 - Mixed and Full GC's

Hi,

In the clean up cycle, about 1.6G is claimed. Those are cleaned because everything in the region is dead. It is possible those are regions for humongous objects.

Wolfgang mentioned this already, in later jdk, certain type of humongous objects can be collected during young gc.

To be more specific on the comments about what is collected during a mixed gc cycle, it is determined after REMARK, not the beginning of the marking cycle.

About the question why mixed gc sees only 29M while full gc can claim 4041.8M, there could be several reasons:

1. -XX:G1MixedGCLiveThresholdPercent=85 is still too low. This means if the region is above 85% full, they are not considered to be candidate of mixed gc. But it might not be the reason, since full gc cleans up 4041.8M.

2. In your PrintAdaptiveSizePolicy output, there is information about how the collection set for mixed gc is decided. Can you post it? Though not likely, there is a chance it is not including some regions since the pause time goal is reached.

3. Full gc cleans everything, including classes and other references. In later jdk, classes are unloaded during marking cycle. But I am not sure if jdk7 has that.

4. As Wolfgang mentioned, maybe the allocation rate is too high for g1 to keep up.

Can you try:

1 increase the region size? It will not help the 15m allocation, but maybe helpful to reduce other humongous objects.

2. lower the  -XX:InitiatingHeapOccupancyPercent=60 (default is 45) to introduce the marking cycle earlier 3. Depends on how long the marking cycle is, maybe increase the concurrent marking threads,

Thanks

Jenny

On 12/15/2016 05:00 AM, Wolfgang Pedot wrote:

> It is my understanding that what gets collected during a mixed cycle

> is determined at the beginning of the cycle so if you have a high

> allocation rate of short lived humongous objects you can fill up the

> heap with stuff the concurrent cycle will not get rid of even though

> they would be reclaimable. Newer JRE-Versions can reclaim humongous

> regions during young generation collects so they can deal better with

> them but I guess its still best to avoid humongous allocations if

> easily possible.

>

> Can you check in your log how long G1 is usually doing mixed collects?

>

> -XX:+G1PrintRegionLivenessInfo may also be an interesting option, it

> lets you see how many humongous objects are in your heap at the start

> of a concurrent cycle and how full your regions are.

>

> regards

> Wolfgang Pedot

>

> Am 15.12.2016 um 13:29 schrieb Gopal, Prasanna CWK:

>>

>> Hi Wolfgang

>>

>> Thanks for your reply. Yes I agree we have high CPU overhead. We have

>> the following settings for CPU-Cores / GC Threads.

>>

>> virtualProcessorCount=48

>>

>> ParallelGCThreads=8

>>

>> *> AFAIR humongous regions are only collected during mixed/full GC in

>> your version.*

>>

>> **This is my understanding as well, but I am not sure why these

>> objects were not collected during Mixed GC cycles.  The Mixed cycle

>> is seeing only 29 MB as claimable, but the Full GC was able to claim

>>  4041.8 MB within 100 ms

>>

>> *45349.970: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason:

>> reclaimable percentage not over threshold, candidate old regions: 46

>> regions, reclaimable: 31277496 bytes (0.42 %), threshold: 10.00 %]*

>>

>> **

>>

>> **

>>

>> Regards

>>

>> Prasanna **

>>

>> -----Original Message-----

>> From: hotspot-gc-use [mailto:hotspot-gc-use-bounces at openjdk.java.net]

>> On Behalf Of Wolfgang Pedot

>> Sent: 15 December 2016 12:10

>> To: hotspot-gc-use at openjdk.java.net<mailto:hotspot-gc-use at openjdk.java.net>

>> Subject: Re: G1 - Mixed and Full GC's

>>

>> Hi,

>>

>> not sure about my "expert"-ness but something caught my eye:

>>

>> you do have a pretty high GC overhead there (recent GC overhead:

>> 30.14 %), it is possible that G1 can simply not keep up with your

>> allocations and that leads to a full GC. How many CPU-cores does your

>> machine have and how many GC threads are used?

>>

>> Can you determine the size of your humongous objects? Getting rid of

>> those by increasing the region-size could help, AFAIR humongous

>> regions are only collected during mixed/full GC in your version.

>>

>> regards

>>

>> Wolfgang Pedot

>>

>> Am 15.12.2016 um 10:44 schrieb Gopal, Prasanna CWK:

