Messages in this thread | | | Date | Mon, 4 Feb 2008 11:33:52 +0100 | From | Jens Axboe <> | Subject | Re: [rfc] direct IO submission and completion scalability issues |
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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
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