On Wed, 2022-03-30 at 11:17 -0700, Linus Torvalds wrote: > On Tue, Mar 29, 2022 at 12:36 PM Trond Myklebust > <trondmy@xxxxxxxxxxxxxxx> wrote: > > > > - Readdir fixes to improve cacheability of large directories when > > there > > are multiple readers and writers. > > So I only took a look at this part now. I've obviously already pulled > it, but that use of 'xxhash()' just made me go "Whaaa?" > > It's claimed that it's used because of its extreme performance, but > the performance numbers come from using it as a block hash. > > That's not what nfs does. > > The nfs code just does > > xxhash(&cookie, sizeof(cookie), 0) & NFS_READDIR_COOKIE_MASK; > > where that "cookie" is just a 64-bit entity. And then it masks off > everything but 18 bits. > > Is that *really* appropriate use of a new hash function? > > Why is this not just doing > > #include <hash.h> > > hash_64(cookie, 18); > > which is a lot more obvious than xxhash(). > > If there really are some serious problems with the perfectly standard > hash() functionality, I think you should document them. > > Because just randomly picking xxhash() without explaining _why_ you > can't just use the same simple thing we use elsewhere is very odd. > > Or rather, when the only documentation is "performance", then I think > the regular "hash_64()" is the obvious and trivial choice. > > Linus My main worry was that hash_64() would have too many collisions. Since this is using cuckoo nesting, that would be a problem. I did some quick studies after I got your email, and it seems as if my concerns were unfounded. I've tested both a linear index and a sample of ext4 getdents offsets. While the sample of ext4 offsets did show a larger number of collisions than a simple linear index, it wasn't too terrible (3 collisions in a sample of 9000 entries). The linear index showed no collisions up to about 100000 entries. So, I'd be OK with changing to hash_64() and will queue up a bugfix for it. I should have done this study earlier... -- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@xxxxxxxxxxxxxxx
