Link libumem to clickhouse

[wfchandler]

Clickhouse heavily contends the system allocator's global lock. Link it with libumem to mitigate this.

Fixes oxidecomputer/omicron#7716

[bnaecker]

Seems fine to me! Just so I understand, this is not to let us swap out libc / umem / jemalloc, but to always pick up the umem-provided allocation symbols, is that right?

[wfchandler]

Seems fine to me! Just so I understand, this is not to let us swap out libc / umem / jemalloc, but to always pick up the umem-provided allocation symbols, is that right?

That's correct. The jemalloc clickhouse bundles doesn't build on Illumos, and given the performance difference with libc I didn't think we'd ever not want to use umem.

[bnaecker]

I agree we probably want to always use libumem. We can figure out how to get jemalloc building later, if / when that becomes relevant. Thanks!

[rmustacc]

Seems fine to me! Just so I understand, this is not to let us swap out libc / umem / jemalloc, but to always pick up the umem-provided allocation symbols, is that right?

Specifically the malloc, free, calloc, and related symbols in libc are marked interposable in the libc mapfile to bypass direct binding (the NODIRECT flag), see malloc() here. By placing -lumem on the line, when the run-time link-editor runs and performs relocations for these symbols, the umem copy of the malloc, free, etc. symbols will end up being used in lieu of those in libc. As clickhouse specifically undoes any LD_PRELOAD methods of changing this, linking it directly is necessary.