G1gc compaction algorithm

Thu Aug 21 14:21:51 UTC 2014

Hi!

Why doesn't the g1gc operate like azul c4 is described to operate:

http://www.azulsystems.com/technology/c4-garbage-collector

"fully concurrent, so it never falls back to a stop-the-world compaction"

?

**
Martin

2014-08-20 11:35 GMT+03:00 Martin Makundi <
martin.makundi at koodaripalvelut.com>:

> Hi!
>
> Here is one more recent log:
>
> http://81.22.250.165/log/gc-2014-08-20.log
>
> with a couple of Full GC's.
>
> **
> Martin
>
>
> 2014-08-19 20:11 GMT+03:00 Martin Makundi <
> martin.makundi at koodaripalvelut.com>:
>
> Hi!
>>
>> I suspect the application does not do the humongous allocations, I
>> suspect it's the gc itself that does these. We have an allocationrecorder
>> that never sees these humongous allocations within the application
>> itself...assuming they are in a form that the allocation hook can detect.
>>
>> From within the application we have lots of objects that are kept in
>> ehcache, so ehcache manages the usage. I am not familiar with ehcache
>> internals but I don't think it uses humongous objects in any way.
>>
>> The max allocation request varies on the objects that are loaded, it is
>> possible that some request is 48m so yes it can vary depending on who logs
>> in and what he/she does... not very reproducible.
>>
>> My main concern is that IF full gc can clean up the memory, there should
>> be a mechanic that does just the same as full gc but without blocking for
>> long time...concurrent full gc, do the 30-60 second operation for example
>> 10% overhead until whole full gc is done (that would take 30-60/10% =
>> 300-600 seconds).
>>
>> And if this is not feasible at the moment...what can we tune to mitigate
>> the peaking in the garbage accumulation and thus avoid the full gc's.
>>
>> Help =)
>>
>>  **
>> Martin
>>
>>
>> 2014-08-19 19:56 GMT+03:00 Yu Zhang <yu.zhang at oracle.com>:
>>
>> Martin,
>>>
>>> Comparing 2 logs 08-15 vs 08-11, the allocation pattern seems changed.
>>> In 08-15, there are 4 requests allocation 109M
>>> ' 112484.582: [G1Ergonomics (Heap Sizing) attempt heap expansion,
>>> reason: humongous allocation request failed, allocation request: 109816768
>>> bytes]'
>>> But in 08-11, the max allocation request is 48M
>>> '117595.621: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason:
>>> humongous allocation request failed, allocation request: 47906832 bytes]'
>>>
>>> Does the application change the allocation when performance changes?
>>>
>>>
>>> Thanks,
>>> Jenny
>>>
>>>
>>> On 8/15/2014 3:20 AM, Martin Makundi wrote:
>>>
>>>> Hi!
>>>>
>>>> Here is our latest logs with bit more heap and suggested parameters.
>>>> We tried eden max 85% with this run because 30% was unsuccessful
>>>> earlier.
>>>>
>>>> Here is the log:http://81.22.250.165/log/gc-16m-2014-08-15.log
>>>>
>>>> It has a couple of full gc hits during a busy day, any new ideas?
>>>>
>>>> **
>>>> Martin
>>>>
>>>>
>>>> 2014-08-13 9:39 GMT+03:00 Martin Makundi<martin.makundi@
>>>> koodaripalvelut.com>:
>>>>
>>>>> Thanks.  At the end, the system cpu is very high.  I guess there are
>>>>>> page
>>>>>> faults due to the heap expansion around timestamp 12002.692.  Is the
>>>>>> memory
>>>>>> tight on your system?
>>>>>>
>>>>> For performance sake we use compressedoops which limits the memory
>>>>> upper
>>>>> bound, this means we could go to bit under 32g, 32255M. Going above
>>>>> compressedoops will increase memory footprint and slow down processing
>>>>> so we
>>>>> would prefer just tuning the gc while within compressedoops.
>>>>>
>>>>>  can you afford to start with -Xms30g -Xmx30g -XX:+AlwaysPreTouch?
>>>>>>
>>>>> Thanks, I'll try -Xms32255M -Xmx32255M -XX:+AlwaysPreTouch
>>>>>
>>>>> **
>>>>> Martin
>>>>>
>>>>>  Thanks,
>>>>>> Jenny
>>>>>>
>>>>>> On 8/11/2014 11:29 PM, Martin Makundi wrote:
>>>>>>
>>>>>> Hi!
>>>>>>
>>>>>> I tried the new parameters:
>>>>>>
>>>>>>  Based on this one, can you do one with -XX:G1MaxNewSizePercent=30
>>>>>>> -XX:InitiatingHeapOccupancyPercent=20 added?
>>>>>>>
>>>>>> This seems to hang the whole system.... we have lots of mostly short
