RFR: 8232722: G1 archive region deallocation may shrink the heap below -Xms

Thomas Schatzl tschatzl at openjdk.org
Thu May 25 09:51:20 UTC 2023 

Hi all,

  can I have reviews for this change that properly deallocates/uncommits CDS archive regions when failing to load the CDS archive?

In particular this caused the nuisance mentioned in the CR where even if -Xms==-Xmx, g1 uncommitted the heap memory anyway.

Testing: gha, manual testing as below

There is no (existing) way to induce CDS load errors easily, so what I did was adding `-XX:+UseNewCode` in `filemap.cpp:2202` to simulate failures when enabled. Obviously I removed the flag in this change.

Here's the problematic case:


$java -XX:GCCardSizeInBytes=128 -Xmx128m -Xms128m -Xlog:gc+region=trace,gc+ergo+heap=debug -XX:+UseNewCode -version

[0.050s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000ffe00000, 0x00000000fff00000, 0x00000000fff00000]
[0.050s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000fff00000, 0x00000000fff06278, 0x0000000100000000]
[0.050s][trace][gc,region   ] G1HR INACTIVE(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.050s][trace][gc,region   ] G1HR INACTIVE(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.050s][debug][gc,ergo,heap] Attempt heap shrinking (CDS archive regions). Total size: 2097152B
[0.050s][trace][gc,region   ] G1HR UNCOMMIT(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.050s][trace][gc,region   ] G1HR UNCOMMIT(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.057s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffd00000, 0x00000000ffd00000, 0x00000000ffe00000]
[0.129s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffc00000, 0x00000000ffc00000, 0x00000000ffd00000]
openjdk version "21-internal" 2023-09-19


(The `GCCardSizeInBytes` option is there to decrease the minimum heap alignment to 512kb/1M so that setting `-Xms` to an odd value in a later test works)

I.e. the CDS regions are unconditionally uncommitted even through `-Xms == -Xmx`.

The next case just illustrates current (pre-existing) behavior with `-Xms != -Xmx`, showing that CDS regions are always committed, leading to higher than `-Xms` memory usage. I will file an enhancement here, as it is acceptable behavior (to me).


$ java -XX:GCCardSizeInBytes=128 -Xmx128m -Xms126m -Xlog:gc+region=trace,gc+ergo+heap=debug -XX:+UseNewCode -version

[0.048s][trace][gc,region   ] G1HR COMMIT(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.048s][trace][gc,region   ] G1HR ACTIVE(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.048s][debug][gc,ergo,heap] Allocate CDS archive regions. Allocated 2 Committed 2
[0.048s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000ffe00000, 0x00000000fff00000, 0x00000000fff00000]
[0.048s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000fff00000, 0x00000000fff05f50, 0x0000000100000000]
[0.063s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffd00000, 0x00000000ffd00000, 0x00000000ffe00000]


I.e. CDS allocation just commits the last few regions without caring about other existing mappings to not fragment the heap.

The next case is the new behavior:

java -XX:GCCardSizeInBytes=128 -Xmx128m -Xms128m -Xlog:gc+region=trace,gc+ergo+heap=debug -XX:+UseNewCode -version

[0.047s][trace][gc,region   ] G1HR ACTIVE(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.047s][trace][gc,region   ] G1HR ACTIVE(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.048s][debug][gc,ergo,heap] Allocate CDS archive regions. Allocated 2 Committed 0
[0.048s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000ffe00000, 0x00000000fff00000, 0x00000000fff00000]
[0.048s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000fff00000, 0x00000000fff05f50, 0x0000000100000000]
[0.049s][trace][gc,region   ] G1HR CLEANUP(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.050s][trace][gc,region   ] G1HR CLEANUP(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.050s][debug][gc,ergo,heap] Deallocate CDS archive regions. Freed 2 Uncommitted 0 regions.
[0.057s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.131s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
openjdk version "21-internal" 2023-09-19

G1 does not uncommit the region blindly any more.

The next test case shows that if `-Xms` is lower than `-Xmx`, the code still properly uncommits (and no only frees the region).

java -XX:GCCardSizeInBytes=128 -Xmx128m -Xms126m -Xlog:gc+region=trace,gc+ergo+heap=debug -XX:+UseNewCode -version

[0.048s][trace][gc,region   ] G1HR COMMIT(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.048s][trace][gc,region   ] G1HR ACTIVE(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.048s][trace][gc,region   ] G1HR COMMIT(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.048s][trace][gc,region   ] G1HR ACTIVE(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.048s][debug][gc,ergo,heap] Allocate CDS archive regions. Allocated 2 Committed 2
[0.048s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000ffe00000, 0x00000000fff00000, 0x00000000fff00000]
[0.048s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000fff00000, 0x00000000fff05f50, 0x0000000100000000]
[0.048s][trace][gc,region   ] G1HR INACTIVE(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.049s][trace][gc,region   ] G1HR INACTIVE(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.049s][debug][gc,ergo,heap] Deallocate CDS archive regions. Freed 2 Uncommitted 2 regions.
[0.049s][trace][gc,region   ] G1HR UNCOMMIT(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.049s][trace][gc,region   ] G1HR UNCOMMIT(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.056s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffd00000, 0x00000000ffd00000, 0x00000000ffe00000]
[0.124s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffc00000, 0x00000000ffc00000, 0x00000000ffd00000]
openjdk version "21-internal" 2023-09-19


And finally a "mixed" case where to allocate the CDS archive G1 uses a mix of already allocated regions and committed regions, and uncommits/gives back regions as expected.


java -XX:GCCardSizeInBytes=128 -Xmx128m -Xms127m -Xlog:gc+region=trace,gc+ergo+heap=debug -XX:+UseNewCode -version

[0.053s][trace][gc,region   ] G1HR COMMIT(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.053s][trace][gc,region   ] G1HR ACTIVE(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.053s][debug][gc,ergo,heap] Allocate CDS archive regions. Allocated 2 Committed 1
[0.053s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000ffe00000, 0x00000000fff00000, 0x00000000fff00000]
[0.053s][trace][gc,region   ] G1HR ALLOC(OLD) [0x00000000fff00000, 0x00000000fff05f50, 0x0000000100000000]
[0.053s][trace][gc,region   ] G1HR INACTIVE(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.054s][trace][gc,region   ] G1HR CLEANUP(FREE) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.054s][debug][gc,ergo,heap] Deallocate CDS archive regions. Freed 2 Uncommitted 1 regions.
[0.054s][trace][gc,region   ] G1HR UNCOMMIT(FREE) [0x00000000ffe00000, 0x00000000ffe00000, 0x00000000fff00000]
[0.061s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000fff00000, 0x00000000fff00000, 0x0000000100000000]
[0.138s][trace][gc,region   ] G1HR ALLOC(EDEN) [0x00000000ffd00000, 0x00000000ffd00000, 0x00000000ffe00000]
openjdk version "21-internal" 2023-09-19


Thanks,
  Thomas

-------------

Commit messages:
 - Some comment changes
 - Remove debug code
 - initial version, uses UseNewCode

Changes: https://git.openjdk.org/jdk/pull/14145/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14145&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8232722
  Stats: 44 lines in 6 files changed: 15 ins; 4 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/14145.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14145/head:pull/14145

PR: https://git.openjdk.org/jdk/pull/14145

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