G1gc compaction algorithm

[Martin Makundi]

>  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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