>>

>> >

>>

>> > Hi Experts

>>

>> >

>>

>> > We have one of our application running  G1 with following

>>

>> > configuration

>>

>> >

>>

>> > -Xmx7G

>>

>> >

>>

>> >  -Xms7G

>>

>> >

>>

>> >  -XX:+UseG1GC

>>

>> >

>>

>> >  -XX:+UnlockExperimentalVMOptions

>>

>> >

>>

>> > -XX:InitiatingHeapOccupancyPercent=60

>>

>> >

>>

>> >  -XX:G1ReservePercent=20

>>

>> >

>>

>> > -XX:G1MixedGCLiveThresholdPercent=85

>>

>> >

>>

>> >  -XX:MaxGCPauseMillis=500

>>

>> >

>>

>> >  -XX:+ParallelRefProcEnabled

>>

>> >

>>

>> >  -XX:+PrintAdaptiveSizePolicy

>>

>> >

>>

>> >  -XX:+PrintHeapAtGC

>>

>> >

>>

>> >  -XX:+PrintReferenceGC

>>

>> >

>>

>> >  -XX:+PrintGCDateStamps

>>

>> >

>>

>> >  -XX:+PrintTenuringDistribution

>>

>> >

>>

>> > -XX:+PrintGCApplicationConcurrentTime

>>

>> >

>>

>> >  -XX:+PrintGCApplicationStoppedTime

>>

>> >

>>

>> > -Duser.timezone=America/Los_Angeles

>>

>> >

>>

>> >  -d64  -Dserver

>>

>> >

>>

>> > JDK : jdk_7u40_x64  ( Yes we need to move to latest JDK)

>>

>> >

>>

>> > We are seeing the following behaviour

>>

>> >

>>

>> > End of a concurrent cycle

>>

>> >

>>

>> > 2016-12-14T12:59:01.307-0500: 45342.193: Total time for which

>>

>> > application threads were stopped: 0.0189970 seconds

>>

>> >

>>

>> > 2016-12-14T12:59:01.307-0500: 45342.193: Application time:

>> > 0.0001190

>>

>> > seconds

>>

>> >

>>

>> > 2016-12-14T12:59:01.308-0500: 45342.193: *[GC cleanup

>>

>> > 5093M->3488M(7168M), 0.0075650 secs] *

>>

>> >

>>

>> > [Times: user=0.06 sys=0.00, real=0.01 secs]

>>

>> >

>>

>> > 2016-12-14T12:59:01.315-0500: 45342.201: Total time for which

>>

>> > application threads were stopped: 0.0081250 seconds

>>

>> >

>>

>> > 2016-12-14T12:59:01.315-0500: 45342.201: [GC

>> > concurrent-cleanup-start]

>>

>> >

>>

>> > 2016-12-14T12:59:01.318-0500: 45342.203: [GC

>> > concurrent-cleanup-end,

>>

>> > 0.0022490 secs]

>>

>> >

>>

>> > After 8 Seconds, Minor GC kicks in and decides not to do any Mixed

>>

>> > GC's

>>

>> >

>>

>> > 2016-12-14T12:59:09.083-0500: 45349.969: [PhantomReference, 0 refs,

>>

>> > 0.0001880 secs]2016-12-14T12:59:09.083-0500: 45349.969: [JNI Weak

>>

>> > Reference, 0.0000050 secs] 45349.970: [G1Ergonomics (Heap Sizing)

>>

>> > attempt heap expansion, reason: recent GC overhead higher than

>>

>> > threshold after GC, recent GC overhead: 30.14 %, threshold: 10.00

>> > %,

>>

>> > uncommitted: 0 bytes, calculated expansion amount: 0 bytes (20.00

>> > %)]

>>

>> >

>>

>> > *45349.970: [G1Ergonomics (Concurrent Cycles) do not request

>>

>> > concurrent cycle initiation, reason: still doing mixed collections,

>>

>> > occupancy: 6926893056 bytes, allocation request: 0 bytes, threshold:

>>

>> > 4509715620 bytes (60.00 %), source: end of GC]*

>>

>> >

>>

>> > *45349.970: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason:

>>

>> > reclaimable percentage not over threshold, candidate old regions:

>> > 46

>>

>> > regions, reclaimable: 31277496 bytes (0.42 %), threshold: 10.00 %]*

>>

>> >

>>

>> > , 0.0765450 secs]

>>

>> >

>>

>> >    [Parallel Time: 74.1 ms, GC Workers: 8]

>>

>> >

>>

>> >       [GC Worker Start (ms): Min: 45349894.0, Avg: 45349894.1, Max:

>>

>> > 45349894.2, Diff: 0.3]

>>

>> >

>>

>> >       [Ext Root Scanning (ms): Min: 4.3, Avg: 5.1, Max: 6.4, Diff:

>>

>> > 2.1, Sum: 41.2]

>>

>> >

>>

>> >       [Update RS (ms): Min: 64.6, Avg: 66.2, Max: 67.1, Diff: 2.5,

>>

>> > Sum: 529.2]

>>

>> >

>>

>> >          [Processed Buffers: Min: 324, Avg: 363.8, Max: 524, Diff:

>>

>> > 200, Sum: 2910]

>>

>> >

>>

>> >       [Scan RS (ms): Min: 0.0, Avg: 0.5, Max: 0.6, Diff: 0.6, Sum:

>>

>> > 3.7]

>>

>> >

>>

>> >       [Object Copy (ms): Min: 1.6, Avg: 2.0, Max: 2.3, Diff: 0.7, Sum:

>>

>> > 15.9]

>>

>> >

>>

>> >       [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum:

>>

>> > 0.0]

>>

>> >

>>

>> >       [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff:

>> > 0.0,

>>

>> > Sum: 0.4]

>>

>> >

>>

>> >       [GC Worker Total (ms): Min: 73.7, Avg: 73.8, Max: 73.9, Diff:

