content/common/inter_process_time_ticks_converter.h

   1 // Copyright (c) 2011 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 #ifndef CONTENT_COMMON_INTER_PROCESS_TIME_TICKS_CONVERTER_H_
   6 #define CONTENT_COMMON_INTER_PROCESS_TIME_TICKS_CONVERTER_H_
   7
   8 #include "base/time/time.h"
   9 #include "content/common/content_export.h"
  10
  11 namespace content {
  12
  13 class LocalTimeDelta;
  14 class LocalTimeTicks;
  15 class RemoteTimeDelta;
  16 class RemoteTimeTicks;
  17
  18 // On Windows, TimeTicks are not consistent between processes. Often, the values
  19 // on one process have a static offset relative to another. Occasionally, these
  20 // offsets shift while running.
  21 //
  22 // To combat this, any TimeTicks values sent from the remote process to the
  23 // local process must be tweaked in order to appear monotonic.
  24 //
  25 // In order to properly tweak ticks, we need 4 reference points:
  26 //
  27 // - |local_lower_bound|:  A known point, recorded on the local process, that
  28 //                         occurs before any remote values that will be
  29 //                         converted.
  30 // - |remote_lower_bound|: The equivalent point on the remote process. This
  31 //                         should be recorded immediately after
  32 //                         |local_lower_bound|.
  33 // - |local_upper_bound|:  A known point, recorded on the local process, that
  34 //                         occurs after any remote values that will be
  35 //                         converted.
  36 // - |remote_upper_bound|: The equivalent point on the remote process. This
  37 //                         should be recorded immediately before
  38 //                         |local_upper_bound|.
  39 //
  40 // Once these bounds are determined, values within the remote process's range
  41 // can be converted to the local process's range. The values are converted as
  42 // follows:
  43 //
  44 // 1. If the remote's range exceeds the local's range, it is scaled to fit.
  45 //    Any values converted will have the same scale factor applied.
  46 //
  47 // 2. The remote's range is shifted so that it is centered within the
  48 //    local's range. Any values converted will be shifted the same amount.
  49 class CONTENT_EXPORT InterProcessTimeTicksConverter {
  50  public:
  51   InterProcessTimeTicksConverter(const LocalTimeTicks& local_lower_bound,
  52                                  const LocalTimeTicks& local_upper_bound,
  53                                  const RemoteTimeTicks& remote_lower_bound,
  54                                  const RemoteTimeTicks& remote_upper_bound);
  55
  56   // Returns the value within the local's bounds that correlates to
  57   // |remote_ms|.
  58   LocalTimeTicks ToLocalTimeTicks(const RemoteTimeTicks& remote_ms) const;
  59
  60   // Returns the equivalent delta after applying remote-to-local scaling to
  61   // |remote_delta|.
  62   LocalTimeDelta ToLocalTimeDelta(const RemoteTimeDelta& remote_delta) const;
  63
  64   // Returns true iff the TimeTicks are converted by adding a constant, without
  65   // scaling. This is the case whenever the remote timespan is smaller than the
  66   // local timespan, which should be the majority of cases due to IPC overhead.
  67   bool IsSkewAdditiveForMetrics() const;
  68
  69   // Returns the (remote time) - (local time) difference estimated by the
  70   // converter. This is the constant that is subtracted from remote TimeTicks to
  71   // get local TimeTicks when no scaling is applied.
  72   base::TimeDelta GetSkewForMetrics() const;
  73
  74  private:
  75   int64 Convert(int64 value) const;
  76
  77   // The local time which |remote_lower_bound_| is mapped to.
  78   int64 local_base_time_;
  79
  80   int64 numerator_;
  81   int64 denominator_;
  82
  83   int64 remote_lower_bound_;
  84   int64 remote_upper_bound_;
  85 };
  86
  87 class CONTENT_EXPORT LocalTimeDelta {
  88  public:
  89   int ToInt32() const { return value_; }
  90
  91  private:
  92   friend class InterProcessTimeTicksConverter;
  93   friend class LocalTimeTicks;
  94
  95   LocalTimeDelta(int value) : value_(value) {}
  96
  97   int value_;
  98 };
  99
 100 class CONTENT_EXPORT LocalTimeTicks {
 101  public:
 102   static LocalTimeTicks FromTimeTicks(const base::TimeTicks& value) {
 103     return LocalTimeTicks(value.ToInternalValue());
 104   }
 105
 106   base::TimeTicks ToTimeTicks() {
 107     return base::TimeTicks::FromInternalValue(value_);
 108   }
 109
 110   LocalTimeTicks operator+(const LocalTimeDelta& delta) {
 111     return LocalTimeTicks(value_ + delta.value_);
 112   }
 113
 114  private:
 115   friend class InterProcessTimeTicksConverter;
 116
 117   LocalTimeTicks(int64 value) : value_(value) {}
 118
 119   int64 value_;
 120 };
 121
 122 class CONTENT_EXPORT RemoteTimeDelta {
 123  public:
 124   static RemoteTimeDelta FromRawDelta(int delta) {
 125     return RemoteTimeDelta(delta);
 126   }
 127
 128  private:
 129   friend class InterProcessTimeTicksConverter;
 130   friend class RemoteTimeTicks;
 131
 132   RemoteTimeDelta(int value) : value_(value) {}
 133
 134   int value_;
 135 };
 136
 137 class CONTENT_EXPORT RemoteTimeTicks {
 138  public:
 139   static RemoteTimeTicks FromTimeTicks(const base::TimeTicks& ticks) {
 140     return RemoteTimeTicks(ticks.ToInternalValue());
 141   }
 142
 143   RemoteTimeDelta operator-(const RemoteTimeTicks& rhs) const {
 144     return RemoteTimeDelta(value_ - rhs.value_);
 145   }
 146
 147  private:
 148   friend class InterProcessTimeTicksConverter;
 149
 150   RemoteTimeTicks(int64 value) : value_(value) {}
 151
 152   int64 value_;
 153 };
 154
 155 }  // namespace content
 156
 157 #endif  // CONTENT_COMMON_INTER_PROCESS_TIME_TICKS_CONVERTER_H_