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nptl: Add backoff mechanism to spinlock loop

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When mutiple threads waiting for lock at the same time, once lock owner
releases the lock, waiters will see lock available and all try to lock,
which may cause an expensive CAS storm.

Binary exponential backoff with random jitter is introduced. As try-lock
attempt increases, there is more likely that a larger number threads
compete for adaptive mutex lock, so increase wait time in exponential.
A random jitter is also added to avoid synchronous try-lock from other
threads.

v2: Remove read-check before try-lock for performance.

v3:
1. Restore read-check since it works well in some platform.
2. Make backoff arch dependent, and enable it for x86_64.
3. Limit max backoff to reduce latency in large critical section.

v4: Fix strict-prototypes error in sysdeps/nptl/pthread_mutex_backoff.h

v5: Commit log updated for regression in large critical section.

Result of pthread-mutex-locks bench

Test Platform: Xeon 8280L (2 socket, 112 CPUs in total)
First Row: thread number
First Col: critical section length
Values: backoff vs upstream, time based, low is better

non-critical-length: 1
	1	2	4	8	16	32	64	112	140
0	0.99	0.58	0.52	0.49	0.43	0.44	0.46	0.52	0.54
1	0.98	0.43	0.56	0.50	0.44	0.45	0.50	0.56	0.57
2	0.99	0.41	0.57	0.51	0.45	0.47	0.48	0.60	0.61
4	0.99	0.45	0.59	0.53	0.48	0.49	0.52	0.64	0.65
8	1.00	0.66	0.71	0.63	0.56	0.59	0.66	0.72	0.71
16	0.97	0.78	0.91	0.73	0.67	0.70	0.79	0.80	0.80
32	0.95	1.17	0.98	0.87	0.82	0.86	0.89	0.90	0.90
64	0.96	0.95	1.01	1.01	0.98	1.00	1.03	0.99	0.99
128	0.99	1.01	1.01	1.17	1.08	1.12	1.02	0.97	1.02

non-critical-length: 32
	1	2	4	8	16	32	64	112	140
0	1.03	0.97	0.75	0.65	0.58	0.58	0.56	0.70	0.70
1	0.94	0.95	0.76	0.65	0.58	0.58	0.61	0.71	0.72
2	0.97	0.96	0.77	0.66	0.58	0.59	0.62	0.74	0.74
4	0.99	0.96	0.78	0.66	0.60	0.61	0.66	0.76	0.77
8	0.99	0.99	0.84	0.70	0.64	0.66	0.71	0.80	0.80
16	0.98	0.97	0.95	0.76	0.70	0.73	0.81	0.85	0.84
32	1.04	1.12	1.04	0.89	0.82	0.86	0.93	0.91	0.91
64	0.99	1.15	1.07	1.00	0.99	1.01	1.05	0.99	0.99
128	1.00	1.21	1.20	1.22	1.25	1.31	1.12	1.10	0.99

non-critical-length: 128
	1	2	4	8	16	32	64	112	140
0	1.02	1.00	0.99	0.67	0.61	0.61	0.61	0.74	0.73
1	0.95	0.99	1.00	0.68	0.61	0.60	0.60	0.74	0.74
2	1.00	1.04	1.00	0.68	0.59	0.61	0.65	0.76	0.76
4	1.00	0.96	0.98	0.70	0.63	0.63	0.67	0.78	0.77
8	1.01	1.02	0.89	0.73	0.65	0.67	0.71	0.81	0.80
16	0.99	0.96	0.96	0.79	0.71	0.73	0.80	0.84	0.84
32	0.99	0.95	1.05	0.89	0.84	0.85	0.94	0.92	0.91
64	1.00	0.99	1.16	1.04	1.00	1.02	1.06	0.99	0.99
128	1.00	1.06	0.98	1.14	1.39	1.26	1.08	1.02	0.98

There is regression in large critical section. But adaptive mutex is
aimed for "quick" locks. Small critical section is more common when
users choose to use adaptive pthread_mutex.

Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
This commit is contained in:
Wangyang Guo 2022-05-06 01:50:10 +00:00 committed by H.J. Lu
parent a2a6bce7d7
commit 8162147872
4 changed files with 89 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
nptl/pthread_mutex_lock.c
View File

@ -138,14 +138,26 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
int cnt = 0;
int max_cnt = MIN (max_adaptive_count (),
mutex->__data.__spins * 2 + 10);
int spin_count, exp_backoff = 1;
unsigned int jitter = get_jitter ();
do
{
if (cnt++ >= max_cnt)
/* In each loop, spin count is exponential backoff plus
random jitter, random range is [0, exp_backoff-1]. */
spin_count = exp_backoff + (jitter & (exp_backoff - 1));
cnt += spin_count;
if (cnt >= max_cnt)
{
/* If cnt exceeds max spin count, just go to wait
queue. */
LLL_MUTEX_LOCK (mutex);
break;
}
atomic_spin_nop ();
do
atomic_spin_nop ();
while (--spin_count > 0);
/* Prepare for next loop. */
exp_backoff = get_next_backoff (exp_backoff);
}
while (LLL_MUTEX_READ_LOCK (mutex) != 0
|| LLL_MUTEX_TRYLOCK (mutex) != 0);

1
sysdeps/nptl/pthreadP.h
View File

@ -33,6 +33,7 @@
#include <kernel-features.h>
#include <errno.h>
#include <internal-signals.h>
#include <pthread_mutex_backoff.h>
#include "pthread_mutex_conf.h"

35
sysdeps/nptl/pthread_mutex_backoff.h Normal file
View File

@ -0,0 +1,35 @@
/* Pthread mutex backoff configuration.
Copyright (C) 2022 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef _PTHREAD_MUTEX_BACKOFF_H
#define _PTHREAD_MUTEX_BACKOFF_H 1
static inline unsigned int
get_jitter (void)
{
/* Arch dependent random jitter, return 0 disables random. */
return 0;
}
static inline int
get_next_backoff (int backoff)
{
/* Next backoff, return 1 disables mutex backoff. */
return 1;
}
#endif

39
sysdeps/x86_64/nptl/pthread_mutex_backoff.h Normal file
View File

@ -0,0 +1,39 @@
/* Pthread mutex backoff configuration.
Copyright (C) 2022 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef _PTHREAD_MUTEX_BACKOFF_H
#define _PTHREAD_MUTEX_BACKOFF_H 1
#include <fast-jitter.h>
static inline unsigned int
get_jitter (void)
{
return get_fast_jitter ();
}
#define MAX_BACKOFF 16
static inline int
get_next_backoff (int backoff)
{
/* Binary expontial backoff. Limiting max backoff
can reduce latency in large critical section. */
return (backoff < MAX_BACKOFF) ? backoff << 1 : backoff;
}
#endif