G1gc compaction algorithm

[Martin Makundi]

Hi!

Here is the most recent attempt, we increased GCPauseIntervalMillis to
10000 so that 10% will be a longer time for doing mixed sets. However, we
still seem to get a Full GC at "[Full GC 20G->17G(30G), 60.0458620 secs]"
due to humongous allocations.

Any suggestions how to mitigate the Full GC? Here is the full log with
parameters:

http://81.22.250.165/log

**
Martin


2014-07-19 5:38 GMT+03:00 Martin Makundi <martin.makundi at koodaripalvelut.com
>:

> BTW: We have 16 cores and we probably need 4 for serving web users, is
> there a way we can utilize more efficiently the excess resources towards gc
> while keeping user experienced pause at minimum?
>
>
> 2014-07-17 22:21 GMT+03:00 Martin Makundi <
> martin.makundi at koodaripalvelut.com>:
>
>
>>  There are 3 factors: MaxGCPauseMilli, MixedGCCountTarget,
>>> G1OldCSetRegionThresholdPercent
>>>
>>> The candidate regions is calculated by some algorithm.
>>> minimum regions = (candidate regions)/MixedGCCountTarget
>>> maximum regions = (heap regions)* G1OldCSetRegionThresholdPercent
>>>
>>
>>> If the estimated mixed gc time is < MaxGCPauseMilli, g1 will try to add
>>> the candidate regions to cset while keeping the estimated time below
>>> MaxGCPauseMilli, as long as it is less than maximum regions, and
>>> reclaimable percentage higher than the waste limit.
>>> If the estimated mixed gc time is > MaxGCPauseMilli, g1 will add minimum
>>> regions to cset.
>>>
>>
>> What's the science behind these equations (in short) or are they purely
>> ad-hoc?
>>
>> **
>> Martin
>>
>>
>>
>>>
>>> In your case, MaxGCPauseMilli is low and MixedGCCountTarget is 80.  So
>>> it can only add 2 when the estimated time >  MaxGCPauseMilli
>>>
>>> Thanks,
>>> Jenny
>>>
>>> On 7/16/2014 9:14 PM, Martin Makundi wrote:
>>>
>>>  >  This is a diagnostic parameter, you need to apply with
>>> > -XX:+UnlockDiagnosticVMOptions -XX:+G1PrintRegionLivenessInfo
>>>
>>>  Thanks, will try that.
>>>
>>>  It will first add old regions if the estimated time is under the
>>>> MaxGCPauseMilli.  If the estimated time is higher than  MaxGCPauseMilli, it
>>>> will add the minimum decided by #candidate-regions/MixedGCCountTarget.  If
>>>> you set MixedGCCountTarget too high, the minimum will be too low.  In your
>>>> case, it is 2.
>>>>
>>>
>>>  Hmm.. what is the logic behind this
>>> candidate-regions/MixedGCCountTarget? Is there no way to tell the gc to
>>> estimate the max number of regions it could maybe achieve in the time
>>> available and do that instead of 2?
>>>
>>>  **
>>> Martin
>>>
>>>>  Thanks,
>>>> Jenny
>>>>
>>>>  On 7/16/2014 6:46 PM, Martin Makundi wrote:
>>>>
>>>>  2. For most of the time, eden size is 1.4g, survivor 150m, the rest
>>>>> is old gen.  I am not sure how much of the old gen is used for humongous
>>>>> allocations.  But it seems there are some tunings you can try to help mixed
>>>>> gc:
>>>>>   - old regions added to cset is 2-14 for mixed gc.  Most of the time
>>>>> the reason is 'predicted time too high'.  You can try either increase
>>>>> -XX:MaxGCPauseMillis to a higher value, or decrease
>>>>> -XX:G1MixedGCCountTarget (currently it is 80) so that more old regions can
>>>>> be added.
>>>>>
>>>>
>>>>  Does it attempt to do any mixed gc if it cannot do
>>>> all G1MixedGCCountTarget or is the value G1MixedGCCountTarget just an upper
>>>> limit? If it just is an upper limit we could keep it at 80 or higher?
>>>>
>>>>
>>>>
>>>
>>>
>>
>
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