docs/analysis/metrics.md

# Trace-based metrics

_The metrics subsystem is a part of the
[trace processor](/docs/analysis/trace-processor.md) which uses traces to
compute reproducible metrics. It can be used in a wide range of situations;
examples include benchmarks, lab tests and on large corpuses of traces._

![Block diagram of metrics](/docs/images/metrics-summary.png)

## Quickstart

The [quickstart](/docs/quickstart/trace-analysis.md) provides a quick overview
on how to compute trace-based metrics traces using trace processor.

## Introduction

### Motivation

Performance metrics are useful to monitor for the health of a system and ensure
that a system does not regress over time as new features are added.

However, metrics retrieved directly from the system have a downside: if there is
a regression, it is difficult to root-cause the issue. Often, the problem may
not be reproducible or may rely on a particular setup.

Trace-based metrics are one possible solution to this problem. Instead of
collecting metrics directly on the system, a trace is collected and metrics are
computed from the trace. If a regression in the metric is spotted, the developer
can look directly at the trace to understand why the regression has occurred
instead of having to reproduce the issue.

### Metric subsystem

The metric subsystem is a part of the
[trace processor](/docs/analysis/trace-processor.md) which executes SQL queries
against traces and produces a metric which summarizes some performance attribute
(e.g. CPU, memory, startup latency etc.).

For example, generating the Android CPU metrics on a trace is as simple as:

```python
> ./trace_processor --run-metrics android_cpu <trace>
android_cpu {
  process_info {
    name: "/system/bin/init"
    threads {
      name: "init"
      core {
        id: 1
        metrics {
          mcycles: 1
          runtime_ns: 570365
          min_freq_khz: 1900800
          max_freq_khz: 1900800
          avg_freq_khz: 1902017
        }
      }
      ...
    }
    ...
  }
  ...
}
```

### Case for upstreaming

Authors are strongly encouraged to add all metrics derived on Perfetto traces to
the Perfetto repo unless there is a clear usecase (e.g. confidentiality) why
these metrics should not be publicly available.

In return for upstreaming metrics, authors will have first class support for
running metrics locally and the confidence that their metrics will remain stable
as trace processor is developed.

As well as scaling upwards while developing from running on a single trace
locally to running on a large set of traces, the reverse is also very useful.
When an anomaly is observed in the metrics of a lab benchmark, a representative
trace can be downloaded and the same metric can be run locally in trace
processor.

Since the same code is running locally and remotely, developers can be confident
in reproducing the issue and use the trace processor and/or the Perfetto UI to
identify the problem.

## Walkthrough: prototyping a metric

TIP: To see how to add to add a new metric to trace processor, see the checklist
[here](/docs/contributing/common-tasks.md#new-metric)

This walkthrough will outline how to prototype a metric locally without needing
to compile trace processor. This metric will compute the CPU time for every
process in the trace and list the names of the top 5 processes (by CPU time) and
the number of threads created by the process.

NOTE: See this [GitHub gist][gist] to see how the code should look at the end of
      the walkthrough. The prerequisites and Step 4 below give instructions on
      how to get trace processor and run the metrics code.

[gist]: https://gist.github.com/tilal6991/c221cf0cae17e298dfa82b118edf9080

### Prerequisites

As a setup step, create a folder to act as a scratch workspace; this folder will be referred to using the env variable `$WORKSPACE` in Step 4.

The other requirement is trace processor. This can downloaded from [here](https://get.perfetto.dev/trace_processor) or can be built from source
using the instructions [here](trace-processor.md). Whichever method is chosen, $TRACE_PROCESSOR env variable will be used to refer to the location of the binary in Step 4.

### Step 1

As all metrics in the metrics platform are defined using protos, the metric needs to be structured as a proto. For this metric, there needs to be some notion of a process name along with its CPU time and number of threads.

Starting off, in a file named `top_five_processes.proto` in our workspace, create a basic proto message called ProcessInfo with those three fields:

```protobuf
message ProcessInfo {
  optional string process_name = 1;
  optional uint64 cpu_time_ms = 2;
  optional uint32 num_threads = 3;
}
```

Next , create a wrapping message which will hold the repeated field containing the top 5 processes.

```protobuf
message TopProcesses {
  repeated ProcessInfo process_info = 1;
}
```

Finally, define an extension to the root proto for all metrics (the [TraceMetrics](https://android.googlesource.com/platform/external/perfetto/+/HEAD/protos/perfetto/metrics/metrics.proto#39) proto).

```protobuf
extend TraceMetrics {
  optional TopProcesses top_processes = 450;
}
```

Adding this extension field allows trace processor to link the newly defined
metric to the `TraceMetrics` proto.

_Notes:_

- The field ids 450-500 are reserved for local development so any of them can be used as the field id for the extension field.
- The choice of field name here is important as the SQL file and the final table generated in SQL will be based on this name.

