On Wed, Feb 11, 2015 at 11:31:43AM -0500, Trond Myklebust wrote: > On Wed, Feb 11, 2015 at 11:22 AM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > On Wed, Feb 11, 2015 at 11:13:38AM -0500, Trond Myklebust wrote: > >> On Wed, Feb 11, 2015 at 11:04 AM, Anna Schumaker > >> <Anna.Schumaker@xxxxxxxxxx> wrote: > >> > I'm not seeing a huge performance increase with READ_PLUS compared to READ (in fact, it's often a bit slower compared to READ, even when using splice). My guess is that the problem is mostly on the client's end since we have to do a memory shift on each segment to get everything lined up properly. I'm playing around with code that cuts down the number of memory shifts, but I still have a few bugs to work out before I'll know if it actually helps. > >> > > >> > >> I'm wondering if the right way to do READ_PLUS would have been to > >> instead have a separate function READ_SPARSE, that will return a list > >> of all sparse areas in the supplied range. We could even make that a > >> READ_SAME, that can do the same for patterned data. > > > > I worry about ending up with incoherent results, but perhaps it's no > > different from the current behavior since we're already piecing together > > our idea of the file content from multiple reads sent in parallel. > > I don't see what the problem is. The client sends a READ_SPARSE, and > caches the existence or not of a hole. How is that in any way > different from caching the results of a read that returns no data? > > >> The thing is that READ works just fine for what we want it to do. The > >> real win here would be if given a very large file, we could request a > >> list of all the sparse areas in, say, a 100GB range, and then use that > >> data to build up a bitmap of unallocated blocks for which we can skip > >> the READ requests. > > > > Can we start by having the server return a single data extent covering > > the whole read request, with the single exception of the case where the > > read falls entirely within a hole? > > > > I think that should help in the case of large holes without interfering > > with the client's zero-copy logic in the case there are no large holes. > > > > That still forces the server to do extra work on each read: it has to > check for the presence of a hole or not instead of just filling the > buffer with data. I guess. Presumably the filesystem already does something like that on read, so I find it hard to imagine that extra check is expensive, especially if it's amortized over large-ish reads. Seems worth checking at least. --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
