Merge branch 'metrics-documentation' into 'master'

Added documentation for GitLab Metrics

This adds documentation for the metrics code I recently added.

Two notes:

* I'm referring to this feature as "GitLab Metrics" everywhere simply
  because the code resides in the `Gitlab::Metrics` namespace. A
  different name is fine, though I think just "Metrics" might be a bit
  too abstract.
* Grafana isn't covered since it's fairly straight-forward to setup
  (there's nothing really special we can cover). I think we _should_
  cover it once we start shipping it with Omnibus.

I assigned @axil since he's our technical writer/documentation guru,
feel free to re-assign if needed.

cc @sytses @axil @dzaporozhets @JobV 

See merge request !2484

doc/README.md

@@ -67,6 +67,7 @@
 - [Reply by email](incoming_email/README.md) Allow users to comment on issues and merge requests by replying to notification emails.
 - [Migrate GitLab CI to CE/EE](migrate_ci_to_ce/README.md) Follow this guide to migrate your existing GitLab CI data to GitLab CE/EE.
 - [Git LFS configuration](workflow/lfs/lfs_administration.md)
+- [GitLab Performance Monitoring](monitoring/performance/introduction.md) Configure GitLab and InfluxDB for measuring performance metrics
 ## Contributor documentation

doc/integration/README.md

@@ -15,6 +15,7 @@ See the documentation below for details on how to configure these services.
 - [OAuth2 provider](oauth_provider.md) OAuth2 application creation
 - [Gmail actions buttons](gmail_action_buttons_for_gitlab.md) Adds GitLab actions to messages
 - [reCAPTCHA](recaptcha.md) Configure GitLab to use Google reCAPTCHA for new users
+- [GitLab Performance Monitoring](metrics/introduction.md) Configure GitLab and InfluxDB for measuring performance metrics
 GitLab Enterprise Edition contains [advanced Jenkins support][jenkins].

doc/monitoring/performance/gitlab_configuration.md

+# GitLab Configuration
+
+GitLab Performance Monitoring is disabled by default. To enable it and change any of its
+settings, navigate to the Admin area in **Settings > Metrics**
+(`/admin/application_settings`).
+
+The minimum required settings you need to set are the InfluxDB host and port.
+Make sure _Enable InfluxDB Metrics_ is checked and hit **Save** to save the
+changes.
+
+---
+
+![GitLab Performance Monitoring Admin Settings](img/metrics_gitlab_configuration_settings.png)
+
+---
+
+Finally, a restart of all GitLab processes is required for the changes to take
+effect:
+
+```bash
+# For Omnibus installations
+sudo gitlab-ctl restart
+
+# For installations from source
+sudo service gitlab restart
+```
+
+## Pending Migrations
+
+When any migrations are pending, the metrics are disabled until the migrations
+have been performed.
+
+---
+
+Read more on:
+
+- [Introduction to GitLab Performance Monitoring](introduction.md)
+- [InfluxDB Configuration](influxdb_configuration.md)
+- [InfluxDB Schema](influxdb_schema.md)

