Hi,
On 23.02.20 22:54, Rémi p wrote:
I am looking into reducing the young collection pause times in my Java
11 application, particularly in the Scan RS step. AFAIK the duration of
this phase is linear in the sum of sizes of the remembered sets of the
regions in the collection set (=old->young region references).
Is there a built-in or recommended way to troubleshoot where all the
old->young references are coming from? I was thinking about patching the
hotspot code to log the object types of randomly sampled old->young
links in order to give me some hints, but I am wondering if there is an
easier/less intrusive solution.
At this time there is no way official way to map remembered set entries
to particular objects. You can get remembered set sizes for each region
with gc+remset=trace and setting G1SummarizeRSetStatsPeriod to something
> 0; the reason for this convoluted process is that in jdk11 getting
remembered set statistics is expensive (fixed in 14 [2]).
If you want to proceed in that direction, I recommend adding the
sampling in the "rebuild remembered sets" concurrent phase [3]; hook
into somewhere in the path starting with
G1RebuildRemSetHeapRegionClosure::scan_for_references().
What you could try in 11 to decrease remembered set memory usage (and
typically Scan RS time a bit) is increasing the use of sparse PRTs by
manually increasing the thresholds when using them. I.e. set
G1RSetSparseRegionEntries to something much higher than the defaults
(4/8/12/16/20/24 entries for 1-32M regions respectively).
The JDK-8223162 change [0] in JDK13 does that for you, selecting better
defaults.
Another option is to increase default region size, but I assume you
already maxed it out (but you did not describe your previous attempts).
If you can try jdk14, we reimplemented the remembered set scan work
distribution method [1], which can yield very significant pause time
improvements. We've seen pause times reduced to 60% of baseline (jdk13
to jdk14) on benchmarks with very large remembered sets.
Hth,
Thomas
[0] https://bugs.openjdk.java.net/browse/JDK-8223162
[1] https://bugs.openjdk.java.net/browse/JDK-8213108
[2] https://bugs.openjdk.java.net/browse/JDK-8233919
[3] https://bugs.openjdk.java.net/browse/JDK-8180415
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