RFR (S): Parallel AlwaysPreTouch

[Aleksey Shipilev]

Hi,

It is not unusual for low-latency customers with large heaps to enable
-XX:+AlwaysPreTouch, that touches every page preemptively during the
allocation (e.g. on startup). This pays the upfront cost during startup,
instead of during the execution.

However, pre-touching on larger heaps may take a while, see e.g. G1 on 1
Tb heap reports 10 minutes (!) to init the heap [1]. So, G1 made the
pre-touch parallel. We can do it too:
 http://cr.openjdk.java.net/~shade/shenandoah/always-pretouch/webrev.01/

On my 4-core desktop, -XX:+AlwaysPreTouch HelloWorld with 20 Gb heap
improves 4.5s -> 1.8s. I would expect this improvement to be larger on
mammoth machines.

Note that shared VM changes match the G1 changes [2], which means we
should nicely fit during the next merge.

Thanks,
-Aleksey


[1] https://bugs.openjdk.java.net/browse/JDK-8157952
[1] http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/bc2c975bc342