doc/monitoring/performance/influxdb_configuration.md

+# InfluxDB Configuration
+
+The default settings provided by [InfluxDB] are not sufficient for a high traffic
+GitLab environment. The settings discussed in this document are based on the
+settings GitLab uses for GitLab.com, depending on your own needs you may need to
+further adjust them.
+
+If you are intending to run InfluxDB on the same server as GitLab, make sure
+you have plenty of RAM since InfluxDB can use quite a bit depending on traffic.
+
+Unless you are going with a budget setup, it's advised to run it separately.
+
+## Requirements
+
+- InfluxDB 0.9.5 or newer
+- A fairly modern version of Linux
+- At least 4GB of RAM
+- At least 10GB of storage for InfluxDB data
+
+Note that the RAM and storage requirements can differ greatly depending on the
+amount of data received/stored. To limit the amount of stored data users can
+look into [InfluxDB Retention Policies][influxdb-retention].
+
+## Installation
+
+Installing InfluxDB is out of the scope of this document. Please refer to the
+[InfluxDB documentation].
+
+## InfluxDB Server Settings
+
+Since InfluxDB has many settings that users may wish to customize themselves
+(e.g. what port to run InfluxDB on), we'll only cover the essentials.
+
+The configuration file in question is usually located at
+`/etc/influxdb/influxdb.conf`. Whenever you make a change in this file,
+InfluxDB needs to be restarted.
+
+### Storage Engine
+
+InfluxDB comes with different storage engines and as of InfluxDB 0.9.5 a new
+storage engine is available, called [TSM Tree]. All users **must** use the new
+`tsm1` storage engine as this [will be the default engine][tsm1-commit] in
+upcoming InfluxDB releases.
+
+Make sure you have the following in your configuration file:
+
+```
+[data]
+  dir = "/var/lib/influxdb/data"
+  engine = "tsm1"
+```
+
+### Admin Panel
+
+Production environments should have the InfluxDB admin panel **disabled**. This
+feature can be disabled by adding the following to your InfluxDB configuration
+file:
+
+```
+[admin]
+  enabled = false
+```
+
+### HTTP
+
+HTTP is required when using the [InfluxDB CLI] or other tools such as Grafana,
+thus it should be enabled. When enabling make sure to _also_ enable
+authentication:
+
+```
+[http]
+  enabled = true
+  auth-enabled = true
+```
+
+_**Note:** Before you enable authentication, you might want to [create an
+admin user](#create-a-new-admin-user)._
+
+### UDP
+
+GitLab writes data to InfluxDB via UDP and thus this must be enabled. Enabling
+UDP can be done using the following settings:
+
+```
+[[udp]]
+  enabled = true
+  bind-address = ":8089"
+  database = "gitlab"
+  batch-size = 1000
+  batch-pending = 5
+  batch-timeout = "1s"
+  read-buffer = 209715200
+```
+
+This does the following:
+
+1. Enable UDP and bind it to port 8089 for all addresses.
+2. Store any data received in the "gitlab" database.
+3. Define a batch of points to be 1000 points in size and allow a maximum of
+   5 batches _or_ flush them automatically after 1 second.
+4. Define a UDP read buffer size of 200 MB.
+
+One of the most important settings here is the UDP read buffer size as if this
+value is set too low, packets will be dropped. You must also make sure the OS
+buffer size is set to the same value, the default value is almost never enough.
+
+To set the OS buffer size to 200 MB, on Linux you can run the following command:
+
+```bash
+sysctl -w net.core.rmem_max=209715200
+```
+
+To make this permanent, add the following to `/etc/sysctl.conf` and restart the
+server:
+
+```bash
+net.core.rmem_max=209715200
+```
+
+It is **very important** to make sure the buffer sizes are large enough to
+handle all data sent to InfluxDB as otherwise you _will_ lose data. The above
+buffer sizes are based on the traffic for GitLab.com. Depending on the amount of
+traffic, users may be able to use a smaller buffer size, but we highly recommend
+using _at least_ 100 MB.
+
+When enabling UDP, users should take care to not expose the port to the public,
+as doing so will allow anybody to write data into your InfluxDB database (as
+[InfluxDB's UDP protocol][udp] doesn't support authentication). We recommend either
+whitelisting the allowed IP addresses/ranges, or setting up a VLAN and only
+allowing traffic from members of said VLAN.
+
+## Create a new admin user
+
+If you want to [enable authentication](#http), you might want to [create an
+admin user][influx-admin]:
+
+```
+influx -execute "CREATE USER jeff WITH PASSWORD '1234' WITH ALL PRIVILEGES"
+```
+
+## Create the `gitlab` database
+
+Once you get InfluxDB up and running, you need to create a database for GitLab.
+Make sure you have changed the [storage engine](#storage-engine) to `tsm1`
+before creating a database.
+
+_**Note:** If you [created an admin user](#create-a-new-admin-user) and enabled
+[HTTP authentication](#http), remember to append the username (`-username <username>`)
+and password (`-password <password>`)  you set earlier to the commands below._
+
+Run the following command to create a database named `gitlab`:
+
+```bash
+influx -execute 'CREATE DATABASE gitlab'
+```
+
+The name **must** be `gitlab`, do not use any other name.
+
+Next, make sure that the database was successfully created:
+
+```bash
+influx -execute 'SHOW DATABASES'
+```
+
+The output should be similar to:
+
+```
+name: databases
+---------------
+name
+_internal
+gitlab
+```
+
+That's it! Now your GitLab instance should send data to InfluxDB.
+
+---
+
+Read more on:
+
+- [Introduction to GitLab Performance Monitoring](introduction.md)
+- [GitLab Configuration](gitlab_configuration.md)
+- [InfluxDB Schema](influxdb_schema.md)
+
+[influxdb-retention]: https://docs.influxdata.com/influxdb/v0.9/query_language/database_management/#retention-policy-management
+[influxdb documentation]: https://docs.influxdata.com/influxdb/v0.9/
+[influxdb cli]: https://docs.influxdata.com/influxdb/v0.9/tools/shell/
+[udp]: https://docs.influxdata.com/influxdb/v0.9/write_protocols/udp/
+[influxdb]: https://influxdata.com/time-series-platform/influxdb/
+[tsm tree]: https://influxdata.com/blog/new-storage-engine-time-structured-merge-tree/
+[tsm1-commit]: https://github.com/influxdata/influxdb/commit/15d723dc77651bac83e09e2b1c94be480966cb0d
+[influx-admin]: https://docs.influxdata.com/influxdb/v0.9/administration/authentication_and_authorization/#create-a-new-admin-user

