net/base/backoff_entry.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "net/base/backoff_entry.h"
   6
   7 #include <algorithm>
   8 #include <cmath>
   9 #include <limits>
  10
  11 #include "base/basictypes.h"
  12 #include "base/logging.h"
  13 #include "base/numerics/safe_math.h"
  14 #include "base/rand_util.h"
  15
  16 namespace net {
  17
  18 BackoffEntry::BackoffEntry(const BackoffEntry::Policy* const policy)
  19     : policy_(policy) {
  20   DCHECK(policy_);
  21   Reset();
  22 }
  23
  24 BackoffEntry::~BackoffEntry() {
  25   // TODO(joi): Remove this once our clients (e.g. URLRequestThrottlerManager)
  26   // always destroy from the I/O thread.
  27   DetachFromThread();
  28 }
  29
  30 void BackoffEntry::InformOfRequest(bool succeeded) {
  31   if (!succeeded) {
  32     ++failure_count_;
  33     exponential_backoff_release_time_ = CalculateReleaseTime();
  34   } else {
  35     // We slowly decay the number of times delayed instead of
  36     // resetting it to 0 in order to stay stable if we receive
  37     // successes interleaved between lots of failures.  Note that in
  38     // the normal case, the calculated release time (in the next
  39     // statement) will be in the past once the method returns.
  40     if (failure_count_ > 0)
  41       --failure_count_;
  42
  43     // The reason why we are not just cutting the release time to
  44     // ImplGetTimeNow() is on the one hand, it would unset a release
  45     // time set by SetCustomReleaseTime and on the other we would like
  46     // to push every request up to our "horizon" when dealing with
  47     // multiple in-flight requests. Ex: If we send three requests and
  48     // we receive 2 failures and 1 success. The success that follows
  49     // those failures will not reset the release time, further
  50     // requests will then need to wait the delay caused by the 2
  51     // failures.
  52     base::TimeDelta delay;
  53     if (policy_->always_use_initial_delay)
  54       delay = base::TimeDelta::FromMilliseconds(policy_->initial_delay_ms);
  55     exponential_backoff_release_time_ = std::max(
  56         ImplGetTimeNow() + delay, exponential_backoff_release_time_);
  57   }
  58 }
  59
  60 bool BackoffEntry::ShouldRejectRequest() const {
  61   return exponential_backoff_release_time_ > ImplGetTimeNow();
  62 }
  63
  64 base::TimeDelta BackoffEntry::GetTimeUntilRelease() const {
  65   base::TimeTicks now = ImplGetTimeNow();
  66   if (exponential_backoff_release_time_ <= now)
  67     return base::TimeDelta();
  68   return exponential_backoff_release_time_ - now;
  69 }
  70
  71 base::TimeTicks BackoffEntry::GetReleaseTime() const {
  72   return exponential_backoff_release_time_;
  73 }
  74
  75 void BackoffEntry::SetCustomReleaseTime(const base::TimeTicks& release_time) {
  76   exponential_backoff_release_time_ = release_time;
  77 }
  78
  79 bool BackoffEntry::CanDiscard() const {
  80   if (policy_->entry_lifetime_ms == -1)
  81     return false;
  82
  83   base::TimeTicks now = ImplGetTimeNow();
  84
  85   int64 unused_since_ms =
  86       (now - exponential_backoff_release_time_).InMilliseconds();
  87
  88   // Release time is further than now, we are managing it.
  89   if (unused_since_ms < 0)
  90     return false;
  91
  92   if (failure_count_ > 0) {
  93     // Need to keep track of failures until maximum back-off period
  94     // has passed (since further failures can add to back-off).
  95     return unused_since_ms >= std::max(policy_->maximum_backoff_ms,
  96                                        policy_->entry_lifetime_ms);
  97   }
  98
  99   // Otherwise, consider the entry is outdated if it hasn't been used for the
 100   // specified lifetime period.
 101   return unused_since_ms >= policy_->entry_lifetime_ms;
 102 }
 103
 104 void BackoffEntry::Reset() {
 105   failure_count_ = 0;
 106
 107   // We leave exponential_backoff_release_time_ unset, meaning 0. We could
 108   // initialize to ImplGetTimeNow() but because it's a virtual method it's
 109   // not safe to call in the constructor (and the constructor calls Reset()).
 110   // The effects are the same, i.e. ShouldRejectRequest() will return false
 111   // right after Reset().
 112   exponential_backoff_release_time_ = base::TimeTicks();
 113 }
 114
 115 base::TimeTicks BackoffEntry::ImplGetTimeNow() const {
 116   return base::TimeTicks::Now();
 117 }
 118
 119 base::TimeTicks BackoffEntry::CalculateReleaseTime() const {
 120   int effective_failure_count =
 121       std::max(0, failure_count_ - policy_->num_errors_to_ignore);
 122
 123   // If always_use_initial_delay is true, it's equivalent to
 124   // the effective_failure_count always being one greater than when it's false.
 125   if (policy_->always_use_initial_delay)
 126     ++effective_failure_count;
 127
 128   if (effective_failure_count == 0) {
 129     // Never reduce previously set release horizon, e.g. due to Retry-After
 130     // header.
 131     return std::max(ImplGetTimeNow(), exponential_backoff_release_time_);
 132   }
 133
 134   // The delay is calculated with this formula:
 135   // delay = initial_backoff * multiply_factor^(
 136   //     effective_failure_count - 1) * Uniform(1 - jitter_factor, 1]
 137   // Note: if the failure count is too high, |delay_ms| will become infinity
 138   // after the exponential calculation, and then NaN after the jitter is
 139   // accounted for. Both cases are handled by using CheckedNumeric<int64> to
 140   // perform the conversion to integers.
 141   double delay_ms = policy_->initial_delay_ms;
 142   delay_ms *= pow(policy_->multiply_factor, effective_failure_count - 1);
 143   delay_ms -= base::RandDouble() * policy_->jitter_factor * delay_ms;
 144
 145   // Do overflow checking in microseconds, the internal unit of TimeTicks.
 146   const int64 kTimeTicksNowUs =
 147       (ImplGetTimeNow() - base::TimeTicks()).InMicroseconds();
 148   base::internal::CheckedNumeric<int64> calculated_release_time_us =
 149       delay_ms + 0.5;
 150   calculated_release_time_us *= base::Time::kMicrosecondsPerMillisecond;
 151   calculated_release_time_us += kTimeTicksNowUs;
 152
 153   base::internal::CheckedNumeric<int64> maximum_release_time_us = kint64max;
 154   if (policy_->maximum_backoff_ms >= 0) {
 155     maximum_release_time_us = policy_->maximum_backoff_ms;
 156     maximum_release_time_us *= base::Time::kMicrosecondsPerMillisecond;
 157     maximum_release_time_us += kTimeTicksNowUs;
 158   }
 159
 160   // Decide between maximum release time and calculated release time, accounting
 161   // for overflow with both.
 162   int64 release_time_us = std::min(
 163       calculated_release_time_us.ValueOrDefault(kint64max),
 164       maximum_release_time_us.ValueOrDefault(kint64max));
 165
 166   // Never reduce previously set release horizon, e.g. due to Retry-After
 167   // header.
 168   return std::max(
 169       base::TimeTicks() + base::TimeDelta::FromMicroseconds(release_time_us),
 170       exponential_backoff_release_time_);
 171 }
 172
 173 }  // namespace net