perf: stop mimalloc asking for transparent huge pages (#1391)

mimalloc madvises its 1 GiB arenas for transparent huge pages, so on
Linux hosts with THP in madvise mode (the Ubuntu default) every partly
used 2 MiB page stayed resident in full. A cold 10,000-model compile
held about 470 MiB of huge pages at its peak. Building mimalloc with
no_thp keeps those arenas on normal pages and lowers that compile's
peak memory by about 85 MB, with unchanged wall and CPU time. Other
platforms are unaffected.

Author: kvlonge

Cargo.toml

@@ -81,7 +81,9 @@ globset = "0.4"
 junction = { version = "2.1.0", default-features = false }
 keyring = { version = "3.6.3", default-features = false }
 # Python loads the extension with dlopen, so mimalloc must not use initial-exec TLS.
-mimalloc = { version = "0.1", default-features = false, features = ["local_dynamic_tls"] }
+# no_thp: mimalloc otherwise asks for transparent huge pages on its 1 GiB arenas, and each
+# partly used 2 MiB page then stays resident in full, which raises peak memory.
+mimalloc = { version = "0.1", default-features = false, features = ["local_dynamic_tls", "no_thp"] }
 proc-macro2 = { version = "1", features = ["span-locations"] }
 pyo3 = { version = "0.26", features = ["abi3-py312"] }
 rpassword = "7.5.4"