Hi,
On 21.04.22 04:05, Tianqi Xia wrote:
Hi,
It seems like currently the only way to log rememebered set usage during
GC is setting the G1SummarizeRSetStatsPeriod flag, which can be quite
expensive. Testing on my local box, running BigRamTester (byThomas
Schatzl) with JDK11 + 10GB heap + 1GB RSet setup, it took hundreds of
milliseconds to process and print those remembered set usage
information, and this happened when the world is stopped.
After a little bit hacking and digging I found most of the time is spent
on the following 2 operations:
1. Calculating the size of PerRegionTable freelist, with 1GB RSet usage,
this freelist can contain more than half a million entries.
2. Calculating the occupancy of OtherRegionTable, which iterates over
all PerRegionTables being used.
After omitting these 2 operations, we can cut down the overall logging
time of remembered set usage to ~1ms, under the same setup.
Can we introduce a "lightweight" mode of remembered set logging that
trims the above mentioned 2 operations? It makes it possible for turning
on remebered set logging in prodcution envrionment, which really helps
diagnose certain performance issues.
a fix for this particular problem has been implemented in JDK-8233919
for JDK 14. Since then that accounting should be very fast (one add per
region, no more iterating through the actual remembered sets), and so
the logging much lighter weight, approximately in the range you state.
Since typically problems with the remembered sets means problems with
remembered set footprint, one option that often helps is to do what
JDK-8223162 (in JDK 13) did, increasing the so-called the sparse entry
sizes via -XX:G1RSetSparseRegionEntries, manually. See the release note
at https://bugs.openjdk.java.net/browse/JDK-8225343 for a comparison of
the old and new default values.
Thanks,
Thomas
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