[adamreeve]

Rationale for this change

Makes ReleaseUnused no longer an unintentional no-op with Jemalloc.

What changes are included in this PR?

Correctly builds the mallctl argument when calling JemallocAllocator::ReleaseUnused, and adds a debug-check of the error code.

Are these changes tested?

Yes. I tested that without the fix, the DCHECK_EQ raised an error for Jemalloc/TestMemoryPool/0.ReleaseUnused:

Check failed: (err) == (0) Failed to purge jemalloc arenas: No such file or directory

With the fix, the tests pass.

I also ran an extra test that dirty pages were now released successfully and reduced to 0, when before they weren't. But I'm not sure this is worth committing as it could be affected by changes to jemalloc internals.

Are there any user-facing changes?

No.

Was AI used for this PR?

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++] ReleaseUnused does nothing with jemalloc due to invalid mallctl name #55362

[Copilot]

🟢 Approval recommended

No active review issues were identified for the jemalloc purge-request change.

No open issues 🎉

What changed in this PR

Fixes GH-55362 by making the jemalloc-backed memory pool’s ReleaseUnused() issue a valid purge request instead of silently doing nothing.

Changes:

• Builds the mallctl name using the numeric value of MALLCTL_ARENAS_ALL.
• Adds a debug assertion with an error message if purging fails.

File	Description
cpp/​src/​arrow/​memory_pool_jemalloc.cc	Corrects the jemalloc purge request and adds debug error checking and required headers.

_{Review effort: Balanced}

[kou]

+1

BTW, are you using jemalloc? If so, we may want to update bundled jemalloc.

[adamreeve]

BTW, are you using jemalloc? If so, we may want to update bundled jemalloc.

No I'm not, I discovered this when testing ReleaseUnused in ParquetSharp, which builds with jemalloc enabled but uses mimalloc by default.