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Make futexes 100x better on x86 MacOS

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Thanks to @autumnjolitz (in #876) the Cosmopolitan codebase is now
acquainted with Apple's outstanding ulock system calls which offer
something much closer to futexes than Grand Central Dispatch which
wasn't quite as good, since its wait function can't be interrupted
by signals (therefore necessitating a busy loop) and it also needs
semaphore objects to be created and freed. Even though ulock is an
internal Apple API, strictly speaking, the benefits of futexes are
so great that it's worth the risk for now especially since we have
the GCD implementation still as a quick escape hatch if it changes

Here's why this change is important for x86 XNU users. Cosmo has a
suboptimal polyfill when the operating system doesn't offer an API
that let's us implement futexes properly. Sadly we had to use that
on X86 XNU until now. The polyfill works using clock_nanosleep, to
poll the futex in a busy loop with exponential backoff. On XNU x86
clock_nanosleep suffers from us not being able to use a fast clock
gettime implementation, which had a compounding effect that's made
the polyfill function even more poorly. On X86 XNU we also need to
polyfill sched_yield() using select(), which made things even more
troublesome. Now that we have futexes we don't have any busy loops
anymore for both condition variables and thread joining so optimal
performance is attained. To demonstrate, consider these benchmarks

Before:

    $ ./lockscale_test.com -b
    consumed 38.8377   seconds real time and
              0.087131 seconds cpu time

After:

    $ ./lockscale_test.com -b
    consumed 0.007955 seconds real time and
             0.011515 seconds cpu time

Fixes #876
This commit is contained in:
Justine Tunney 2023-10-03 14:47:20 -07:00
parent ff250a0c10
commit 85f64f3851
No known key found for this signature in database
GPG key ID: BE714B4575D6E328
21 changed files with 267 additions and 34 deletions
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15
third_party/nsync/mu_semaphore.c vendored
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@ -20,6 +20,11 @@
#include "libc/dce.h"
#include "third_party/nsync/mu_semaphore.internal.h"
/* Apple's ulock (part by Cosmo futexes) is an internal API, but:
1. Unlike GCD it's cancellable, i.e. can be EINTR'd by signals
2. We currently always use ulock anyway for joining threads */
#define PREFER_GCD_OVER_ULOCK 0
asm(".ident\t\"\\n\\n\
*NSYNC (Apache 2.0)\\n\
Copyright 2016 Google, Inc.\\n\
@ -28,7 +33,7 @@ https://github.com/google/nsync\"");
/* Initialize *s; the initial value is 0. */
void nsync_mu_semaphore_init (nsync_semaphore *s) {
if (IsXnuSilicon ()) {
if (PREFER_GCD_OVER_ULOCK && IsXnuSilicon ()) {
return nsync_mu_semaphore_init_gcd (s);
} else if (IsNetbsd ()) {
return nsync_mu_semaphore_init_sem (s);
@ -39,7 +44,7 @@ void nsync_mu_semaphore_init (nsync_semaphore *s) {
/* Releases system resources associated with *s. */
void nsync_mu_semaphore_destroy (nsync_semaphore *s) {
if (IsXnuSilicon ()) {
if (PREFER_GCD_OVER_ULOCK && IsXnuSilicon ()) {
return nsync_mu_semaphore_destroy_gcd (s);
} else if (IsNetbsd ()) {
return nsync_mu_semaphore_destroy_sem (s);
@ -53,7 +58,7 @@ void nsync_mu_semaphore_destroy (nsync_semaphore *s) {
errno_t nsync_mu_semaphore_p (nsync_semaphore *s) {
errno_t err;
BEGIN_CANCELLATION_POINT;
if (IsXnuSilicon ()) {
if (PREFER_GCD_OVER_ULOCK && IsXnuSilicon ()) {
err = nsync_mu_semaphore_p_gcd (s);
} else if (IsNetbsd ()) {
err = nsync_mu_semaphore_p_sem (s);
@ -71,7 +76,7 @@ errno_t nsync_mu_semaphore_p (nsync_semaphore *s) {
errno_t nsync_mu_semaphore_p_with_deadline (nsync_semaphore *s, nsync_time abs_deadline) {
errno_t err;
BEGIN_CANCELLATION_POINT;
if (IsXnuSilicon ()) {
if (PREFER_GCD_OVER_ULOCK && IsXnuSilicon ()) {
err = nsync_mu_semaphore_p_with_deadline_gcd (s, abs_deadline);
} else if (IsNetbsd ()) {
err = nsync_mu_semaphore_p_with_deadline_sem (s, abs_deadline);
@ -84,7 +89,7 @@ errno_t nsync_mu_semaphore_p_with_deadline (nsync_semaphore *s, nsync_time abs_d
/* Ensure that the count of *s is at least 1. */
void nsync_mu_semaphore_v (nsync_semaphore *s) {
if (IsXnuSilicon ()) {
if (PREFER_GCD_OVER_ULOCK && IsXnuSilicon ()) {
return nsync_mu_semaphore_v_gcd (s);
} else if (IsNetbsd ()) {
return nsync_mu_semaphore_v_sem (s);