Re: [dpdk-dev] [PATCH v1 0/3] MCS queued lock implementation

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed,  5 Jun 2019 23:58:45 +0800
Phil Yang <phil.yang@arm.com> wrote:

> This patch set added MCS lock library and its unit test.
> 
> The MCS lock (proposed by JOHN M. MELLOR-CRUMMEY and MICHAEL L. SCOTT) provides
> scalability by spinning on a CPU/thread local variable which avoids expensive
> cache bouncings. It provides fairness by maintaining a list of acquirers and
> passing the lock to each CPU/thread in the order they acquired the lock.
> 
> References:
> 1. http://web.mit.edu/6.173/www/currentsemester/readings/R06-scalable-synchronization-1991.pdf
> 2. https://lwn.net/Articles/590243/
> 
> Mirco-benchmarking result:
> ------------------------------------------------------------------------------------------------
> MCS lock                      | spinlock                       | ticket lock
> ------------------------------+--------------------------------+--------------------------------
> Test with lock on 13 cores... |  Test with lock on 14 cores... |  Test with lock on 14 cores...
> Core [15] Cost Time = 22426 us|  Core [14] Cost Time = 47974 us|  Core [14] cost time = 66761 us
> Core [16] Cost Time = 22382 us|  Core [15] Cost Time = 46979 us|  Core [15] cost time = 66766 us
> Core [17] Cost Time = 22294 us|  Core [16] Cost Time = 46044 us|  Core [16] cost time = 66761 us
> Core [18] Cost Time = 22412 us|  Core [17] Cost Time = 28793 us|  Core [17] cost time = 66767 us
> Core [19] Cost Time = 22407 us|  Core [18] Cost Time = 48349 us|  Core [18] cost time = 66758 us
> Core [20] Cost Time = 22436 us|  Core [19] Cost Time = 19381 us|  Core [19] cost time = 66766 us
> Core [21] Cost Time = 22414 us|  Core [20] Cost Time = 47914 us|  Core [20] cost time = 66763 us
> Core [22] Cost Time = 22405 us|  Core [21] Cost Time = 48333 us|  Core [21] cost time = 66766 us
> Core [23] Cost Time = 22435 us|  Core [22] Cost Time = 38900 us|  Core [22] cost time = 66749 us
> Core [24] Cost Time = 22401 us|  Core [23] Cost Time = 45374 us|  Core [23] cost time = 66765 us
> Core [25] Cost Time = 22408 us|  Core [24] Cost Time = 16121 us|  Core [24] cost time = 66762 us
> Core [26] Cost Time = 22380 us|  Core [25] Cost Time = 42731 us|  Core [25] cost time = 66768 us
> Core [27] Cost Time = 22395 us|  Core [26] Cost Time = 29439 us|  Core [26] cost time = 66768 us
>                               |  Core [27] Cost Time = 38071 us|  Core [27] cost time = 66767 us
> ------------------------------+--------------------------------+--------------------------------
> Total Cost Time = 291195 us   |  Total Cost Time = 544403 us   |  Total cost time = 934687 us
> ------------------------------------------------------------------------------------------------

From the user point of view there needs to be clear recommendations
on which lock type to use.

And if one of the lock types is always slower it should be deprecated long term.