On Thu, 2019-09-19 at 16:19 -0500, Daniel Forrest wrote: > On Thu, Sep 19, 2019 at 08:40:41PM +0000, Trond Myklebust wrote: > > On Thu, 2019-09-19 at 23:20 +0300, Alkis Georgopoulos wrote: > > > On 9/19/19 11:05 PM, Trond Myklebust wrote: > > > > There are plenty of operations that can take longer than 700 ms > > > > to > > > > complete. Synchronous writes to disk are one, but COMMIT (i.e. > > > > the > > > > NFS > > > > equivalent of fsync()) can often take much longer even though > > > > it > > > > has no > > > > payload. > > > > > > > > So the problem is not the size of the WRITE payload. The real > > > > problem > > > > is the timeout. > > > > > > > > The bottom line is that if you want to keep timeo=7 as a mount > > > > option > > > > for TCP, then you are on your own. > > > > > > > > > > The problem isn't timeo at all. > > > If I understand it correctly, when I try to launch firefox over > > > nfsroot, > > > NFS will wait until it fills 1M before "replying" to the > > > application. > > > Thus the applications will launch a lot slower, as they get > > > "disk > > > feedback" in larger chunks and not "snappy". > > > > > > In numbers: > > > timeo=600,rsize=1M => firefox opens in 30 secs > > > timeo=600,rsize=32k => firefox opens in 20 secs > > > > > > > That's a different problem, and is most likely due to readahead > > causing > > your client to read more data than it needs to. It is also true > > that > > the maximum readahead size is proportional to the rsize and that > > maybe > > it shouldn't be. > > However the VM layer is supposed to ensure that the kernel doesn't > > try > > to read ahead more than necessary. It is bounded by the maximum we > > set > > in the NFS layer, but it isn't supposed to hit that maximum unless > > the > > readahead heuristics show that the application may need it. > > > > -- > > Trond Myklebust > > Linux NFS client maintainer, Hammerspace > > trond.myklebust@xxxxxxxxxxxxxxx > > What may be happening here is something I have noticed with glibc. > > - statfs reports the rsize/wsize as the block size of the filesystem. > > - glibc uses the block size as the default buffer size for > fread/fwrite. > > If an application is using fread/fwrite on an NFS mounted file with > an > rsize/wsize of 1M it will try to fill a 1MB buffer. > > I have often changed mounts to use rsize/wsize=64K to alleviate this. > That sounds like an abuse of the filesystem block size. There is nothing in the POSIX definition of either fread() or fwrite() that requires glibc to do this: https://pubs.opengroup.org/onlinepubs/9699919799/functions/fread.html https://pubs.opengroup.org/onlinepubs/9699919799/functions/fwrite.html -- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@xxxxxxxxxxxxxxx