>>>>>> lived
>>>>>> (ehcache timeToIdleSeconds="900") large java object trees 1M-10M each
>>>>>> (data
>>>>>> reports loaded into cache).
>>>>>>
>>>>>> Maybe eden should be even bigger instead of smaller?
>>>>>>
>>>>>> Here is the log from today, it hung up quite early, I suspect the gc:
>>>>>> http://81.22.250.165/log/gc-16m-2014-08-12.log
>>>>>>
>>>>>> The process ate most of the cpu cacacity and we had to kill it and
>>>>>> restart
>>>>>> without -XX:G1MaxNewSizePercent=30.
>>>>>>
>>>>>> What you suggest?
>>>>>>
>>>>>> **
>>>>>> Martin
>>>>>>
>>>>>>  The reason for G1MaxNewSizePercent(default=60) is to set an upper
>>>>>>> limit
>>>>>>> to Eden size.  It seems the Eden size grows to 17g before Full gc,
>>>>>>> then a
>>>>>>> bunch of humongous allocation happened, and there is not enough old
>>>>>>> gen.
>>>>>>>
>>>>>>> The following log entry seems not right: The Eden Size is over 60%
>>>>>>> of the
>>>>>>> heap.
>>>>>>> "2014-08-11T11:13:05.487+0300: 193238.308: [GC pause (young)
>>>>>>> (initial-mark) 193238.308: [G1Ergonomics (CSet Construction) start
>>>>>>> choosing
>>>>>>> CSet, _pending_cards: 769041, predicted base time: 673.25 ms,
>>>>>>> remaining
>>>>>>> time: 326.75 ms, target pause time: 1000.00 ms] 193238.308:
>>>>>>> [G1Ergonomics
>>>>>>> (CSet Construction) add young regions to CSet, eden: 1 regions,
>>>>>>> survivors:
>>>>>>> 21 regions, predicted young region time: 145.63 ms] 193238.308:
>>>>>>> [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1
>>>>>>> regions,
>>>>>>> survivors: 21 regions, old: 0 regions, predicted pause time: 818.88
>>>>>>> ms,
>>>>>>> target pause time: 1000.00 ms], 0.7559550 secs]   [Parallel Time:
>>>>>>> 563.9 ms,
>>>>>>> GC Workers: 13]      [GC Worker Start (ms): Min: 193238308.1, Avg:
>>>>>>> 193238318.0, Max: 193238347.6, Diff: 39.5]      [Ext Root Scanning
>>>>>>> (ms):
>>>>>>> Min: 0.0, Avg: 13.0, Max: 35.8, Diff: 35.8, Sum: 168.4]      [Update
>>>>>>> RS
>>>>>>> (ms): Min: 399.2, Avg: 416.8, Max: 442.8, Diff: 43.6, Sum: 5418.0]
>>>>>>> [Processed Buffers: Min: 162, Avg: 232.0, Max: 326, Diff: 164, Sum:
>>>>>>> 3016]
>>>>>>> [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
>>>>>>> [Object Copy (ms): Min: 79.9, Avg: 104.8, Max: 152.4, Diff: 72.5,
>>>>>>> Sum:
>>>>>>> 1363.0]      [Termination (ms): Min: 0.0, Avg: 19.1, Max: 27.3,
>>>>>>> Diff: 27.3,
>>>>>>> Sum: 248.9]      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max:
>>>>>>> 0.0, Diff:
>>>>>>> 0.0, Sum: 0.3]      [GC Worker Total (ms): Min: 524.1, Avg: 553.8,
>>>>>>> Max:
>>>>>>> 563.7, Diff: 39.6, Sum: 7198.8]      [GC Worker End (ms): Min:
>>>>>>> 193238871.7,
>>>>>>> Avg: 193238871.8, Max: 193238871.8, Diff: 0.1]
>>>>>>>     [Code Root Fixup: 0.0 ms]
>>>>>>>     [Clear CT: 0.3 ms]
>>>>>>>     [Other: 191.7 ms]
>>>>>>>        [Choose CSet: 0.0 ms]      [Ref Proc: 190.1 ms]      [Ref
>>>>>>> Enq: 0.3
>>>>>>> ms]      [Free CSet: 0.2 ms]
>>>>>>>     [Eden: 16.0M(2464.0M)->0.0B(22.9G) Survivors: 336.0M->240.0M
>>>>>>> Heap:
>>>>>>> 14.1G(28.7G)->14.1G(28.7G)]
>>>>>>>   [Times: user=8.45 sys=0.04, real=0.75 secs]"
>>>>>>>
>>>>>>> The reason for increasing InitiatingHeapOccupancyPercent to 20 from
>>>>>>> 10 is
>>>>>>> we are wasting some concurrent cycles.
>>>>>>>
>>>>>>> We will see how this goes.  We might increase G1ReservePercent to
>>>>>>> handle
>>>>>>> this kind of allocation if it is not enough.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Jenny
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Jenny
>>>>>>>
>>>>>>> On 8/11/2014 10:46 AM, Martin Makundi wrote:
>>>>>>>
>>>>>>> Hi!
>>>>>>>
>>>>>>> Here is our latest log with one Full GC @ 2014-08-11T11:20:02 which
>>>>>>> is
>>>>>>> caused by heap full and allocation request: 144 bytes.
>>>>>>>
>>>>>>> http://81.22.250.165/log/gc-16m-2014-08-11.log
>>>>>>>
>>>>>>> Any ideas how to mitigate this kind of situation? The Full GC makes
>>>>>>> quite
>>>>>>> a difference to the situation but causes a painful pause also.
>>>>>>>
>>>>>>> **
>>>>>>> Martin
>>>>>>>
>>>>>>>
>>>>>>>
>>>
>>
>
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