Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

[Waiman Long <waiman.long@hp.com>]

On 02/02/2014 04:03 AM, Ingo Molnar wrote:
> * Waiman Long<waiman.long@hp.com>  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 
data.

-Longman