Expose the transparent hugepages (THP) configuration so the `-XX:+UseTransparentHugePages` java setting works

lordofpipes commented 4 months ago

Closes #61

Java is able to make use of hugepages to greatly reduce pagetable lookups by allowing the process to map larger chunks of RAM at a time. This is known to boost Minecraft performance on both the server and client. The Linux kernel in particular offers transparent hugepages, which allows userspace processes to specify whether they prefer hugepages, without needing root access. In order for Java to detect this, it needs to see the kernel-side switches in /sys/kernel/mm/transparent_hugepage/.

Because the Flatpak sandboxing restricts access to some parts of /sys/, enabling THP in Minecraft results in this error:

[0.016s][warning][pagesize] UseTransparentHugePages disabled, transparent huge pages are not supported by the operating system.

Thankfully, Java is designed to fallback to non-THP, so adding the flag doesn't cause the user's instances to crash, but it silently disables an option that otherwise could work. The /sys/ files in question are accessed in jdk21u/src/hotspot/os/linux/hugepages.cpp.

These can be exposed as read-only in the Flatpak, to enable the JVM to properly go into THP mode. We can also expose the manually configured hugepages, for whoever is still using this. On a typical system, the contents of the files exposed might be:

/sys/kernel/mm/hugepages/hugepages-2048kB/nr_overcommit_hugepages
0

/sys/kernel/mm/transparent_hugepage/hugepages-128kB/enabled
always inherit madvise [never]

/sys/kernel/mm/hugepages/hugepages-2048kB/surplus_hugepages
0

/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
0

/sys/kernel/mm/transparent_hugepage/hpage_pmd_size
2097152

/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages_mempolicy
0

/sys/kernel/mm/hugepages/hugepages-2048kB/resv_hugepages
0

/sys/kernel/mm/hugepages/hugepages-2048kB/free_hugepages
0

/sys/kernel/mm/transparent_hugepage/defrag
always defer defer+madvise [madvise] never

/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
always [inherit] madvise never

/sys/kernel/mm/transparent_hugepage/hugepages-256kB/enabled
always inherit madvise [never]

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

/sys/kernel/mm/transparent_hugepage/shmem_enabled
always within_size advise [never] deny force

/sys/kernel/mm/transparent_hugepage/hugepages-32kB/enabled
always inherit madvise [never]

/sys/kernel/mm/transparent_hugepage/use_zero_page
1

/sys/kernel/mm/transparent_hugepage/hugepages-512kB/enabled
always inherit madvise [never]

/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
1

/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
45

/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
4096

/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
10000

/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
851

/sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled
always inherit madvise [never]

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
256

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
511

/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
60000

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
64

/sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
always inherit madvise [never]

/sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled
always inherit madvise [never]

Doing this allows the usage of -XX:+UseTransparentHugePages in Minecraft and makes the warning go away. I have verified that this makes the Java process show AnonHugePages in /proc/$pid/smaps, indicating that THP is now working properly through the sandboxing.