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Commit fd55798d authored by Darin Petkov's avatar Darin Petkov
Browse files

Metrics: Update README to reflect recent changes.

The updates include:

- Mention the need for "Consent To Send Stats" to exists and the new
AreMetricsEnabled API.

- Replace wiki with XML references (need to add an internal only document
somewhere too).

- Reformat to take full advantage of 80-character lines.

BUG=none
TEST=none

Change-Id: Idbe06e70d5d15f1f8dbbb9475f38a1b0becbca32

Review URL: http://codereview.chromium.org/3573007
parent dd6a8db0
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+70 −76
Original line number Diff line number Diff line
@@ -2,35 +2,32 @@ Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.

The Chrome OS "metrics" package contains utilities for client-side
user metric collection. The collected data is sent to Chrome for
transport to the UMA server.
The Chrome OS "metrics" package contains utilities for client-side user metric
collection. The collected data is sent to Chrome for transport to the UMA
server.


================================================================================
The Metrics Library: libmetrics
================================================================================

libmetrics is a small library that implements the basic C and C++ API
for metrics collection. All metrics collection is funneled through
this library. The easiest and recommended way for a client-side module
to collect user metrics is to link libmetrics and use its APIs to send
metrics to Chrome for transport to UMA. In order to use the library in
a module, you need to do the following:
libmetrics is a small library that implements the basic C and C++ API for
metrics collection. All metrics collection is funneled through this library. The
easiest and recommended way for a client-side module to collect user metrics is
to link libmetrics and use its APIs to send metrics to Chrome for transport to
UMA. In order to use the library in a module, you need to do the following:

- Add a dependence (DEPEND and RDEPEND) on chromeos-base/metrics to
  the module's ebuild.
- Add a dependence (DEPEND and RDEPEND) on chromeos-base/metrics to the module's
  ebuild.

- Link the module with libmetrics (for example, by passing -lmetrics
  to the module's link command). Both libmetrics.so and libmetrics.a
  are built and installed under $SYSROOT/usr/lib/. Note that by
  default -lmetrics will link against libmetrics.so, which is
  preferred.
- Link the module with libmetrics (for example, by passing -lmetrics to the
  module's link command). Both libmetrics.so and libmetrics.a are built and
  installed under $SYSROOT/usr/lib/. Note that by default -lmetrics will link
  against libmetrics.so, which is preferred.

- To access the metrics library API in the module, include the
  <metrics/metrics_library.h> header file. The file is installed in
  $SYSROOT/usr/include/ when the metrics library is built and
  installed.
  $SYSROOT/usr/include/ when the metrics library is built and installed.

- The API includes two methods:

@@ -42,15 +39,18 @@ a module, you need to do the following:
                                     int max)
  sends a sample for an enumeration (linear) histogram.

  Before using these methods, a MetricsLibrary object needs to be
  constructed and initialized through its Init method. See the
  complete API documentation in metrics_library.h under
  src/platform/metrics/.
  Before using these methods, a MetricsLibrary object needs to be constructed
  and initialized through its Init method. See the complete API documentation in
  metrics_library.h under src/platform/metrics/.

  For more information on the C API see c_metrics_library.h.

- On the target platform, shortly after the sample is sent it should
  be visible in Chrome through "about:histograms".
- Samples are sent to Chrome only if the "/home/chronos/Consent To Send Stats"
  file exists (see the AreMetricsEnabled API method). Normally, this file is
  created when the user opts into metrics collection.

- On the target platform, shortly after the sample is sent, it should be visible
  in Chrome through "about:histograms".


================================================================================
@@ -67,55 +67,51 @@ Network.TimeToDrop
Server Side
================================================================================

If the histogram data is visible in about:histograms, it will be sent
by an official Chrome build to UMA, assuming the user has opted into
metrics collection. To make the histogram visible on
"chromedashboard", the histogram wiki needs to be updated (steps 2 and
3 after following the "Details on how to add your own histograms" link
under the Histograms tab).  Include the string "Chrome OS" in the
histogram description so that it's easier to distinguish Chrome OS
specific metrics from general Chrome histograms.
If the histogram data is visible in about:histograms, it will be sent by an
official Chrome build to UMA, assuming the user has opted into metrics
collection. To make the histogram visible on "chromedashboard", the histogram
description XML file needs to be updated (steps 2 and 3 after following the
"Details on how to add your own histograms" link under the Histograms tab).
Include the string "Chrome OS" in the histogram description so that it's easier
to distinguish Chrome OS specific metrics from general Chrome histograms.

