Re: [RFC][PATCH] make global bitlock waitqueues per-node

[Nicholas Piggin <npiggin@gmail.com>]

On Wed, 21 Dec 2016 10:12:36 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Dec 21, 2016 at 4:30 AM, Nicholas Piggin <npiggin@gmail.com> wrote:
> >
> > I've been doing a bit of testing, and I don't know why you're seeing
> > this.
> >
> > I don't think I've been able to trigger any actual page lock contention
> > so nothing gets put on the waitqueue to really bounce cache lines around
> > that I can see.  
> 
> The "test is the waitqueue is empty" is going to cause cache misses
> even if there is no contention.
> 
> In fact, that's why I want the contention bit in the struct page - not
> because of any NUMA issues, but simply due to cache misses.
> 
> And yes, with no contention the bit waiting should hopefully be able
> to cache things shared - which should make the bouncing much less -
> but there's going to be a shitload of false sharing with any actual
> IO, so you will get bouncing due to that.

Well that's what I'm actually interested in, but I could not get it to
do much bouncing at all. There was a significant amount of writes going
through when having the backing store files on writeback filesystem,
but even that was not really triggering a lot of actual waiters.

Not that I don't believe it could happen, and Dave's system is a lot
bigger and faster and more NUMA than the one I was testing on. I'm
just curious.

Thanks,
Nick