On Fri, Feb 06, 2015 at 03:54:56AM -0800, Christoph Hellwig wrote: > On Thu, Feb 05, 2015 at 11:43:46AM -0500, Anna Schumaker wrote: > > > The problem is that the typical case of all data won't use splice > > > every with your patches as the 4.2 client will always send a READ_PLUS. > > > > > > So we'll have to find a way to use it where it helps. While we might be > > > able to add some hacks to only use splice for the first segment I guess > > > we just need to make the splice support generic enough in the long run. > > > > > > > I should be able to use splice if I detect that we're only returning a single DATA segment easily enough. > > You could also elect to never return more than one data segment as a > start: > > In all situations, the > server may choose to return fewer bytes than specified by the client. > The client needs to check for this condition and handle the > condition appropriately. Yeah, I think that was more-or-less what Anna's first attempt did and I said "what if that means more round trips"? The client can't anticipate the short reads so it can't make up for this with parallelism. > But doing any of these for a call that's really just an optimization > soudns odd. I'd really like to see an evaluation of the READ_PLUS > impact on various workloads before offering it. Yes, unfortunately I don't see a way to make this just an obvious win. (Is there any way we could make it so with better protocol? Maybe RDMA could help get the alignment right in multiple-segment cases? But then I think there needs to be some sort of language about RDMA, or else we're stuck with: https://tools.ietf.org/html/rfc5667#section-5 which I think forces us to return READ_PLUS data inline, another possible READ_PLUS regression.) Anyway, ignoring RDMA, I guess the interesting questions to me are: - How much does READ_PLUS actually help in the cases it was meant to? Perhaps create and age a VM image somehow, and then time a read of the entire image? - What are the worst-case regressions? Read a large file with a lot of small holes and measure time and cpu usage? --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
