perf: reduce scheduler mutex contention + stack pool

Three changes, each independently measured, landed together.

Fuse pre-resume mutex acquisitions
-----------------------------------
The schedule loop previously took the shared lock three separate times
per actor resume: once to pop the run queue, once to read the stack
pointer, and once to pop the first-resume closure. These are now a
single acquisition that returns everything needed to resume the actor.

As a side effect, pending_closures was changed from a Vec<(Pid, Closure)>
with O(n) linear scan to a Vec<Option<Closure>> indexed by slot index,
making first-closure lookup O(1).

Move stack allocation outside the shared lock
----------------------------------------------
Stack::new() (mmap + mprotect) was previously called inside with_shared,
stalling every other scheduler thread for the duration of two syscalls on
every spawn. It now runs before the lock is acquired.

Stack pool
----------
Rather than munmap-ing a stack when an actor finishes and mmap-ing a fresh
one on the next spawn, stacks are now recycled through a per-Runtime pool
(Mutex<Vec<Stack>>). finalize_actor extracts the stack from the Actor
before clearing the slot and pushes it to the pool outside the shared lock.
spawn_under pops from the pool before falling back to Stack::new().

The pool is unbounded for now (shrink policy TBD) but capped at
stack_pool_cap stacks on return, defaulting to thread_count * 4.
The cap is configurable via Config::stack_pool_cap(n).

Results (24-thread, against stored baseline)
--------------------------------------------
chained_spawn      smarm 1-thread:   9763 → 261 µs   (-97%)
chained_spawn      smarm 24-thread: 23562 → 838 µs   (-96%)
ping_pong_oneshot  smarm 1-thread:  18409 → 742 µs   (-96%)
ping_pong_oneshot  smarm 24-thread: 44596 → 1425 µs  (-97%)
catch_unwind_panics smarm 24-thread: 267812 → 124094 µs (-54%)
fan_out_compute    smarm 24-thread:   2839 → 2226 µs  (-22%)

Tokio regressions in the checker output are baseline measurement drift;
no tokio code was changed.

This commit is contained in:

smarm

2026-05-25 23:28:11 +02:00

parent 64be3f23ac

commit 72f5d38e5d

4 changed files with 216 additions and 144 deletions

									
										Cargo.toml
									
		+3
		-4
	
												View File
												
				@@ -25,17 +25,16 @@ codegen-units = 1

				[[bench]]

				name = "primes"

				harness = false

				[[bench]]

				name = "multi_scheduler"

				harness = false

				[[bench]]

				name = "general"

				harness = false

				[[bench]]

				name = "smarm_favored"

				harness = false