Commits on May 24, 2019

1. Reduce nfs max_session_lots from 64 to 4 …

   The default number of session slots (nfs/max_session_lots) is 64. This
   is the maximum number of parallel requests a nfs client can have
   outstanding to a nfs server (per mount). Because we have 64 nfsd threads
   on our servers, a single client is able to dominate all nsfd threads of
   a server.
   
   There seems to be a bad interaction between nfs and readahead. The
   readahead size is set to max_session_lots-1 and is doubled when
   posix_fadvice(..,POSIX_FADV_SEQUENTIAL) is used, which "cp" and "cat"
   do.  Although the exact mechanism is not yet fully understood, the
   readahead io-requests seem to be handled by multiple nfsd thread in
   parallel, which are blocking each other while compeeting for the same
   block device. Because all nfsd threads are used up (by a single client
   doing a single copy) other nfs clients stall and we've even seen them
   being pushed over their lease timeouts and losing their state.
   
   Reduce this value via modprobe.d to avoid overload situations.
   
   This parameter can also be read and written on a running system via
   /sys/module/nfs/parameters/max_session_slots if nfs is loaded.  Changes
   apply to future mounts only.

   

   donald committed May 24, 2019
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