feat: add jemalloc as optional custom allocator

[mbutrovich]

Which issue does this PR close?

Closes #1648.

Rationale for this change

Comet currently supports mimalloc to override the system memory allocator (make release COMET_FEATURES=mimalloc or cargo build --features=mimalloc). We want to explore jemalloc (tikv-jemallocator crate) as an alternative to mimalloc for our custom allocator. I will bring some discussion from #1648:

#1648 (comment)

FYI: We found mimalloc to be unstable for running long term (keeps memory allocated but seems doesn't release it over time => running into OOMs much more easily). Switching to jemalloc fixed this.

#1648 (comment)

We prefer to use jemalloc with Comet by setting LD_PRELOAD, as it returns freed memory to the operating system more aggressively than ptmalloc, resulting in a more predictable RSS for the Spark executor process. Additionally, jemalloc's memory profiler is helpful for diagnosing OOM issues.

However, I'm uncertain whether jemalloc can still be used with LD_PRELOAD when Comet is compiled with mimalloc. The last time I attempted to dynamically override malloc it didn't work.

#1648 (comment)

I've also only ever used jemalloc in the past. I'm not sure what the discussion was at the time to pursue mimalloc for Comet. @mdcallag has some recent writing on the topic, althrough RocksDB is a different workload than what we're doing:

https://smalldatum.blogspot.com/2025/04/battle-of-mallocators.html https://smalldatum.blogspot.com/2025/04/battle-of-mallocators-part-2.html

The Germans 🇩🇪 did a bake-off a few years ago and they chose jemalloc for Umbra (and likely CedarDB): https://www.adms-conf.org/2019-camera-ready/durner_adms19.pdf

Other relevant reading:

• https://github.com/jemalloc/jemalloc
• https://github.com/jemalloc/jemalloc/wiki/Background#adoption
• https://internals.rust-lang.org/t/jemalloc-was-just-removed-from-the-standard-library/8759 (Rust used to implicitly link jemalloc at compilation. Now it is still used in the toolchain like rustc)
• https://kerkour.com/rust-jemalloc
• https://magiroux.com/rust-jemalloc-profiling

What changes are included in this PR?

Add jemalloc as a feature accessible through make release COMET_FEATURES=jemalloc. I had tcmalloc (Google) and snmalloc (Microsoft Research) as well, but the former failed to link at compilation time and the latter didn't like being loaded later as a dynamic library with Comet.

How are these changes tested?

[andygrove]

LGTM. Thanks @mbutrovich

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.81%. Comparing base (f09f8af) to head (2fc309e).
Report is 163 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1679      +/-   ##
============================================
+ Coverage     56.12%   58.81%   +2.68%     
- Complexity      976     1082     +106     
============================================
  Files           119      125       +6     
  Lines         11743    12597     +854     
  Branches       2251     2362     +111     
============================================
+ Hits           6591     7409     +818     
- Misses         4012     4016       +4     
- Partials       1140     1172      +32     

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[mbutrovich]

The Spark SQL test does not seem related to the change.

[parthchandra]

Restarted the test