On Mon, Jan 06, 2014 at 01:47:26PM -0800, Kent Overstreet wrote: > Ok, so I hadn't really given any thought to that kind of use case; insofar as > I > had I would've been skeptical percpu tag allocation made sense for 32 > different > tags at all. > > We really don't want to screw over the users that aren't so constrained by the > size of their tag space; there really is a huge performance tradeoff here > (otherwise you're stealing tags and bouncing cachelines for _every_ tag > allocation when the queue is full, and your percpu tag allocation is no longer > very percpu). > > I'm not sure what the best strategy is for NCQ-type max nr_tags, though - > thoughts? > > Easy thing to do for now is just to add another parameter to percpu_ida_init() > for the number of tags that are allowed to sit unused on other cpu's > freelists - > users that have large relatively unbounded nr_tags can set that to nr_tags / > 2, > for NCQ you'd set it to 0. > > I suspect we can do better for NCQ though, w.r.t. worst case performance.
Yeah, that was my first thought when I posted "percpu_ida: Allow variable maximum number of cached tags" patch some few months ago. But I am back- pedalling as it does not appear solves the fundamental problem - what is the best threshold? May be we can walk off with a per-cpu timeout that flushes batch nr of tags from local caches to the pool? Each local allocation would restart the timer, but once allocation requests stopped coming on a CPU the tags would not gather dust in local caches. -- Regards, Alexander Gordeev [email protected] -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
