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Date:	Wed, 05 Feb 2014 22:08:35 -0500
From:	Waiman Long <>
To:	Ingo Molnar <>
CC:	Peter Zijlstra <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	"H. Peter Anvin" <>, Arnd Bergmann <>,,,, Steven Rostedt <>,
	Andrew Morton <>,
	Michel Lespinasse <>,
	Andi Kleen <>,
	Rik van Riel <>,
	"Paul E. McKenney" <>,
	Linus Torvalds <>,
	Raghavendra K T <>,
	George Spelvin <>,
	Tim Chen <>,,
	Scott J Norton <>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

On 02/02/2014 04:03 AM, Ingo Molnar wrote:
> * Waiman Long<>  wrote:
>> How about making the selection of MCS or ticket queuing either user
>> configurable or depending on the setting of NR_CPUS, NUMA, etc?
> No!
> There are lots of disadvantages to adding such CONFIG_NUMA Kconfig
> variants for locking primitives:
>   - an doubling of the test matrix
>   - an doubling of the review matrix and a halving of effective review
>     capacity: we've just about go the capacity to review and validate
>     patches like this. Splitting out a 'only NUMA cares' variant is a
>     non-starter really.
>   - but most importantly, there's absolutely no reason to not be fast
>     on 128 CPU systems in the low contended case either! Sacrificing
>     the low contended case with 'on 128 CPU systems it is the contended
>     path that matters' is an idiotic argument.
> Essentially the only area were we allow Kconfig dependencies are
> unyielding physical forces: such as lots of CPUs needing a wider CPU
> mask.
> As Peter said it, the right solution is to fix the contended case. If
> that also happens to speed up or better organize the uncondended code
> then that's good, but it should not make it worse.
> Thanks,
> 	Ingo

You are right. I am trying to measure the performance impact of MCS 
queuing has on a lightly contended system. I need to write some custom 
test code to get that information. With that information, I may be able 
to tune it to perform more or less on par with ticket lock.

As for the additional cache line access of the MCS lock, I don't think 
it is really an issued as the MCS node is allocated on local stack which 
is likely to be in the cache anyway. I will report back when I have more 


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