Summary

Introduce new option -XX:ZAllowHeapOnFileSystem to allocate heap for ZGC to filesystem.

Problem

ZGC supports tmpfs and hugetlbfs as Java heap backing storage. We can allocate Java heap on NV-DIMM by JEP 316, but ZGC does not support it.

Solution

Introduce new option -XX:ZAllowHeapOnFileSystem to allocate heap for ZGC to filesystem.

According to Linux kernel document for DAX, DAX is supported in ext2, ext4, and xfs. Also we need to mount it with -o dax. So we need to support any filesystems for it.

This option need to be set combination with -XX:AllocateHeapAt.

Specification

--- old/src/hotspot/share/gc/z/z_globals.hpp    2020-02-14 17:56:55.619510600 +0900
+++ new/src/hotspot/share/gc/z/z_globals.hpp    2020-02-14 17:56:54.823184100 +0900
@@ -58,6 +58,10 @@
           "Uncommit memory if it has been unused for the specified "        \
           "amount of time (in seconds)")                                    \
                                                                             \
+  experimental(bool, ZAllowHeapOnFileSystem, false,                         \
+          "Allow to use filesystem as Java heap backing storage "           \
+          "specified by -XX:AllocateHeapAt")                                \
+                                                                            \
   diagnostic(uint, ZStatisticsInterval, 10,                                 \
           "Time between statistics print outs (in seconds)")                \
           range(1, (uint)-1)                                                \