The UMA server logs and keeps the collected field data even if the
metric's name is not added to the histogram wiki. However, the
dashboard histogram for that metric will show field data as of the
histogram wiki update date; it will not include data for older
dates. If past data needs to be displayed, manual server-side
intervention is required. In other words, one should assume that field
data collection starts only after the histogram wiki has been updated.
The UMA server logs and keeps the collected field data even if the metric's name
is not added to the histogram XML. However, the dashboard histogram for that
metric will show field data as of the histogram XML update date; it will not
include data for older dates. If past data needs to be displayed, manual
server-side intervention is required. In other words, one should assume that
field data collection starts only after the histogram XML has been updated.


================================================================================
The Metrics Client: metrics_client
================================================================================

metrics_client is a simple shell command-line utility for sending
histogram samples. It's installed under /usr/bin on the target
platform and uses libmetrics to send the data to Chrome. The utility
is useful for generating metrics from shell scripts.
metrics_client is a simple shell command-line utility for sending histogram
samples. It's installed under /usr/bin on the target platform and uses
libmetrics to send the data to Chrome. The utility is useful for generating
metrics from shell scripts.

For usage information and command-line options, run "metrics_client"
on the target platform or look for "Usage:" in metrics_client.cc.
For usage information and command-line options, run "metrics_client" on the
target platform or look for "Usage:" in metrics_client.cc.


================================================================================
The Metrics Daemon: metrics_daemon
================================================================================

metrics_daemon is a daemon that runs in the background on the target
platform and is intended for passive or ongoing metrics collection, or
metrics collection requiring feedback from multiple modules. For
example, it listens to D-Bus signals related to the user session and
screen saver states to determine if the user is actively using the
device or not and generates the corresponding data. The metrics daemon
uses libmetrics to send the data to Chrome.
metrics_daemon is a daemon that runs in the background on the target platform
and is intended for passive or ongoing metrics collection, or metrics collection
requiring feedback from multiple modules. For example, it listens to D-Bus
signals related to the user session and screen saver states to determine if the
user is actively using the device or not and generates the corresponding
data. The metrics daemon uses libmetrics to send the data to Chrome.

The recommended way to generate metrics data from a module is to link
and use libmetrics directly. However, the module could instead send
signals to or communicate in some alternative way with the metrics
daemon. Then the metrics daemon needs to monitor for the relevant
events and take appropriate action -- for example, aggregate data and
send the histogram samples.
The recommended way to generate metrics data from a module is to link and use
libmetrics directly. However, the module could instead send signals to or
communicate in some alternative way with the metrics daemon. Then the metrics
daemon needs to monitor for the relevant events and take appropriate action --
for example, aggregate data and send the histogram samples.


================================================================================
@@ -124,28 +120,26 @@ FAQ

Q. What should my histogram's |min| and |max| values be set at?

A. You should set the values to a range that covers the vast majority
   of samples that would appear in the field. Note that samples below
   the |min| will still be collected in the underflow bucket and
   samples above the |max| will end up in the overflow bucket. Also,
   the reported mean of the data will be correct regardless of the
   range.
A. You should set the values to a range that covers the vast majority of samples
   that would appear in the field. Note that samples below the |min| will still
   be collected in the underflow bucket and samples above the |max| will end up
   in the overflow bucket. Also, the reported mean of the data will be correct
   regardless of the range.

Q. How many buckets should I use in my histogram?

A. You should allocate as many buckets as necessary to perform proper
   analysis on the collected data. Note, however, that the memory
   allocated in Chrome for each histogram is proportional to the
   number of buckets. Therefore, it is strongly recommended to keep
   this number low (e.g., 50 is normal, while 100 is probably high).
A. You should allocate as many buckets as necessary to perform proper analysis
   on the collected data. Note, however, that the memory allocated in Chrome for
   each histogram is proportional to the number of buckets. Therefore, it is
   strongly recommended to keep this number low (e.g., 50 is normal, while 100
   is probably high).

Q. When should I use an enumeration (linear) histogram vs. a regular
   (exponential) histogram?

A. Enumeration histograms should really be used only for sampling
   enumerated events and, in some cases, percentages. Normally, you
   should use a regular histogram with exponential bucket layout that
   provides higher resolution at the low end of the range and lower
   resolution at the high end. Regular histograms are generally used
   for collecting performance data (e.g., timing, memory usage, power)
   as well as aggregated event counts.
A. Enumeration histograms should really be used only for sampling enumerated
   events and, in some cases, percentages. Normally, you should use a regular
   histogram with exponential bucket layout that provides higher resolution at
   the low end of the range and lower resolution at the high end. Regular
   histograms are generally used for collecting performance data (e.g., timing,
   memory usage, power) as well as aggregated event counts.