Wither transparent huge pages for the heap?

Thu Nov 6 00:00:14 UTC 2025

The title of JDK-8366434<https://bugs.openjdk.org/browse/JDK-8366434> suggests the issue only affects G1, but I find it also affects ParallelGC:

$ "${OPENJDK_DIR}"/ jdk-25.0.1/bin/java -showversion -XX:+UseParallelGC -XX:+UseTransparentHugePages -Xms12g -Xmx12g -Xlog:pagesize=info -cp ~/Play/Java HelloWorld
[0.001s][info][pagesize] Explicit hugepage support:
[0.001s][info][pagesize]   hugepage size: 2M
[0.001s][info][pagesize]   hugepage size: 512M
[0.001s][info][pagesize]   hugepage size: 16G
[0.001s][info][pagesize]   default hugepage size: 512M
[0.001s][info][pagesize] Transparent hugepage (THP) support:
[0.001s][info][pagesize]   THP mode: always
[0.001s][info][pagesize]   THP pagesize: 512M
[0.001s][info][pagesize] Shared memory transparent hugepage (THP) support:
[0.001s][info][pagesize]   Shared memory THP mode: never
[0.001s][info][pagesize] JVM will attempt to prevent THPs in thread stacks.
[0.001s][info][pagesize] UseLargePages=1, UseTransparentHugePages=1
[0.001s][info][pagesize] Large page support enabled. Usable page sizes: 64k, 512M. Default large page size: 512M.
[0.002s][info][pagesize] CodeHeap 'profiled nmethods':  min=2496K max=118272K base=0x00004000108d0000 size=118272K page_size=64K
[0.002s][info][pagesize] CodeHeap 'non-nmethods':  min=2496K max=9152K base=0x0000400017c50000 size=9152K page_size=64K
[0.002s][info][pagesize] CodeHeap 'non-profiled nmethods':  min=2496K max=118336K base=0x0000400018540000 size=118336K page_size=64K
[0.002s][info][pagesize] Heap:  min=12G max=12G base=0x0000000500000000 size=12G page_size=64K
[0.002s][info][pagesize] Card Table:  min=24M max=24M base=0x0000400001ee0000 size=24M page_size=64K
[0.004s][info][pagesize] Mark Bitmap:  min=192M max=192M base=0x0000400025000000 size=192M page_size=64K
[0.004s][info][pagesize] Parallel Compact Data:  min=960K max=960K base=0x00004000038f0000 size=960K page_size=64K
openjdk version "25.0.1" 2025-10-21
OpenJDK Runtime Environment (build 25.0.1+8-27)
OpenJDK 64-Bit Server VM (build 25.0.1+8-27, mixed mode, sharing)
Hello world!

The Release Note<https://bugs.openjdk.org/browse/JDK-8366564> says

A workaround to allow the use of transparent huge pages with G1 is to configure the THP mode as `always`:
```
# echo always > /sys/kernel/mm/transparent_hugepage/enabled
```

but I observe the issue (see above) even though my system has transparent huge pages configured to "always":

$ cat /sys/kernel/mm/transparent_hugepage/enabled
[always] madvise never

It looks like code changes<https://github.com/openjdk/jdk25u/commit/a66fe2a8765c085b8faeb9c89751306b834935ce> affect code shared by all (?) the collectors, so a fix for G1GC probably fixes ParallelGC, even if the description only covers G1G1.  The tests in there seem capable of testing SerialGC, ParallelGC, and G1GC, so that is reassuring.

I will wait patiently for JDK 25.0.2.

                                                ... peter

From: Stefan Johansson <stefan.johansson at oracle.com>
Date: Tuesday, November 4, 2025 at 01:38
To: Peter Kessler OS <peter.kessler at os.amperecomputing.com>, "hotspot-gc-use at openjdk.java.net" <hotspot-gc-use at openjdk.java.net>
Subject: Re: Wither transparent huge pages for the heap?

Hi Peter,

This is a known issue [1], and we found the problem a bit to late for the fix to be included in the initial release of JDK 25. See the JBS issue [2] for more details.

The fix for the problem is scheduled to be released in JDK 25.0.2. The workaround (which is not optimal) is to configure the THP mode to 'always' in:
/sys/kernel/mm/transparent_hugepage/enabled

This will make the heap be backed by transparent huge pages. When configured as 'always' you don't need the -XX:+UseTransparentHugePages flag. The logs won't reflect the actual page size used if I'm not mistaken, but you can use OS tools to verify that transparent huge pages are used for the heap.

