From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org, Phil Auld <pauld@redhat.com>
Subject: Re: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning
Date: Fri, 20 Nov 2020 15:44:08 +0100 [thread overview]
Message-ID: <20201120144408.GF3040@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20201118053556.3fmmtat7upv6dtvd@linux-p48b.lan>
On Tue, Nov 17, 2020 at 09:35:56PM -0800, Davidlohr Bueso wrote:
> On Tue, 17 Nov 2020, Waiman Long wrote:
>
> > The column "CS Load" represents the number of pause instructions issued
> > in the locking critical section. A CS load of 1 is extremely short and
> > is not likey in real situations. A load of 20 (moderate) and 100 (long)
> > are more realistic.
> >
> > It can be seen that the previous patches in this series have reduced
> > performance in general except in highly contended cases with moderate
> > or long critical sections that performance improves a bit. This change
> > is mostly caused by the "Prevent potential lock starvation" patch that
> > reduce reader optimistic spinning and hence reduce reader fragmentation.
> >
> > The patch that further limit reader optimistic spinning doesn't seem to
> > have too much impact on overall performance as shown in the benchmark
> > data.
> >
> > The patch that disables reader optimistic spinning shows reduced
> > performance at lightly loaded cases, but comparable or slightly better
> > performance on with heavier contention.
>
> I'm not overly worried about the lightly loaded cases here as the users
> (mostly thinking mmap_sem) most likely won't care for real workloads,
> not, ie: will-it-scale type things.
>
> So at SUSE we also ran into this very same problem with reader optimistic
> spinning and considering the fragmentation went with disabling it, much
> like this patch - but without the reader optimistic lock stealing bits
> you have. So far nothing has really shown to fall out in our performance
> automation. And per your data a single reader spinner does not seem to be
> worth the added complexity of keeping reader spinning vs ripping it out.
I'm fine with ripping it... It was finnicky to begin with.
next prev parent reply other threads:[~2020-11-20 14:44 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-18 3:04 [PATCH 0/5] locking/rwsem: Rework reader optimistic spinning Waiman Long
2020-11-18 3:04 ` [PATCH 1/5] locking/rwsem: Pass the current atomic count to rwsem_down_read_slowpath() Waiman Long
2020-11-18 3:04 ` [PATCH 2/5] locking/rwsem: Prevent potential lock starvation Waiman Long
2020-11-20 14:44 ` Peter Zijlstra
2020-11-20 17:27 ` Waiman Long
2020-11-18 3:04 ` [PATCH 3/5] locking/rwsem: Enable reader optimistic lock stealing Waiman Long
2020-11-20 14:36 ` Peter Zijlstra
2020-11-20 17:26 ` Waiman Long
2020-12-08 3:53 ` Davidlohr Bueso
2020-11-18 3:04 ` [PATCH 4/5] locking/rwsem: Wake up all waiting readers if RWSEM_WAKE_READ_OWNED Waiman Long
2020-11-18 4:53 ` Davidlohr Bueso
2020-11-19 18:37 ` Waiman Long
2020-11-18 3:04 ` [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning Waiman Long
2020-11-18 5:35 ` Davidlohr Bueso
2020-11-19 18:40 ` Waiman Long
2020-11-20 13:11 ` David Laight
2020-11-20 17:04 ` Waiman Long
2020-11-20 17:37 ` David Laight
2020-11-20 21:38 ` Davidlohr Bueso
2020-11-21 11:50 ` David Laight
2020-11-20 14:44 ` Peter Zijlstra [this message]
2020-11-20 22:39 ` Waiman Long
2020-11-20 14:42 ` Peter Zijlstra
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