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

Nick Piggin
2008-02-04T10:31:46Z

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.