Hope this helps,
Stefan

[1] https://www.oracle.com/java/technologies/javase/25-relnote-issues.html
[2] https://bugs.openjdk.org/browse/JDK-8366434
On 2025-11-03 21:55, Peter Kessler OS wrote:
Between JDK-24 (24.0.2+12-54, dba 24.0.2) and JDK-25 (25.0.1+8-27, dba 25.0.1), running an application on a machine with 64KiB system page size and 512MiB transparent huge pages, I see this difference:

$ "${OPENJDK_DIR}"/jdk-24.0.2/bin/java -showversion -XX:+UseTransparentHugePages -Xmx12g -Xms12g -Xlog:pagesize=info HelloWorld
[0.001s][info][pagesize] Explicit hugepage support:
[0.001s][info][pagesize]   hugepage size: 2M
[0.001s][info][pagesize]   hugepage size: 512M
[0.001s][info][pagesize]   hugepage size: 16G
[0.001s][info][pagesize]   default hugepage size: 512M
[0.001s][info][pagesize] Transparent hugepage (THP) support:
[0.001s][info][pagesize]   THP mode: always
[0.001s][info][pagesize]   THP pagesize: 512M
[0.001s][info][pagesize] Shared memory transparent hugepage (THP) support:
[0.001s][info][pagesize]   Shared memory THP mode: never
[0.001s][info][pagesize] JVM will attempt to prevent THPs in thread stacks.
[0.001s][info][pagesize] UseLargePages=1, UseTransparentHugePages=1
[0.001s][info][pagesize] Large page support enabled. Usable page sizes: 64k, 512M. Default large page size: 512M.
[0.002s][info][pagesize] CodeHeap 'profiled nmethods':  min=2496K max=118272K base=0x00004000108d0000 size=118272K page_size=64K
[0.002s][info][pagesize] CodeHeap 'non-nmethods':  min=2496K max=9152K base=0x0000400017c50000 size=9152K page_size=64K
[0.002s][info][pagesize] CodeHeap 'non-profiled nmethods':  min=2496K max=118336K base=0x0000400018540000 size=118336K page_size=64K
[0.003s][info][pagesize] Heap:  min=12G max=12G base=0x0000000500000000 size=12G page_size=512M
[0.003s][info][pagesize] Block Offset Table: req_size=24M req_page_size=64K base=0x0000400024440000 size=24M page_size=64K
[0.003s][info][pagesize] Card Table: req_size=24M req_page_size=64K base=0x0000400025c40000 size=24M page_size=64K
[0.003s][info][pagesize] Mark Bitmap: req_size=192M req_page_size=64K base=0x0000400027440000 size=192M page_size=64K
openjdk version "24.0.2" 2025-07-15
OpenJDK Runtime Environment (build 24.0.2+12-54)
OpenJDK 64-Bit Server VM (build 24.0.2+12-54, mixed mode, sharing)
Hello world!

versus

$ "${OPENJDK_DIR}"/jdk-25.0.1/bin/java -showversion -XX:+UseTransparentHugePages -Xmx12g -Xms12g -Xlog:pagesize=info HelloWorld
[0.001s][info][pagesize] Explicit hugepage support:
[0.001s][info][pagesize]   hugepage size: 2M
[0.001s][info][pagesize]   hugepage size: 512M
[0.001s][info][pagesize]   hugepage size: 16G
[0.001s][info][pagesize]   default hugepage size: 512M
[0.001s][info][pagesize] Transparent hugepage (THP) support:
[0.001s][info][pagesize]   THP mode: always
[0.001s][info][pagesize]   THP pagesize: 512M
[0.001s][info][pagesize] Shared memory transparent hugepage (THP) support:
[0.001s][info][pagesize]   Shared memory THP mode: never
[0.001s][info][pagesize] JVM will attempt to prevent THPs in thread stacks.
[0.001s][info][pagesize] UseLargePages=1, UseTransparentHugePages=1
[0.001s][info][pagesize] Large page support enabled. Usable page sizes: 64k, 512M. Default large page size: 512M.
[0.002s][info][pagesize] CodeHeap 'profiled nmethods':  min=2496K max=118272K base=0x00004000108d0000 size=118272K page_size=64K
[0.002s][info][pagesize] CodeHeap 'non-nmethods':  min=2496K max=9152K base=0x0000400017c50000 size=9152K page_size=64K
[0.002s][info][pagesize] CodeHeap 'non-profiled nmethods':  min=2496K max=118336K base=0x0000400018540000 size=118336K page_size=64K
[0.003s][info][pagesize] Heap:  min=12G max=12G base=0x0000000500000000 size=12G page_size=64K
[0.003s][info][pagesize] Block Offset Table: req_size=24M req_page_size=64K base=0x0000400024550000 size=24M page_size=64K
[0.003s][info][pagesize] Card Table: req_size=24M req_page_size=64K base=0x0000400025d50000 size=24M page_size=64K
[0.003s][info][pagesize] Mark Bitmap: req_size=192M req_page_size=64K base=0x0000400027550000 size=192M page_size=64K
openjdk version "25.0.1" 2025-10-21
OpenJDK Runtime Environment (build 25.0.1+8-27)
OpenJDK 64-Bit Server VM (build 25.0.1+8-27, mixed mode, sharing)
Hello world!

Why am I not getting 512MiB pages for my heap in JDK-25, when I was getting 512MiB pages for my heap in JDK-24?  Maybe more to the point: How do I get transparent huge pages for my heap in JDK-25?

                                                ... peter

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