G1gc compaction algorithm

[Yu Zhang]

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.

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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