linux-kernel - Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation

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Message-ID: <20130722103402.GA1991@gmail.com>
Date:	Mon, 22 Jul 2013 12:34:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Catalin Marinas <catalin.marinas@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Akinobu Mita <akinobu.mita@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	George Spelvin <linux@...izon.com>
Subject: Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation


* Waiman Long <waiman.long@...com> wrote:

> I had run some performance tests using the fserver and new_fserver 
> benchmarks (on ext4 filesystems) of the AIM7 test suite on a 80-core 
> DL980 with HT on. The following kernels were used:
> 
> 1. Modified 3.10.1 kernel with mb_cache_spinlock in fs/mbcache.c
>    replaced by a rwlock
> 2. Modified 3.10.1 kernel + modified __read_lock_failed code as suggested
>    by Ingo
> 3. Modified 3.10.1 kernel + queue read/write lock
> 4. Modified 3.10.1 kernel + queue read/write lock in classic read/write
>    lock behavior
> 
> The last one is with the read lock stealing flag set in the qrwlock
> structure to give priority to readers and behave more like the classic
> read/write lock with less fairness.
> 
> The following table shows the averaged results in the 200-1000
> user ranges:
> 
> +-----------------+--------+--------+--------+--------+
> |  Kernel         |    1   |    2   |    3   |   4    |
> +-----------------+--------+--------+--------+--------+
> | fserver JPM     | 245598 | 274457 | 403348 | 411941 |
> | % change from 1 |   0%   | +11.8% | +64.2% | +67.7% |
> +-----------------+--------+--------+--------+--------+
> | new-fserver JPM | 231549 | 269807 | 399093 | 399418 |
> | % change from 1 |   0%   | +16.5% | +72.4% | +72.5% |
> +-----------------+--------+--------+--------+--------+

So it's not just herding that is a problem.

I'm wondering, how sensitive is this particular benchmark to fairness? 
I.e. do the 200-1000 simulated users each perform the same number of ops, 
so that any smearing of execution time via unfairness gets amplified?

I.e. does steady-state throughput go up by 60%+ too with your changes?

Thanks,

	Ingo
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