How should I use -XX:PreTouchParallelChunkSize=?

[Peter Kessler OS]

How should I use -XX:PreTouchParallelChunkSize=?

I am using an Ampere Altra, running Oracle Linux Server release 8.5, that has 64KB pages and 524288KB huge pages.  If I run a program with enough logging, I see, for example,

    $ ${OpenJDKDir}/ jdk-19-ea+8-441/bin/java -Xms112g -Xmx112g -Xmn12g -XX:SurvivorRatio=10 -XX:+AlwaysPreTouch -XX:+UseTransparentHugePages -XX:+UseParallelGC -XX:ParallelGCThreads=80 -Xlog:gc+init=info,gc+heap=debug,gc=info:stdout HelloWorld
    ...
    [0.035s][debug][gc,heap] Running ParallelGC PreTouch head with 20 workers for 20 work units pre-touching 10737418240B.
    [0.114s][debug][gc,heap] Running ParallelGC PreTouch head with 2 workers for 2 work units pre-touching 1073741824B.
    [0.130s][debug][gc,heap] Running ParallelGC PreTouch head with 2 workers for 2 work units pre-touching 1073741824B.
    [0.160s][debug][gc,heap] Running ParallelGC PreTouch head with 80 workers for 200 work units pre-touching 107374182400B.
    [0.937s][info ][gc,init] Version: 19-ea+8-441 (release)
    ...

showing the parallel pretouch at work, and I can infer from the time stamps how long it takes to pretouch the 10GB of the eden, the 1GB of each of the survivors, and the 100GB of the old generation.  If I do not specify a -XX:PreTouchParallelChunkSize=, or specify a size smaller than the huge page size, logging says that the work units are the huge page size.  The default size (src/hotspot/os/linux/globals_linux.hpp#L93) is 4MB, which does not apply if I have huge pages of 512MB.

If I gather the times to the "Version" line for various releases of the OpenJDK and various sizes of -XX:PreTouchParallelChunkSize=, I get

                                -XX:PreTouchParallelChunkSize=
    JDK                   4k     64k   2048k   4096k   524288k  1048576k
    -----------------  -----   -----   -----   -----   -------  --------
    jdk-15.0.2         3.434   3.469   3.469   3.508     3.467     3.434
    jdk-17.0.2         0.806   0.806   0.805   0.804     0.836     0.844
    jdk-18-ea+34-2083  0.803   0.802   0.802   0.833     0.802     0.868
    jdk-19-ea+8-441    0.806   0.849   0.803   0.803     0.803     0.846

showing seconds for the median of 7 runs.  It looks like jdk-15.0.2 was not good at parallelizing pretouching of the heap.  Later JDKs look like they parallelize the pretouching of the heap, but it does not matter much what the setting, or non-setting, of -XX:PreTouchParallelChunkSize= is.

My question is: what difference does setting -XX:PreTouchParallelChunkSize= make?  That is: is there some point to setting it at all?

This is a somewhat academic question, since if I have specified -XX:+AlwaysPreTouch I am not that interested in start-up time.  But thank you for getting back the 2.5 seconds from jdk-15.0.2 start-up.

Thank you for your help in explaining -XX:PreTouchParallelChunkSize=.

                  ... peter

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