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From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v4 10/16] locking/rwsem: Wake up almost all readers in wait queue
Date: Wed, 17 Apr 2019 13:16:39 -0400
Message-ID: <8644ca1b-caa3-0a20-efd7-826ad1cbddd9@redhat.com> (raw)
In-Reply-To: <20190417133937.GG4038@hirez.programming.kicks-ass.net>

On 04/17/2019 09:39 AM, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:53PM -0400, Waiman Long wrote:
>> When the front of the wait queue is a reader, other readers
>> immediately following the first reader will also be woken up at the
>> same time. However, if there is a writer in between. Those readers
>> behind the writer will not be woken up.
>>
>> Because of optimistic spinning, the lock acquisition order is not FIFO
>> anyway. The lock handoff mechanism will ensure that lock starvation
>> will not happen.
>>
>> Assuming that the lock hold times of the other readers still in the
>> queue will be about the same as the readers that are being woken up,
>> there is really not much additional cost other than the additional
>> latency due to the wakeup of additional tasks by the waker. Therefore
>> all the readers up to a maximum of 256 in the queue are woken up when
>> the first waiter is a reader to improve reader throughput.
>>
>> With a locking microbenchmark running on 5.1 based kernel, the total
>> locking rates (in kops/s) on a 8-socket IvyBridge-EX system with
>> equal numbers of readers and writers before and after this patch were
>> as follows:
>>
>>    # of Threads  Pre-Patch   Post-patch
>>    ------------  ---------   ----------
>>         4          1,641        1,674
>>         8            731        1,062
>>        16            564          924
>>        32             78          300
>>        64             38          195
>>       240             50          149
>>
>> There is no performance gain at low contention level. At high contention
>> level, however, this patch gives a pretty decent performance boost.
> Right, so this basically completes the convertion from task-fair (FIFO)
> to phase-fair.
>
> https://cs.unc.edu/~anderson/papers/rtsj10-for-web.pdf

Right, the changes that I am making is similar in concept to the
phase-fair rwlock mentioned in the article. That is an interesting
article even though I was not aware of it before you brought it up.