>>

>> > 0.2, Sum: 590.3]

>>

>> >

>>

>> >       [GC Worker End (ms): Min: 45349967.8, Avg: 45349967.9, Max:

>>

>> > 45349967.9, Diff: 0.0]

>>

>> >

>>

>> >    [Code Root Fixup: 0.0 ms]

>>

>> >

>>

>> >    [Clear CT: 0.1 ms]

>>

>> >

>>

>> >    [Other: 2.3 ms]

>>

>> >

>>

>> >       [Choose CSet: 0.0 ms]

>>

>> >

>>

>> >       [Ref Proc: 1.2 ms]

>>

>> >

>>

>> >       [Ref Enq: 0.2 ms]

>>

>> >

>>

>> >       [Free CSet: 0.6 ms]

>>

>> >

>>

>> >    [Eden: 310.0M(1382.0M)->0.0B(352.0M) Survivors: 4096.0K->6144.0K

>>

>> > Heap: 6603.4M(7168.0M)->6297.9M(7168.0M)]

>>

>> >

>>

>> > Heap after GC invocations=4395 (full 207):

>>

>> >

>>

>> > garbage-first heap   total 7340032K, used 6449092K

>>

>> > [0x000000063ae00000, 0x00000007fae00000, 0x00000007fae00000)

>>

>> >

>>

>> >   region size 2048K, 3 young (6144K), 3 survivors (6144K)

>>

>> >

>>

>> > compacting perm gen  total 77824K, used 77354K [0x00000007fae00000,

>>

>> > 0x00000007ffa00000, 0x0000000800000000)

>>

>> >

>>

>> >    the space 77824K,  99% used [0x00000007fae00000,

>>

>> > 0x00000007ff98a840, 0x00000007ff98aa00, 0x00000007ffa00000)

>>

>> >

>>

>> > No shared spaces configured.

>>

>> >

>>

>> > }

>>

>> >

>>

>> > [Times: user=0.60 sys=0.00, real=0.08 secs]

>>

>> >

>>

>> > Immediately after this another concurrent cycle kicks in

>>

>> >

>>

>> > 45349.974: [G1Ergonomics (Concurrent Cycles) request concurrent

>> > cycle

>>

>> > initiation, reason: occupancy higher than threshold, occupancy:

>>

>> > 6943670272 bytes, allocation request: 15728656 bytes, threshold:

>>

>> > 4509715620 bytes (60.00 %), source: concurrent humongous

>> > allocation]

>>

>> >

>>

>> > 2016-12-14T12:59:09.088-0500: 45349.974: Application time:

>> > 0.0004930

>>

>> > seconds

>>

>> >

>>

>> > 45349.974: [G1Ergonomics (Concurrent Cycles) request concurrent

>> > cycle

>>

>> > initiation, reason: requested by GC cause, GC cause: G1 Humongous

>>

>> > Allocation]

>>

>> >

>>

>> > Before the concurrent cycle can complete, Full GC kicks in

>>

>> >

>>

>> > 2016-12-14T12:59:09.195-0500: 45350.081: [*Full

>>

>> > GC2016-12-14T12:59:11.445-0500: 45352.331*: [SoftReference, 765

>> > refs,

>>

>> > 0.0001920 secs]2016-12-14T12:59:11.445-0500: 45352.331:

>>

>> > [WeakReference, 2417 refs, 0.0004400

>>

>> > secs]2016-12-14T12:59:11.446-0500: 45352.331: [FinalReference, 898

>>

>> > refs, 0.0001680 secs]2016-12-14T12:59:11.446-0500: 45352.332:

>>

>> > [PhantomReference, 10 refs, 0.0000060

>>

>> > secs]2016-12-14T12:59:11.446-0500: 45352.332: [JNI Weak Reference,

>>

>> > 0.0000190 secs] 6557M->2515M(7168M), 6.1312980 secs]

>>

>> >

>>

>> > [Eden: 2048.0K(348.0M)->0.0B(3070.0M) Survivors: 10.0M->0.0B Heap/:

>>

>> > 6557.1M(7168.0M)->2515.3M(7168.0M)]/

>>

>> >

>>

>> > It will be helpful for us to understand,

>>

>> >

>>

>> > 1)Why If  Full GC can clean up nearly 4 GB (:

>>

>> > 6557.1M(7168.0M)->2515.3M(7168.0M)]) , why didn't the mixed GC

>> > kicked

>>

>> > in earlier ?. Does this mean all our old regions are tightly packed

>>

>> > (greater than G1MixedGCLiveThresholdPercent=85  )  ?

>>

>> >

>>

>> > 2)We also see , high amount of Humongous object allocation. Could

>> > it

>>

>> > be the case that Only Full GC's can remove the Humongous objects ?

>>

>> >

>>

>> > 3)Is there any other tuning parameters we can use to mitigate this

>>

>> > scenario ?

>>

>> >

>>

>> > Please do let me know, if you need any more information. Thanks in

>>

>> > advance for your help.

>>

>> >

>>

>> > Regards

>>

>> >

>>

>> > Prasanna

>>

>> >

>>

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