On Feb 6, 2015, at 11:46 AM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > On Feb 6, 2015, at 11:08 AM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > >> 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. > > I don’t think a “win” is necessary. It simply needs to be no worse than > READ for current use cases. > > READ_PLUS should be a win for the particular use cases it was > designed for (large sparsely-populated datasets). Without a > demonstrated benefit I think there’s no point in keeping it. > >> (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.) Btw, if I understand this correctly: Without a spec update, a large NFS READ_PLUS reply would be returned in a reply list, which is moved via RDMA WRITE, just like READ replies. The difference is NFS READ payload is placed directly into the client’s page cache by the adapter. With a reply list, the client transport would need to copy the returned data into the page cache. And a large reply buffer would be needed. So, slower, yes. But not inline. > NFSv4.2 currently does not have a binding to RPC/RDMA. Right, this means a spec update is needed. I agree with you, and it’s on our list. > It’s hard to > say at this point what a READ_PLUS on RPC/RDMA might look like. > > RDMA clearly provides no advantage for moving a pattern that a > client must re-inflate into data itself. I can guess that only the > CONTENT_DATA case is interesting for RDMA bulk transfers. > > But don’t forget that NFSv4.1 and later don’t yet work over RDMA, > thanks to missing support for bi-directional RPC/RDMA. I wouldn’t > worry about special cases for it at this point. > > -- > Chuck Lever > chuck[dot]lever[at]oracle[dot]com > > > > -- > 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 -- Chuck Lever chuck[dot]lever[at]oracle[dot]com -- 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
