Re: [rfc] direct IO submission and completion scalability issues

Jens Axboe
2008-02-04T10:34:08Z

On Mon, Feb 04 2008, Nick Piggin wrote:
> On Mon, Feb 04, 2008 at 11:12:44AM +0100, Jens Axboe wrote:
> > On Sun, Feb 03 2008, Nick Piggin wrote:
> > > On Fri, Jul 27, 2007 at 06:21:28PM -0700, Suresh B wrote:
> > >
> > > Hi guys,
> > >
> > > Just had another way we might do this. Migrate the completions out to
> > > the submitting CPUs rather than migrate submission into the completing
> > > CPU.
> > >
> > > I've got a basic patch that passes some stress testing. It seems fairly
> > > simple to do at the block layer, and the bulk of the patch involves
> > > introducing a scalable smp_call_function for it.
> > >
> > > Now it could be optimised more by looking at batching up IPIs or
> > > optimising the call function path or even mirating the completion event
> > > at a different level...
> > >
> > > However, this is a first cut. It actually seems like it might be taking
> > > slightly more CPU to process block IO (~0.2%)... however, this is on my
> > > dual core system that shares an llc, which means that there are very few
> > > cache benefits to the migration, but non-zero overhead. So on multisocket
> > > systems hopefully it might get to positive territory.
> >
> > That's pretty funny, I did pretty much the exact same thing last week!
>
> Oh nice ;)
>
>
> > The primary difference between yours and mine is that I used a more
> > private interface to signal a softirq raise on another CPU, instead of
> > allocating call data and exposing a generic interface. That put the
> > locking in blk-core instead, turning blk_cpu_done into a structure with
> > a lock and list_head instead of just being a list head, and intercepted
> > at blk_complete_request() time instead of waiting for an already raised
> > softirq on that CPU.
>
> Yeah I was looking at that... didn't really want to add the spinlock
> overhead to the non-migration case. Anyway, I guess that sort of
> fine implementation details is going to have to be sorted out with
> results.

As Andi mentions, we can look into making that lockless. For the initial
implementation I didn't really care, just wanted something to play with
that would nicely allow me to control both the submit and complete side
of the affinity issue.

--
Jens Axboe