Summary

Make MetaspaceReclaimPolicy a diagnostic switch

Problem

JEP-387 revamped Metaspace and, among other things, introduced the product switch MetaspaceReclaimPolicy. The switch switches between three different behavioral modes:

1) MetaspaceReclaimPolicy=balanced (default) attempts to reclaim Metaspace upon class loader death

2) MetaspaceReclaimPolicy=aggressive does the same but more enthusiastically;

The difference between them is minor; Metaspace has an inbuilt granularity with which memory is committed and uncommitted - 64K by default, 16K in aggressive mode. In practice, it did not matter much: aggressive mode would lead to slightly more reclaimed memory at the cost of slightly more VMA fragmentation.

3) MetaspaceReclaimPolicy=none forfeits memory reclamation altogether. This is a larger break from the other two modes and leads to a bunch of special-path coding in metaspace.

These three modes lead to different code paths that all need to get tested separately to prevent regressions. MetaspaceReclaimPolicy=none, in particular, also makes the code more complex.

The original purpose of this switch, especially the "none" mode, was to have a fallback in case of errors. But post-JEP-387-Metaspace has been very stable and robust; there was never a need for this fallback.

Solution

I would like to make MetaspaceReclaimPolicy a diagnostic switch, with the expressed freedom to decrease later the number of permutations and/or to completely remove the switch as a user-facing control, without further CSRs.

Specification

--- a/src/hotspot/share/runtime/globals.hpp
+++ b/src/hotspot/share/runtime/globals.hpp
@@ -1423,7 +1423,7 @@ const int ObjectAlignmentInBytes = 8;
           "Force the class space to be allocated at this address or "       \
           "fails VM initialization (requires -Xshare=off.")                 \
                                                                             \
-  product(ccstr, MetaspaceReclaimPolicy, "balanced",                        \
+  product(ccstr, MetaspaceReclaimPolicy, "balanced", DIAGNOSTIC,            \
           "options: balanced, aggressive, none")                            \
                                                                             \
   product(bool, PrintMetaspaceStatisticsAtExit, false, DIAGNOSTIC,          \