On Fri, Jan 31, 2014 at 01:59:02PM -0500, Waiman Long wrote:

On 01/31/2014 04:26 AM, Peter Zijlstra wrote:

quoted

On Thu, Jan 30, 2014 at 04:17:15PM +0100, Peter Zijlstra wrote:

quoted

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.

But but but, just fix such heavily contended locks. Don't make sensible
code that is lightly contended run slower because of it.