Document /proc/fs/nfsd/pool_stats

Kernel NFS Server Statistics

============================

This document describes the format and semantics of the statistics

which the kernel NFS server makes available to userspace.  These

statistics are available in several text form pseudo files, each of

which is described separately below.

In most cases you don't need to know these formats, as the nfsstat(8)

program from the nfs-utils distribution provides a helpful command-line

interface for extracting and printing them.

All the files described here are formatted as a sequence of text lines,

separated by newline '\n' characters.  Lines beginning with a hash

'#' character are comments intended for humans and should be ignored

by parsing routines.  All other lines contain a sequence of fields

separated by whitespace.

/proc/fs/nfsd/pool_stats

------------------------

This file is available in kernels from 2.6.30 onwards, if the

/proc/fs/nfsd filesystem is mounted (it almost always should be).

The first line is a comment which describes the fields present in

all the other lines.  The other lines present the following data as

a sequence of unsigned decimal numeric fields.  One line is shown

for each NFS thread pool.

All counters are 64 bits wide and wrap naturally.  There is no way

to zero these counters, instead applications should do their own

rate conversion.

pool

	The id number of the NFS thread pool to which this line applies.

	This number does not change.

	Thread pool ids are a contiguous set of small integers starting

	at zero.  The maximum value depends on the thread pool mode, but

	currently cannot be larger than the number of CPUs in the system.

	Note that in the default case there will be a single thread pool

	which contains all the nfsd threads and all the CPUs in the system,

	and thus this file will have a single line with a pool id of "0".

packets-arrived

	Counts how many NFS packets have arrived.  More precisely, this

	is the number of times that the network stack has notified the

	sunrpc server layer that new data may be available on a transport

	(e.g. an NFS or UDP socket or an NFS/RDMA endpoint).

	Depending on the NFS workload patterns and various network stack

	effects (such as Large Receive Offload) which can combine packets

	on the wire, this may be either more or less than the number

	of NFS calls received (which statistic is available elsewhere).

	However this is a more accurate and less workload-dependent measure

	of how much CPU load is being placed on the sunrpc server layer

	due to NFS network traffic.

sockets-enqueued

	Counts how many times an NFS transport is enqueued to wait for

	an nfsd thread to service it, i.e. no nfsd thread was considered

	available.

	The circumstance this statistic tracks indicates that there was NFS

	network-facing work to be done but it couldn't be done immediately,

	thus introducing a small delay in servicing NFS calls.  The ideal

	rate of change for this counter is zero; significantly non-zero

	values may indicate a performance limitation.

	This can happen either because there are too few nfsd threads in the

	thread pool for the NFS workload (the workload is thread-limited),

	or because the NFS workload needs more CPU time than is available in

	the thread pool (the workload is CPU-limited).  In the former case,

	configuring more nfsd threads will probably improve the performance

	of the NFS workload.  In the latter case, the sunrpc server layer is

	already choosing not to wake idle nfsd threads because there are too

	many nfsd threads which want to run but cannot, so configuring more

	nfsd threads will make no difference whatsoever.  The overloads-avoided

	statistic (see below) can be used to distinguish these cases.

threads-woken

	Counts how many times an idle nfsd thread is woken to try to

	receive some data from an NFS transport.

	This statistic tracks the circumstance where incoming

	network-facing NFS work is being handled quickly, which is a good

	thing.  The ideal rate of change for this counter will be close

	to but less than the rate of change of the packets-arrived counter.

overloads-avoided

	Counts how many times the sunrpc server layer chose not to wake an

	nfsd thread, despite the presence of idle nfsd threads, because

	too many nfsd threads had been recently woken but could not get

	enough CPU time to actually run.

	This statistic counts a circumstance where the sunrpc layer

	heuristically avoids overloading the CPU scheduler with too many

	runnable nfsd threads.  The ideal rate of change for this counter

	is zero.  Significant non-zero values indicate that the workload

	is CPU limited.  Usually this is associated with heavy CPU usage

	on all the CPUs in the nfsd thread pool.

	If a sustained large overloads-avoided rate is detected on a pool,

	the top(1) utility should be used to check for the following

	pattern of CPU usage on all the CPUs associated with the given

	nfsd thread pool.

	 - %us ~= 0 (as you're *NOT* running applications on your NFS server)

	 - %wa ~= 0

	 - %id ~= 0

	 - %sy + %hi + %si ~= 100

	If this pattern is seen, configuring more nfsd threads will *not*

	improve the performance of the workload.  If this patten is not

	seen, then something more subtle is wrong.

threads-timedout

	Counts how many times an nfsd thread triggered an idle timeout,

	i.e. was not woken to handle any incoming network packets for

	some time.

	This statistic counts a circumstance where there are more nfsd

	threads configured than can be used by the NFS workload.  This is

	a clue that the number of nfsd threads can be reduced without

	affecting performance.  Unfortunately, it's only a clue and not

	a strong indication, for a couple of reasons:

	 - Currently the rate at which the counter is incremented is quite

	   slow; the idle timeout is 60 minutes.  Unless the NFS workload

	   remains constant for hours at a time, this counter is unlikely

	   to be providing information that is still useful.

	 - It is usually a wise policy to provide some slack,

	   i.e. configure a few more nfsds than are currently needed,

	   to allow for future spikes in load.

Note that incoming packets on NFS transports will be dealt with in

one of three ways.  An nfsd thread can be woken (threads-woken counts

this case), or the transport can be enqueued for later attention

(sockets-enqueued counts this case), or the packet can be temporarily

deferred because the transport is currently being used by an nfsd

thread.  This last case is not very interesting and is not explicitly

counted, but can be inferred from the other counters thus:

packets-deferred = packets-arrived - ( sockets-enqueued + threads-woken )

More

----

Descriptions of the other statistics file should go here.

Greg Banks <gnb@sgi.com>

26 Mar 2009