On Wed, May 15, 2013 at 03:34:27PM +0100, James Vanns wrote: > > > On Wed, May 15, 2013 at 02:42:42PM +0100, James Vanns wrote: > > > > fs/nfsd/nfssvc.c:nfsd_get_default_maxblksize() is probably a good > > > > starting point. Its caller, nfsd_create_serv(), calls > > > > svc_create_pooled() with the result that's calculated. > > > > > > Hmm. If I've read this section of code correctly, it seems to me > > > that on most modern NFS servers (using TCP as the transport) the > > > default > > > and preferred blocksize negotiated with clients will almost always > > > be > > > 1MB - the maximum RPC payload. The nfsd_get_default_maxblksize() > > > function > > > seems obsolete for modern 64-bit servers with at least 4G of RAM as > > > it'll > > > always prefer this upper bound instead of any value calculated > > > according to > > > available RAM. > > > > Well, "obsolete" is an odd way to put it--the code is still expected > > to work on smaller machines. > > Poor choice of words perhaps. I guess I'm just used to NFS servers being > pretty hefty pieces of kit and 'small' workstations having a couple of GB > of RAM too. > > > Arguments welcome about the defaults, thoodd ugh I wonder whether it > > would be better to be doing this sort of calculation in user space. > > See below. > > > > For what it's worth (not sure if I specified this) I'm running > > > kernel 2.6.32. > > > > > > Anyway, this file/function appears to set the default *max* > > > blocksize. I haven't > > > read all the related code yet, but does the preferred block size > > > derive > > > from this maximum too? > > > > See > > > > For finfo see fs/nfsd/nfs3proc.c:nfsd3_proc_fsinfo, which uses > > > > svc_max_payload(). > > I've just returned from nfsd3_proc_fsinfo() and found what I would > consider an odd decision - perhaps nothing better was suggested at > the time. It seems to me that in response to an FSINFO call the reply > stuffs the max_block_size value in both the maximum *and* preferred > block sizes for both read and write. A 1MB block size for a preferred > default is a little high! If a disk is reading at 33MB/s and we have just > a single server running 64 knfsd and each READ call is requesting 1MB of > data then all of a sudden we have an aggregate read speed of ~512k/s I lost you here. > and > that is without network latencies. And of course we will probably have 100s of > requests queued behind each knfsd waiting for these 512k reads to finish. All of a > sudden our user experience is rather poor :( Note the preferred size is not a minimum--the client isn't forced to do 1MB reads if it really only wants 1 page, for example, if that's what you mean. (I haven't actually looked at how typical clients used rt/wtpref.) --b. > Perhaps a better suggestion would be to at least expose the maximum and preferred > block sizes (for both read and write) via a sysctl key so an administrator can set > it to the underlying block sizes of the file system or physical device? > > Perhaps the defaults should at least be a smaller multiple of the page size or somewhere > between that and the PDU of the network layer the service is bound too. > > Just my tuppence - and my maths might be flawed ;) > > Jim > > > I'm not sure what the history is behind that logic, though. > > > > --b. > > > > -- > Jim Vanns > Senior Software Developer > Framestore -- 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
