From: Olivier Dion <odion@efficios.com>
To: libc-help@sourceware.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Dead code in pthread_cond_wait() for spin-wait
Date: Wed, 11 Oct 2023 14:30:37 -0400 [thread overview]
Message-ID: <87a5spvwoi.fsf@laura> (raw)
Hi,
Commit ed19993b5b0d05d62cc883571519a67dae481a14 from 2016 introduce "a
new implementation of condition variables". This new implementation
seems to introduce a spin-wait fast-path to avoid taking the underlying
futex when doing a pthread_cond_wait().
However, looking at the following snippet in nptl/pthread_cond_wait.c:
static __always_inline int
__pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
clockid_t clockid, const struct __timespec64 *abstime)
{
const int maxspin = 0;
[...]
/* Spin-wait first. [...] */
unsigned int spin = maxspin;
while (signals == 0 && spin > 0) {
[...]
}
[...]
}
it seems to me that the try-spinning portion is dead code in
pthread_cond_wait_common(). I was wondering why this was the case?
I also think that there would maybe some interest in using the Userspace
RCU wait algorithm [0] for this.
[0] https://github.com/urcu/userspace-rcu/blob/master/src/urcu-wait.h
Thanks,
Olivier
--
Olivier Dion
EfficiOS Inc.
https://www.efficios.com
next reply other threads:[~2023-10-11 18:30 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-11 18:30 Olivier Dion [this message]
2023-10-11 18:36 ` Mathieu Desnoyers
2023-10-11 18:57 ` Adhemerval Zanella Netto
2023-10-12 15:45 ` Olivier Dion
2023-10-12 16:40 ` Florian Weimer
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