Why does G1GC do Full GC when it only needs 40 bytes?

Tue Jun 10 01:09:43 UTC 2014

>
> t means that there is a increasing object allocation in a short amount of
> time and G1 think it needs more heap. However, the following GC indicates
> live object size is only ~15G, half of your heap. Combine these two, it
> means that probably we need to ask G1 to do more clean up for your
> application to make sure there is enough space to handle such allocation
> traffic.
>
> 2014-06-06T13:48:17.164+0300: 21811.068: [Full GC 29G->15G(30G), 53.9089520 secs]
>    [Eden: 0.0B(5456.0M)->0.0B(1536.0M) Survivors: 0.0B->0.0B Heap: 29.9G(30.0G)->15.1G(30.0G)]
>
> To avoid full GC, here are two important G1 parameters you can tune
> (suggested numbers are based on my personal experience)
>
> -XX:*G1ReservePercent*=20 (default: 10)
> Sets the percentage of reserve memory to keep free so as to reduce the
> risk of to-space overflows. The default is 10 percent. When you increase or
> decrease the percentage, make sure to adjust the total Java heap by the
> same amount. This setting is not available in Java HotSpot VM, build 23.
>

Thanks. If possible, I would try to avoid this because we try to maximize
the amount of -XX:+UseCompressedOops memory we can have. Otherwise we would
probably need to double our memory and also there is some overhead
(processing power wise) with non compressed oops. I assume not
using -XX:+UseCompressedOops will somewhat double our hw costs...

> -XX:*G1HeapWastePercent*=5 (default: 10)
> Sets the percentage of heap that you are willing to waste. The Java
> HotSpot VM does not initiate the mixed garbage collection cycle when the
> reclaimable percentage is less than the heap waste percentage. The default
> is 10 percent. This setting is not available in Java HotSpot VM, build 23.
>
> In this way, G1 will reserve some heap (20%) for emergency and do more
> clean up, which should help with your situation.
>
> In addition, even if you want to make sure GC is always "on", it is still
> better to set InitiatingHeapOccupancyPercent slight higher than your live
> object ratio. For example, you may want to set it to 55% (slightly larger
> than 15.1 / 30).
>

Thanks, will try these.

>
> The good news is, we know one of our production services running with
> 100GB heap under G1 but there is no full GC at all and its throughput is
> also better than CMS. Please try to tune these parameters and see if it
> helps. I found this page is also super useful:
> http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html
>

Thanks.

**
Martin

>
>
> Best
> Shengzhe
>
>
> On Mon, Jun 9, 2014 at 7:29 AM, Martin Makundi <
> martin.makundi at koodaripalvelut.com> wrote:
>
>> 1. "-XX:InitiatingHeapOccupancyPercent=0 should trigger concurrent gcs
>> too much. "
>>
>> We use InitiatingHeapOccupancyPercent to make sure the gc is always
>> active. How is InitiatingHeapOccupancyPercent related to Full GC:s?
>>
>> 2. "What is the jdk version?
>>
>> java version "1.7.0_55"
>> Java(TM) SE Runtime Environment (build 1.7.0_55-b13)
>> Java HotSpot(TM) 64-Bit Server VM (build 24.55-b03, mixed mode)
>>
>>
>> 3. Can you share the gc log?"
>>
>> Log is available at 81.22.250.165/log
>>
>> 4. I wonder if it is possible to have mixed set of
>> different/automatically varying  sizes of G1HeapRegionSize's small for
>> smaller objects and larger for larger objects?
>>
>>
>> **
>> Martin
>>
>>
>> 2014-06-07 11:03 GMT+03:00 Martin Makundi <
>> martin.makundi at koodaripalvelut.com>:
>>
>> Why does G1GC do Full GC when it only needs only 40 bytes?
>>>
>>> Is there a way to tune this so that it would try to free up "some" chunk
>>> of memory and escape the full gc when enough memory has been freed? This
>>>  would lower the freeze time?
>>>
>>> See logs:
>>>
>>>  [Times: user=12.89 sys=0.16, real=1.25 secs]
>>> 2014-06-06T17:11:48.727+0300: 34022.631: [GC
>>> concurrent-root-region-scan-start]
>>> 2014-06-06T17:11:48.727+0300: 34022.631: [GC
>>> concurrent-root-region-scan-end, 0.0000180 secs]
>>> 2014-06-06T17:11:48.727+0300: 34022.631: [GC concurrent-mark-start]
>>>  34022.632: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason:
>>> allocation request failed, allocation request: 40 bytes]
>>>  34022.632: [G1Ergonomics (Heap Sizing) expand the heap, requested
>>> expansion amount: 8388608 bytes, attempted expansion amount: 8388608 bytes]
>>>  34022.632: [G1Ergonomics (Heap Sizing) did not expand the heap, reason:
>>> heap expansion operation failed]
>>> {Heap before GC invocations=1867 (full 1):
>>>  garbage-first heap   total 31457280K, used 31453781K
>>> [0x00007f1724000000, 0x00007f1ea4000000, 0x00007f1ea4000000)
>>>   region size 8192K, 0 young (0K), 0 survivors (0K)
>>>  compacting perm gen  total 524288K, used 166271K [0x00007f1ea4000000,
>>> 0x00007f1ec4000000, 0x00007f1ec4000000)
>>>    the space 524288K,  31% used [0x00007f1ea4000000, 0x00007f1eae25fdc8,
>>> 0x00007f1eae25fe00, 0x00007f1ec4000000)
>>> No shared spaces configured.
>>> 2014-06-06T17:11:48.728+0300: 34022.632: [Full GC 29G->13G(30G),
>>> 47.7918670 secs]
>>>
>>>
>>> Settings:
>>>
>>> -server -XX:InitiatingHeapOccupancyPercent=0 -Xss4096k
>>> -XX:MaxPermSize=512m -XX:PermSize=512m -Xms20G -Xmx30G -Xnoclassgc
>>> -XX:-OmitStackTraceInFastThrow -XX:+UseNUMA -XX:+UseFastAccessorMethods
>>> -XX:ReservedCodeCacheSize=128m -XX:-UseStringCache -XX:+UseGCOverheadLimit
>>> -Duser.timezone=EET -XX:+UseCompressedOops -XX:+DisableExplicitGC
>>> -XX:+AggressiveOpts -XX:CMSInitiatingOccupancyFraction=70
>>> -XX:+ParallelRefProcEnabled -XX:+UseAdaptiveSizePolicy
>>> -XX:MaxGCPauseMillis=500 -XX:+UseG1GC -XX:G1HeapRegionSize=8M
>>> -XX:GCPauseIntervalMillis=10000 -XX:+PrintGCDetails -XX:+PrintHeapAtGC
>>> -XX:+PrintAdaptiveSizePolicy -XX:+PrintGCDateStamps -XX:+PrintGC
>>> -Xloggc:gc.log
>>>
>>
>>
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>
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