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. -- Dan Forrest Space Science and Engineering Center, University of Wisconsin, Madison dforrest@xxxxxxxx
