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

[Waiman Long]

On 01/31/2014 04:26 AM, Peter Zijlstra wrote:
> On Thu, Jan 30, 2014 at 04:17:15PM +0100, Peter Zijlstra wrote:
> > The below is still small and actually works.
> OK, so having actually worked through the thing; I realized we can
> actually do a version without MCS lock and instead use a ticket lock for
> the waitqueue.
>
> This is both smaller (back to 8 bytes for the rwlock_t), and should be
> faster under moderate contention for not having to touch extra
> cachelines.
>
> Completely untested and with a rather crude generic ticket lock
> implementation to illustrate the concept:

Using a ticket lock instead will have the same scalability problem as 
the ticket spinlock as all the waiting threads will spin on the lock 
cacheline causing a lot of cache bouncing traffic. That is the reason 
why I want to replace ticket spinlock with queue spinlock. If the 
16-byte size is an issue, I can use the same trick in the queue spinlock 
patch to reduce its size down to 8 bytes with a bit more overhead in the 
slowpath.

Another thing I want to discuss about is whether a bit more overhead in 
moderate contention cases is really such a bit deal. With moderate 
contention, I suppose the amount of time spent in the locking functions 
will be just a few percent at most for real workloads. It won't really 
be noticeable if the locking functions take, maybe, 50% more time to 
finish. Anyway, I am going to do more performance testing on low end 
machines.

-Longman
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