From: Waiman Long <waiman.long@hpe.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, <linux-kernel@vger.kernel.org>,
<x86@kernel.org>, <linux-alpha@vger.kernel.org>,
<linux-ia64@vger.kernel.org>, <linux-s390@vger.kernel.org>,
<linux-arch@vger.kernel.org>, Davidlohr Bueso <dave@stgolabs.net>,
Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP
Date: Wed, 15 Jun 2016 15:17:50 -0400 [thread overview]
Message-ID: <5761A9DE.6040702@hpe.com> (raw)
In-Reply-To: <20160615172242.GQ30921@twins.programming.kicks-ass.net>
On 06/15/2016 01:22 PM, Peter Zijlstra wrote:
> On Tue, Jun 14, 2016 at 06:48:05PM -0400, Waiman Long wrote:
>> Currently, when down_read() fails, the active read locking isn't undone
>> until the rwsem_down_read_failed() function grabs the wait_lock. If the
>> wait_lock is contended, it may takes a while to get the lock. During
>> that period, writer lock stealing will be disabled because of the
>> active read lock.
>>
>> This patch will release the active read lock ASAP so that writer lock
>> stealing can happen sooner. The only downside is when the reader is
>> the first one in the wait queue as it has to issue another atomic
>> operation to update the count.
>>
>> On a 4-socket Haswell machine running on a 4.7-rc1 tip-based kernel,
>> the fio test with multithreaded randrw and randwrite tests on the
>> same file on a XFS partition on top of a NVDIMM with DAX were run,
>> the aggregated bandwidths before and after the patch were as follows:
>>
>> Test BW before patch BW after patch % change
>> ---- --------------- -------------- --------
>> randrw 1210 MB/s 1352 MB/s +12%
>> randwrite 1622 MB/s 1710 MB/s +5.4%
>>
>> The write-only microbench also showed improvement because some read
>> locking was done by the XFS code.
> How does a reader only micro-bench react? I'm thinking the extra atomic
> might hurt a bit.
>
A reader only benchmark will not go into the slow path at all. It is
only when there is a mix of readers and writers will the reader slowpath
be executed.
I think there will be a little bit of performance impact for a workload
that produce just the right amount of rwsem contentions. However, it is
hard to produce a microbenchmark to create such a right amount of
contention. As the amount of contention increases, I believe this patch
will help performance instead of hurting it. Even then, the amount of
performance degradation in that particular case will be pretty small.
Cheers,
Longman
next prev parent reply other threads:[~2016-06-15 19:18 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-14 22:48 [RFC PATCH-tip v2 0/6] locking/rwsem: Enable reader optimistic spinning Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Waiman Long
2016-06-15 8:04 ` Boqun Feng
2016-06-15 17:18 ` Peter Zijlstra
2016-06-15 19:01 ` Waiman Long
2016-06-16 2:19 ` Boqun Feng
2016-06-16 10:16 ` Will Deacon
2016-06-16 21:35 ` Waiman Long
2016-06-17 0:48 ` Boqun Feng
2016-06-17 15:26 ` Waiman Long
2016-06-17 15:45 ` Will Deacon
2016-06-17 18:17 ` Waiman Long
2016-06-18 8:46 ` Boqun Feng
2016-06-20 7:59 ` Will Deacon
2016-06-15 16:56 ` Davidlohr Bueso
2016-06-15 17:12 ` Peter Zijlstra
2016-06-15 18:27 ` Davidlohr Bueso
2016-06-15 18:40 ` Peter Zijlstra
2016-06-15 18:56 ` Davidlohr Bueso
2016-06-17 1:11 ` Davidlohr Bueso
2016-06-17 14:28 ` Waiman Long
2016-06-17 16:29 ` Davidlohr Bueso
2016-06-17 16:46 ` Davidlohr Bueso
2016-06-15 19:08 ` Waiman Long
2016-06-15 20:04 ` Waiman Long
2016-06-15 21:59 ` Peter Zijlstra
2016-06-14 22:48 ` [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP Waiman Long
2016-06-15 17:22 ` Peter Zijlstra
2016-06-15 19:17 ` Waiman Long [this message]
2016-06-16 2:14 ` Davidlohr Bueso
2016-06-16 21:25 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 3/6] locking/rwsem: Enable count-based spinning on reader Waiman Long
2016-06-15 17:38 ` Peter Zijlstra
2016-06-15 19:28 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 4/6] locking/rwsem: move down rwsem_down_read_failed function Waiman Long
2016-06-15 17:40 ` Peter Zijlstra
2016-06-15 19:21 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation Waiman Long
2016-06-15 17:43 ` Peter Zijlstra
2016-06-15 19:31 ` Waiman Long
2016-06-15 21:57 ` Peter Zijlstra
2016-06-15 17:45 ` Peter Zijlstra
2016-06-15 19:35 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 6/6] locking/rwsem: Enable spinning readers Waiman Long
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