doc/monitoring/performance/influxdb_schema.md

+# InfluxDB Schema
+
+The following measurements are currently stored in InfluxDB:
+
+- `PROCESS_file_descriptors`
+- `PROCESS_gc_statistics`
+- `PROCESS_memory_usage`
+- `PROCESS_method_calls`
+- `PROCESS_object_counts`
+- `PROCESS_transactions`
+- `PROCESS_views`
+
+Here, `PROCESS` is replaced with either `rails` or `sidekiq` depending on the
+process type. In all series, any form of duration is stored in milliseconds.
+
+## PROCESS_file_descriptors
+
+This measurement contains the number of open file descriptors over time. The
+value field `value` contains the number of descriptors.
+
+## PROCESS_gc_statistics
+
+This measurement contains Ruby garbage collection statistics such as the amount
+of minor/major GC runs (relative to the last sampling interval), the time spent
+in garbage collection cycles, and all fields/values returned by `GC.stat`.
+
+## PROCESS_memory_usage
+
+This measurement contains the process' memory usage (in bytes) over time. The
+value field `value` contains the number of bytes.
+
+## PROCESS_method_calls
+
+This measurement contains the methods called during a transaction along with
+their duration, and a name of the transaction action that invoked the method (if
+available). The method call duration is stored in the value field `duration`,
+while the method name is stored in the tag `method`. The tag `action` contains
+the full name of the transaction action. Both the `method` and `action` fields
+are in the following format:
+
+```
+ClassName#method_name
+```
+
+For example, a method called by the `show` method in the `UsersController` class
+would have `action` set to `UsersController#show`.
+
+## PROCESS_object_counts
+
+This measurement is used to store retained Ruby objects (per class) and the
+amount of retained objects. The number of objects is stored in the `count` value
+field while the class name is stored in the `type` tag.
+
+## PROCESS_transactions
+
+This measurement is used to store basic transaction details such as the time it
+took to complete a transaction, how much time was spent in SQL queries, etc. The
+following value fields are available:
+
+| Value | Description |
+| ----- | ----------- |
+| `duration`  | The total duration of the transaction |
+| `allocated_memory` | The amount of bytes allocated while the transaction was running. This value is only reliable when using single-threaded application servers |
+| `method_duration` | The total time spent in method calls |
+| `sql_duration` | The total time spent in SQL queries |
+| `view_duration` | The total time spent in views |
+
+## PROCESS_views
+
+This measurement is used to store view rendering timings for a transaction. The
+following value fields are available:
+
+| Value | Description |
+| ----- | ----------- |
+| `duration` | The rendering time of the view |
+| `view` | The path of the view, relative to the application's root directory |
+
+The `action` tag contains the action name of the transaction that rendered the
+view.
+
+---
+
+Read more on:
+
+- [Introduction to GitLab Performance Monitoring](introduction.md)
+- [GitLab Configuration](gitlab_configuration.md)
+- [InfluxDB Configuration](influxdb_configuration.md)

doc/monitoring/performance/introduction.md

+# GitLab Performance Monitoring
+
+GitLab comes with its own application performance measuring system as of GitLab
+8.4, simply called "GitLab Performance Monitoring". GitLab Performance Monitoring is available in both the
+Community and Enterprise editions.
+
+Apart from this introduction, you are advised to read through the following
+documents in order to understand and properly configure GitLab Performance Monitoring:
+
+- [GitLab Configuration](gitlab_configuration.md)
+- [InfluxDB Configuration](influxdb_configuration.md)
+- [InfluxDB Schema](influxdb_schema.md)
+
+## Introduction to GitLab Performance Monitoring
+
+GitLab Performance Monitoring makes it possible to measure a wide variety of statistics
+including (but not limited to):
+
+- The time it took to complete a transaction (a web request or Sidekiq job).
+- The time spent in running SQL queries and rendering HAML views.
+- The time spent executing (instrumented) Ruby methods.
+- Ruby object allocations, and retained objects in particular.
+- System statistics such as the process' memory usage and open file descriptors.
+- Ruby garbage collection statistics.
+
+Metrics data is written to [InfluxDB][influxdb] over [UDP][influxdb-udp]. Stored
+data can be visualized using [Grafana][grafana] or any other application that
+supports reading data from InfluxDB. Alternatively data can be queried using the
+InfluxDB CLI.
+
+## Metric Types
+
+Two types of metrics are collected:
+
+1. Transaction specific metrics.
+1. Sampled metrics, collected at a certain interval in a separate thread.
+
+### Transaction Metrics
+
+Transaction metrics are metrics that can be associated with a single
+transaction. This includes statistics such as the transaction duration, timings
+of any executed SQL queries, time spent rendering HAML views, etc. These metrics
+are collected for every Rack request and Sidekiq job processed.
+
+### Sampled Metrics
+
+Sampled metrics are metrics that can't be associated with a single transaction.
+Examples include garbage collection statistics and retained Ruby objects. These
+metrics are collected at a regular interval. This interval is made up out of two
+parts:
+
+1. A user defined interval.
+1. A randomly generated offset added on top of the interval, the same offset
+   can't be used twice in a row.
+
+The actual interval can be anywhere between a half of the defined interval and a
+half above the interval. For example, for a user defined interval of 15 seconds
+the actual interval can be anywhere between 7.5 and 22.5. The interval is
+re-generated for every sampling run instead of being generated once and re-used
+for the duration of the process' lifetime.
+
+[influxdb]: https://influxdata.com/time-series-platform/influxdb/
+[influxdb-udp]: https://docs.influxdata.com/influxdb/v0.9/write_protocols/udp/
+[grafana]: http://grafana.org/