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-
-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