Summary

Update PreTouchParallelChunkSize default value for improved performance.

Problem

The current value of PreTouchParallelChunkSize has originally been set according to some ad-hoc testing. Further, more extensive testing showed that the time to pre-touch memory can be improved by almost 15% by adjusting this value.

This can have significant impact on startup time with -XX:+AlwaysPreTouch enabled.

Solution

Set new default value for PreTouchParallelChunkSize to 4MB. JDK-8254699 contains a spreadsheet with an exhaustive performance test showing this value to be best overall.

Also change minimal size of this value to a (small) page size.

Specification

                                                                          \
-  product(size_t, PreTouchParallelChunkSize, 1 * G,                      \
+  product(size_t, PreTouchParallelChunkSize, 4 * M,                      \
           "Per-thread chunk size for parallel memory pre-touch.")        \
-          range(1, SIZE_MAX / 2)                                         \
+          range(4*K, SIZE_MAX / 2)                                       \
                                                                          \