linux/net/sunrpc/timer.c
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   1/*
   2 * linux/net/sunrpc/timer.c
   3 *
   4 * Estimate RPC request round trip time.
   5 *
   6 * Based on packet round-trip and variance estimator algorithms described
   7 * in appendix A of "Congestion Avoidance and Control" by Van Jacobson
   8 * and Michael J. Karels (ACM Computer Communication Review; Proceedings
   9 * of the Sigcomm '88 Symposium in Stanford, CA, August, 1988).
  10 *
  11 * This RTT estimator is used only for RPC over datagram protocols.
  12 *
  13 * Copyright (C) 2002 Trond Myklebust <trond.myklebust@fys.uio.no>
  14 */
  15
  16#include <asm/param.h>
  17
  18#include <linux/types.h>
  19#include <linux/unistd.h>
  20#include <linux/module.h>
  21
  22#include <linux/sunrpc/clnt.h>
  23
  24#define RPC_RTO_MAX (60*HZ)
  25#define RPC_RTO_INIT (HZ/5)
  26#define RPC_RTO_MIN (HZ/10)
  27
  28/**
  29 * rpc_init_rtt - Initialize an RPC RTT estimator context
  30 * @rt: context to initialize
  31 * @timeo: initial timeout value, in jiffies
  32 *
  33 */
  34void rpc_init_rtt(struct rpc_rtt *rt, unsigned long timeo)
  35{
  36        unsigned long init = 0;
  37        unsigned int i;
  38
  39        rt->timeo = timeo;
  40
  41        if (timeo > RPC_RTO_INIT)
  42                init = (timeo - RPC_RTO_INIT) << 3;
  43        for (i = 0; i < 5; i++) {
  44                rt->srtt[i] = init;
  45                rt->sdrtt[i] = RPC_RTO_INIT;
  46                rt->ntimeouts[i] = 0;
  47        }
  48}
  49EXPORT_SYMBOL_GPL(rpc_init_rtt);
  50
  51/**
  52 * rpc_update_rtt - Update an RPC RTT estimator context
  53 * @rt: context to update
  54 * @timer: timer array index (request type)
  55 * @m: recent actual RTT, in jiffies
  56 *
  57 * NB: When computing the smoothed RTT and standard deviation,
  58 *     be careful not to produce negative intermediate results.
  59 */
  60void rpc_update_rtt(struct rpc_rtt *rt, unsigned int timer, long m)
  61{
  62        long *srtt, *sdrtt;
  63
  64        if (timer-- == 0)
  65                return;
  66
  67        /* jiffies wrapped; ignore this one */
  68        if (m < 0)
  69                return;
  70
  71        if (m == 0)
  72                m = 1L;
  73
  74        srtt = (long *)&rt->srtt[timer];
  75        m -= *srtt >> 3;
  76        *srtt += m;
  77
  78        if (m < 0)
  79                m = -m;
  80
  81        sdrtt = (long *)&rt->sdrtt[timer];
  82        m -= *sdrtt >> 2;
  83        *sdrtt += m;
  84
  85        /* Set lower bound on the variance */
  86        if (*sdrtt < RPC_RTO_MIN)
  87                *sdrtt = RPC_RTO_MIN;
  88}
  89EXPORT_SYMBOL_GPL(rpc_update_rtt);
  90
  91/**
  92 * rpc_calc_rto - Provide an estimated timeout value
  93 * @rt: context to use for calculation
  94 * @timer: timer array index (request type)
  95 *
  96 * Estimate RTO for an NFS RPC sent via an unreliable datagram.  Use
  97 * the mean and mean deviation of RTT for the appropriate type of RPC
  98 * for frequently issued RPCs, and a fixed default for the others.
  99 *
 100 * The justification for doing "other" this way is that these RPCs
 101 * happen so infrequently that timer estimation would probably be
 102 * stale.  Also, since many of these RPCs are non-idempotent, a
 103 * conservative timeout is desired.
 104 *
 105 * getattr, lookup,
 106 * read, write, commit     - A+4D
 107 * other                   - timeo
 108 */
 109unsigned long rpc_calc_rto(struct rpc_rtt *rt, unsigned int timer)
 110{
 111        unsigned long res;
 112
 113        if (timer-- == 0)
 114                return rt->timeo;
 115
 116        res = ((rt->srtt[timer] + 7) >> 3) + rt->sdrtt[timer];
 117        if (res > RPC_RTO_MAX)
 118                res = RPC_RTO_MAX;
 119
 120        return res;
 121}
 122EXPORT_SYMBOL_GPL(rpc_calc_rto);
 123
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