Why is G1GC collection usage threshold not updated early?

[Bernd Eckenfels]

I guess G1 started much later with Mixed Collections compared to CMS. And when no GC happens the cu is not 0. you should maybe log the collection count as well.

There is BTW A GC User mailinglist as well.

Gruss
Bernd
--
http://bernd.eckenfels.net
________________________________
From: hotspot-gc-dev <hotspot-gc-dev-bounces at openjdk.java.net> on behalf of Sundara Mohan M <email.sundarms at gmail.com>
Sent: Wednesday, June 28, 2017 8:54:36 PM
To: hotspot-gc-dev at openjdk.java.net
Subject: Why is G1GC collection usage threshold not updated early?

I am trying to estimate the free memory using metrics from MemoryPoolMxBean.getCollectionUsage().

I am observing following behavior with G1GC

iteration=    0 - G1 Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  100 - G1 Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  200 - G1 Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  300 - G1 Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  400 - G1 Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  500 - G1 Old Gen: u=  1% cu=  0% uth=75% cuth=75%
iteration=  600 - G1 Old Gen: u=  4% cu=  0% uth=75% cuth=75%
iteration=  700 - G1 Old Gen: u=  9% cu=  0% uth=75% cuth=75%
iteration=  800 - G1 Old Gen: u= 16% cu=  0% uth=75% cuth=75%
iteration=  900 - G1 Old Gen: u= 25% cu=  0% uth=75% cuth=75%
iteration= 1000 - G1 Old Gen: u= 34% cu=  0% uth=75% cuth=75%
iteration= 1100 - G1 Old Gen: u= 45% cu=  0% uth=75% cuth=75%
iteration= 1200 - G1 Old Gen: u= 38% cu=  0% uth=75% cuth=75%
iteration= 1300 - G1 Old Gen: u= 46% cu=  0% uth=75% cuth=75%
iteration= 1400 - G1 Old Gen: u= 52% cu=  0% uth=75% cuth=75%
iteration= 1500 - G1 Old Gen: u= 45% cu=  0% uth=75% cuth=75%
iteration= 1600 - G1 Old Gen: u= 67% cu=  0% uth=75% cuth=75%
iteration= 1700 - G1 Old Gen: u= 56% cu=  0% uth=75% cuth=75%
iteration= 1800 - G1 Old Gen: u= 55% cu=  0% uth=75% cuth=75%
iteration= 1900 - G1 Old Gen: u= 61% cu=  0% uth=75% cuth=75%
iteration= 2000 - G1 Old Gen: u= 56% cu=  0% uth=75% cuth=75%
iteration= 2100 - G1 Old Gen: u= 76% cu=  0% uth=75% cuth=75%
iteration= 2200 - G1 Old Gen: u= 65% cu=  0% uth=75% cuth=75%
iteration= 2300 - G1 Old Gen: u= 62% cu=  0% uth=75% cuth=75%
iteration= 2400 - G1 Old Gen: u= 75% cu=  0% uth=75% cuth=75%
iteration= 2500 - G1 Old Gen: u= 75% cu=  0% uth=75% cuth=75%
iteration= 2600 - G1 Old Gen: u= 72% cu=  0% uth=75% cuth=75%
iteration= 2700 - G1 Old Gen: u= 69% cu=  0% uth=75% cuth=75%
iteration= 2800 - G1 Old Gen: u= 74% cu=  0% uth=75% cuth=75%
iteration= 2900 - G1 Old Gen: u= 80% cu=  0% uth=75% cuth=75%
iteration= 3000 - G1 Old Gen: u= 83% cu=  0% uth=75% cuth=75%
iteration= 3100 - G1 Old Gen: u= 89% cu=  0% uth=75% cuth=75%
iteration= 3200 - G1 Old Gen: u= 71% cu= 59% uth=75% cuth=75%
iteration= 3300 - G1 Old Gen: u= 90% cu= 59% uth=75% cuth=75%
iteration= 3400 - G1 Old Gen: u= 76% cu= 62% uth=75% cuth=75%
iteration= 3500 - G1 Old Gen: u= 65% cu= 65% uth=75% cuth=75%


CMS GC

iteration=    0 - CMS Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  100 - CMS Old Gen: u=  0% cu=  0% uth=75% cuth=75%
iteration=  200 - CMS Old Gen: u=  1% cu=  0% uth=75% cuth=75%
iteration=  300 - CMS Old Gen: u=  3% cu=  0% uth=75% cuth=75%
iteration=  400 - CMS Old Gen: u= 12% cu=  0% uth=75% cuth=75%
iteration=  500 - CMS Old Gen: u= 19% cu=  0% uth=75% cuth=75%
iteration=  600 - CMS Old Gen: u= 34% cu=  0% uth=75% cuth=75%
iteration=  700 - CMS Old Gen: u= 43% cu=  0% uth=75% cuth=75%
iteration=  800 - CMS Old Gen: u= 63% cu=  0% uth=75% cuth=75%
iteration=  900 - CMS Old Gen: u= 48% cu= 37% uth=75% cuth=75%
iteration= 1000 - CMS Old Gen: u= 60% cu= 37% uth=75% cuth=75%
iteration= 1100 - CMS Old Gen: u= 58% cu= 45% uth=75% cuth=75%
iteration= 1200 - CMS Old Gen: u= 71% cu= 45% uth=75% cuth=75%
iteration= 1300 - CMS Old Gen: u= 66% cu= 53% uth=75% cuth=75%
iteration= 1400 - CMS Old Gen: u= 80% cu= 53% uth=75% cuth=75%

u = usage(getUsage), cu = collectionUsage (getCollectionUsage), uth = usage threshold %, cuth = collection usage threshold %
my program just keeps allocating string and frees some strings.

1. Why does G1GC doesn't update it's collection usage till 59% whereas in CMSGC it is updated at 37% itself?

Can someone shed more light on this?

Thanks,
Sundar
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