From: "H.J. Lu" <hjl.tools@gmail.com>
To: libc-alpha@sourceware.org
Cc: Florian Weimer <fweimer@redhat.com>,
Oleh Derevenko <oleh.derevenko@gmail.com>,
Arjan van de Ven <arjan@linux.intel.com>,
Andreas Schwab <schwab@linux-m68k.org>
Subject: [PATCH v4 0/3] Optimize CAS [BZ #28537]
Date: Tue, 9 Nov 2021 16:16:11 -0800 [thread overview]
Message-ID: <20211110001614.2087610-1-hjl.tools@gmail.com> (raw)
CAS instruction is expensive. From the x86 CPU's point of view, getting
a cache line for writing is more expensive than reading. See Appendix
A.2 Spinlock in:
https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf
The full compare and swap will grab the cache line exclusive and cause
excessive cache line bouncing.
Optimize CAS in low level locks and pthread_mutex_lock.c:
1. Do an atomic load and skip CAS if compare may fail to reduce cache
line bouncing on contended locks.
2. Replace atomic_compare_and_exchange_bool_acq with
atomic_compare_and_exchange_val_acq to avoid the extra load.
3. Drop __glibc_unlikely in __lll_trylock and lll_cond_trylock since we
don't know if it's actually rare; in the contended case it is clearly not
rare.
This is the first patch set to optimize CAS. I will investigate the rest
CAS usages in glibc after this patch set has been accepted.
H.J. Lu (3):
Reduce CAS in low level locks [BZ #28537]
Reduce CAS in __pthread_mutex_lock_full [BZ #28537]
Optimize CAS in __pthread_mutex_lock_full [BZ #28537]
nptl/lowlevellock.c | 12 ++++-----
nptl/pthread_mutex_lock.c | 53 ++++++++++++++++++++++++++++---------
sysdeps/nptl/lowlevellock.h | 29 +++++++++++++-------
3 files changed, 67 insertions(+), 27 deletions(-)
--
2.33.1
next reply other threads:[~2021-11-10 0:16 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-10 0:16 H.J. Lu [this message]
2021-11-10 0:16 ` [PATCH v4 1/3] Reduce CAS in low level locks " H.J. Lu
2021-11-10 1:56 ` Noah Goldstein
2021-11-10 0:16 ` [PATCH v4 2/3] Reduce CAS in __pthread_mutex_lock_full " H.J. Lu
2021-11-10 0:16 ` [PATCH v4 3/3] Optimize " H.J. Lu
2021-11-10 14:26 ` [PATCH v4 0/3] Optimize CAS " Paul E Murphy
2021-11-10 20:07 ` Paul A. Clarke
2021-11-10 21:33 ` H.J. Lu
2021-11-11 0:30 ` Paul A. Clarke
2021-11-10 23:34 ` H.J. Lu
2021-11-10 15:35 ` Paul A. Clarke
2021-11-10 15:42 ` H.J. Lu
2021-11-10 15:50 ` Paul A. Clarke
2021-11-10 15:52 ` H.J. Lu
2021-11-10 15:52 ` Florian Weimer
2021-11-10 16:03 ` H.J. Lu
2021-11-10 16:04 ` Paul A. Clarke
2021-11-10 16:14 ` Andreas Schwab
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