Cheers,
Longman


  reply index

Thread overview: 88+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-13 17:22 [PATCH v4 00/16] locking/rwsem: Rwsem rearchitecture part 2 Waiman Long
2019-04-13 17:22 ` [PATCH v4 01/16] locking/rwsem: Prevent unneeded warning during locking selftest Waiman Long
2019-04-18  8:04   ` [tip:locking/core] " tip-bot for Waiman Long
2019-04-13 17:22 ` [PATCH v4 02/16] locking/rwsem: Make owner available even if !CONFIG_RWSEM_SPIN_ON_OWNER Waiman Long
2019-04-13 17:22 ` [PATCH v4 03/16] locking/rwsem: Remove rwsem_wake() wakeup optimization Waiman Long
2019-04-13 17:22 ` [PATCH v4 04/16] locking/rwsem: Implement a new locking scheme Waiman Long
2019-04-16 13:22   ` Peter Zijlstra
2019-04-16 13:32     ` Waiman Long
2019-04-16 14:18       ` Peter Zijlstra
2019-04-16 14:42         ` Peter Zijlstra
2019-04-13 17:22 ` [PATCH v4 05/16] locking/rwsem: Merge rwsem.h and rwsem-xadd.c into rwsem.c Waiman Long
2019-04-13 17:22 ` [PATCH v4 06/16] locking/rwsem: Code cleanup after files merging Waiman Long
2019-04-16 16:01   ` Peter Zijlstra
2019-04-16 16:17     ` Peter Zijlstra
2019-04-16 19:45       ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation Waiman Long
2019-04-16 14:12   ` Peter Zijlstra
2019-04-16 20:26     ` Waiman Long
2019-04-16 21:07       ` Waiman Long
2019-04-17  7:13         ` Peter Zijlstra
2019-04-17 16:22           ` Waiman Long
2019-04-16 15:49   ` Peter Zijlstra
2019-04-16 16:15     ` Peter Zijlstra
2019-04-16 18:41       ` Waiman Long
2019-04-16 18:16     ` Waiman Long
2019-04-16 18:32       ` Peter Zijlstra
2019-04-17  7:35       ` Peter Zijlstra
2019-04-17 16:35         ` Waiman Long
2019-04-17  8:05       ` Peter Zijlstra
2019-04-17 16:39         ` Waiman Long
2019-04-18  8:22           ` Peter Zijlstra
2019-04-17  8:17   ` Peter Zijlstra
2019-04-13 17:22 ` [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on_owner() return owner state Waiman Long
2019-04-17  9:00   ` Peter Zijlstra
2019-04-17 16:42     ` Waiman Long
2019-04-17 10:19   ` Peter Zijlstra
2019-04-17 16:53     ` Waiman Long
2019-04-17 12:41   ` Peter Zijlstra
2019-04-17 12:47     ` Peter Zijlstra
2019-04-17 18:29       ` Waiman Long
2019-04-18  8:39         ` Peter Zijlstra
2019-04-17 13:00     ` Peter Zijlstra
2019-04-17 18:50       ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 09/16] locking/rwsem: Ensure an RT task will not spin on reader Waiman Long
2019-04-17 13:18   ` Peter Zijlstra
2019-04-17 18:47     ` Waiman Long
2019-04-18  8:52       ` Peter Zijlstra
2019-04-18 13:27         ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 10/16] locking/rwsem: Wake up almost all readers in wait queue Waiman Long
2019-04-16 16:50   ` Davidlohr Bueso
2019-04-16 17:37     ` Waiman Long
2019-04-17 13:39   ` Peter Zijlstra
2019-04-17 17:16     ` Waiman Long [this message]
2019-04-13 17:22 ` [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer Waiman Long
2019-04-17 13:56   ` Peter Zijlstra
2019-04-17 17:34     ` Waiman Long
2019-04-18  8:57       ` Peter Zijlstra
2019-04-18 14:35         ` Waiman Long
2019-04-17 13:58   ` Peter Zijlstra
2019-04-17 17:45     ` Waiman Long
2019-04-18  9:00       ` Peter Zijlstra
2019-04-18 13:40         ` Waiman Long
2019-04-17 14:05   ` Peter Zijlstra
2019-04-17 17:51     ` Waiman Long
2019-04-18  9:11       ` Peter Zijlstra
2019-04-18 14:37         ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 12/16] locking/rwsem: Enable time-based spinning on reader-owned rwsem Waiman Long
2019-04-18 13:06   ` Peter Zijlstra
2019-04-18 15:15     ` Waiman Long
2019-04-19  7:56       ` Peter Zijlstra
2019-04-19 14:33         ` Waiman Long
2019-04-19 15:36           ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 13/16] locking/rwsem: Add more rwsem owner access helpers Waiman Long
2019-04-13 17:22 ` [PATCH v4 14/16] locking/rwsem: Guard against making count negative Waiman Long
2019-04-18 13:51   ` Peter Zijlstra
2019-04-18 14:08     ` Waiman Long
2019-04-18 14:30       ` Peter Zijlstra
2019-04-18 14:40       ` Peter Zijlstra
2019-04-18 14:54         ` Waiman Long
2019-04-19 10:26           ` Peter Zijlstra
2019-04-19 12:02             ` Peter Zijlstra
2019-04-19 13:03               ` Peter Zijlstra
2019-04-19 13:15                 ` Peter Zijlstra
2019-04-19 19:39                   ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 15/16] locking/rwsem: Merge owner into count on x86-64 Waiman Long
2019-04-18 14:28   ` Peter Zijlstra
2019-04-18 14:40     ` Waiman Long
2019-04-13 17:22 ` [PATCH v4 16/16] locking/rwsem: Remove redundant computation of writer lock word Waiman Long

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