1
  2Kernel NFS Server Statistics
  3============================
  4
  5This document describes the format and semantics of the statistics
  6which the kernel NFS server makes available to userspace.  These
  7statistics are available in several text form pseudo files, each of
  8which is described separately below.
  9
 10In most cases you don't need to know these formats, as the nfsstat(8)
 11program from the nfs-utils distribution provides a helpful command-line
 12interface for extracting and printing them.
 13
 14All the files described here are formatted as a sequence of text lines,
 15separated by newline '\n' characters.  Lines beginning with a hash
 16'#' character are comments intended for humans and should be ignored
 17by parsing routines.  All other lines contain a sequence of fields
 18separated by whitespace.
 19
 20/proc/fs/nfsd/pool_stats
 21------------------------
 22
 23This file is available in kernels from 2.6.30 onwards, if the
 24/proc/fs/nfsd filesystem is mounted (it almost always should be).
 25
 26The first line is a comment which describes the fields present in
 27all the other lines.  The other lines present the following data as
 28a sequence of unsigned decimal numeric fields.  One line is shown
 29for each NFS thread pool.
 30
 31All counters are 64 bits wide and wrap naturally.  There is no way
 32to zero these counters, instead applications should do their own
 33rate conversion.
 34
 35pool
 36	The id number of the NFS thread pool to which this line applies.
 37	This number does not change.
 38
 39	Thread pool ids are a contiguous set of small integers starting
 40	at zero.  The maximum value depends on the thread pool mode, but
 41	currently cannot be larger than the number of CPUs in the system.
 42	Note that in the default case there will be a single thread pool
 43	which contains all the nfsd threads and all the CPUs in the system,
 44	and thus this file will have a single line with a pool id of "0".
 45
 46packets-arrived
 47	Counts how many NFS packets have arrived.  More precisely, this
 48	is the number of times that the network stack has notified the
 49	sunrpc server layer that new data may be available on a transport
 50	(e.g. an NFS or UDP socket or an NFS/RDMA endpoint).
 51
 52	Depending on the NFS workload patterns and various network stack
 53	effects (such as Large Receive Offload) which can combine packets
 54	on the wire, this may be either more or less than the number
 55	of NFS calls received (which statistic is available elsewhere).
 56	However this is a more accurate and less workload-dependent measure
 57	of how much CPU load is being placed on the sunrpc server layer
 58	due to NFS network traffic.
 59
 60sockets-enqueued
 61	Counts how many times an NFS transport is enqueued to wait for
 62	an nfsd thread to service it, i.e. no nfsd thread was considered
 63	available.
 64
 65	The circumstance this statistic tracks indicates that there was NFS
 66	network-facing work to be done but it couldn't be done immediately,
 67	thus introducing a small delay in servicing NFS calls.  The ideal
 68	rate of change for this counter is zero; significantly non-zero
 69	values may indicate a performance limitation.
 70
 71	This can happen either because there are too few nfsd threads in the
 72	thread pool for the NFS workload (the workload is thread-limited),
 73	or because the NFS workload needs more CPU time than is available in
 74	the thread pool (the workload is CPU-limited).  In the former case,
 75	configuring more nfsd threads will probably improve the performance
 76	of the NFS workload.  In the latter case, the sunrpc server layer is
 77	already choosing not to wake idle nfsd threads because there are too
 78	many nfsd threads which want to run but cannot, so configuring more
 79	nfsd threads will make no difference whatsoever.  The overloads-avoided
 80	statistic (see below) can be used to distinguish these cases.
 81
 82threads-woken
 83	Counts how many times an idle nfsd thread is woken to try to
 84	receive some data from an NFS transport.
 85
 86	This statistic tracks the circumstance where incoming
 87	network-facing NFS work is being handled quickly, which is a good
 88	thing.  The ideal rate of change for this counter will be close
 89	to but less than the rate of change of the packets-arrived counter.
 90
 91overloads-avoided
 92	Counts how many times the sunrpc server layer chose not to wake an
 93	nfsd thread, despite the presence of idle nfsd threads, because
 94	too many nfsd threads had been recently woken but could not get
 95	enough CPU time to actually run.
 96
 97	This statistic counts a circumstance where the sunrpc layer
 98	heuristically avoids overloading the CPU scheduler with too many
 99	runnable nfsd threads.  The ideal rate of change for this counter
100	is zero.  Significant non-zero values indicate that the workload
101	is CPU limited.  Usually this is associated with heavy CPU usage
102	on all the CPUs in the nfsd thread pool.
103
104	If a sustained large overloads-avoided rate is detected on a pool,
105	the top(1) utility should be used to check for the following
106	pattern of CPU usage on all the CPUs associated with the given
107	nfsd thread pool.
108
109	 - %us ~= 0 (as you're *NOT* running applications on your NFS server)
110
111	 - %wa ~= 0
112
113	 - %id ~= 0
114
115	 - %sy + %hi + %si ~= 100
116
117	If this pattern is seen, configuring more nfsd threads will *not*
118	improve the performance of the workload.  If this patten is not
119	seen, then something more subtle is wrong.
120
121threads-timedout
122	Counts how many times an nfsd thread triggered an idle timeout,
123	i.e. was not woken to handle any incoming network packets for
124	some time.
125
126	This statistic counts a circumstance where there are more nfsd
127	threads configured than can be used by the NFS workload.  This is
128	a clue that the number of nfsd threads can be reduced without
129	affecting performance.  Unfortunately, it's only a clue and not
130	a strong indication, for a couple of reasons:
131
132	 - Currently the rate at which the counter is incremented is quite
133	   slow; the idle timeout is 60 minutes.  Unless the NFS workload
134	   remains constant for hours at a time, this counter is unlikely
135	   to be providing information that is still useful.
136
137	 - It is usually a wise policy to provide some slack,
138	   i.e. configure a few more nfsds than are currently needed,
139	   to allow for future spikes in load.
140
141
142Note that incoming packets on NFS transports will be dealt with in
143one of three ways.  An nfsd thread can be woken (threads-woken counts
144this case), or the transport can be enqueued for later attention
145(sockets-enqueued counts this case), or the packet can be temporarily
146deferred because the transport is currently being used by an nfsd
147thread.  This last case is not very interesting and is not explicitly
148counted, but can be inferred from the other counters thus:
149
150packets-deferred = packets-arrived - ( sockets-enqueued + threads-woken )
151
152
153More
154----
155Descriptions of the other statistics file should go here.
156
157
158Greg Banks <gnb@sgi.com>
15926 Mar 2009