perf(core): cut indexing RSS by 70% and peak by 30%

README.md

@@ -575,6 +575,7 @@ Run `:FFFScan` to force a rescan.
 - `:FFFOpenLog` opens the current session's log file.
 - Historical log files are stored near the main log file `<state>/log/fff+<UTC-timestamp>+<pid>.log` (up to 20 files)
 - For a crash backtrace, run `lldb -- nvim` or `gdb -- nvim` and reproduce
+- fff keeps its allocator off transparent huge pages to keep the index RSS low (~15-20% on large repos); set `MIMALLOC_ALLOW_LARGE_OS_PAGES=2` before starting Neovim to trade that memory back for a slightly faster initial index build
 
 </details>
 

crates/fff-core/src/file_picker.rs

@@ -2385,6 +2385,21 @@ fn common_dir_prefix_len(a: &str, b: &str) -> usize {
     last_sep
 }
 
+/// Keep mimalloc off 2 MiB huge pages: with THP the arena is resident at
+/// 2 MiB granularity and idle index memory inflates RSS by ~2x. Env overrides win.
+/// Must run before the first allocation (see `fff_nvim`'s init-array hook).
+#[cfg(feature = "mimalloc-collect")]
+pub extern "C" fn tune_mimalloc() {
+    // SAFETY: getenv/mi_option_set touch static tables only; no allocation happens here.
+    unsafe {
+        let user_set = !libc::getenv(c"MIMALLOC_ALLOW_LARGE_OS_PAGES".as_ptr()).is_null()
+            || !libc::getenv(c"MIMALLOC_LARGE_OS_PAGES".as_ptr()).is_null();
+        if !user_set {
+            libmimalloc_sys::mi_option_set(libmimalloc_sys::mi_option_large_os_pages, 0);
+        }
+    }
+}
+
 /// Ask the global allocator to return freed pages to the OS.
 /// Enabled via the `mimalloc-collect` feature (set by fff-nvim).
 /// No-op when the feature is off (tests, system allocator).