Putting everything together, along with some boilerplate preamble gives:

```protobuf
syntax = "proto2";

package perfetto.protos;

import "protos/perfetto/metrics/metrics.proto";

message ProcessInfo {
  optional string process_name = 1;
  optional int64 cpu_time_ms = 2;
  optional uint32 num_threads = 3;
}

message TopProcesses {
  repeated ProcessInfo process_info = 1;
}

extend TraceMetrics {
  optional TopProcesses top_processes = 450;
}
```

### Step 2

Next, write the SQL to generate the table of the top 5 processes ordered by the
sum of the CPU time they ran for and the number of threads which were associated
with the process.

The following SQL should added to a file called `top_five_processes.sql` in the
workspace:

```sql
CREATE VIEW top_five_processes_by_cpu
SELECT
  process.name as process_name,
  CAST(SUM(sched.dur) / 1e6 as INT64) as cpu_time_ms,
  COUNT(DISTINCT utid) as num_threads
FROM sched
INNER JOIN thread USING(utid)
INNER JOIN process USING(upid)
GROUP BY process.name
ORDER BY cpu_time_ms DESC
LIMIT 5;
```

Let's break this query down:

1. The first table used is the `sched` table. This contains all the scheduling
   data available in the trace. Each scheduling "slice" is associated with a
   thread which is uniquely identified in Perfetto traces using its `utid`. The
   two pieces of information needed from the sched table are the `dur` -
   short for duration, this is the amount of time the slice lasted - and the
   `utid` which will be used to join with the thread table.
2. The next table is the thread table. This gives us a lot of information which
   is not particularly interesting (including its thread name) but it does give
   us the `upid`. Similar to `utid`, `upid` is the unique identifier for a
   process in a Perfetto trace. In this case, `upid` will refer to the process
   which hosts the thread given by `utid`.
3. The final table is the process table. This gives the name of the process
   associated with the original sched slice.
4. With the process, thread and duration for each sched slice, all the slices
   for a single processes are collected and their durations summed to get the
   CPU time (dividing by 1e6 as sched's duration is in nanoseconds) and
   the number of distinct threads.
5. Finally, we order by the cpu time and limit to the top 5 results.

### Step 3

Now that the result of the metric has been expressed as an SQL table, it needs
to be converted to a proto. The metrics platform has built-in support for emitting
protos using SQL functions; something which is used extensively in this step.

Let's look at how it works for our table above.

```sql
CREATE VIEW top_processes_output AS
SELECT TopProcesses(
  'process_info', (
    SELECT RepeatedField(
      ProcessInfo(
        'process_name', process_name,
        'cpu_time_ms', cpu_time_ms,
        'num_threads', num_threads
      )
    )
    FROM top_five_processes_by_cpu
  )
);
```

Breaking this down again:

1. Starting from the inner-most SELECT statement, there is what looks like
   a function call to the ProcessInfo function; in fact this is no coincidence.
   For each proto that the metrics platform knows about, an SQL function is
   generated with the same name as the proto. This function takes key value
   pairs with the key as the name of the proto field to fill and the value being
   the data to store in the field. The output is the proto created by writing
   the fields described in the function. (\*)

   In this case, this function is called once for each row in the
   `top_five_processes_by_cpu` table. The output will be the fully filled
   ProcessInfo proto.

   The call to the `RepeatedField` function is the most interesting part and
   also the most important. In technical terms, `RepeatedField` is an aggregate
   function. Practically, this means that it takes a full table of values and
   generates a single array which contains all the values passed to it.

   Therefore, the output of this whole SELECT statement is an array of 5
   ProcessInfo protos.

2. Next is creation of the `TopProcesses` proto. By now, the syntax should
   already feel somewhat familiar; the proto builder function is called to fill
   in the `process_info` field with the array of protos from the inner function.

   The output of this SELECT is a single `TopProcesses` proto containing the
   ProcessInfos as a repeated field.

3. Finally, the view is created. This view is specially named to allow the
   metrics platform to query it to obtain the root proto for each metric
   (in this case `TopProcesses`). See the note below as to the pattern behind
   this view's name.

(\*) _This is not strictly true. To type-check the protos, some metadata
is returned about the type of the proto but this is unimportant for metric
authors._

NOTE: It is important that the views be named {name of TraceMetrics extension
      field}\_output. This is the pattern used and expected by the metrics
      platform for all metrics.

The final file should look like so:

```sql
CREATE VIEW top_five_processes_by_cpu AS
SELECT
  process.name as process_name,
  CAST(SUM(sched.dur) / 1e6 as INT64) as cpu_time_ms,
  COUNT(DISTINCT utid) as num_threads
FROM sched
INNER JOIN thread USING(utid)
INNER JOIN process USING(upid)
GROUP BY process.name
ORDER BY cpu_time_ms DESC
LIMIT 5;

CREATE top_processes_output AS
SELECT TopProcesses(
  'process_info', (
    SELECT RepeatedField(
      ProcessInfo(
        'process_name', process_name,
        'cpu_time_ms', cpu_time_ms,
        'num_threads', num_threads
      )
    )
    FROM top_five_processes_by_cpu
  )
);
```

NOTE: The name of the SQL file should be the same as the name of TraceMetrics
      extension field. This is to allow the metrics platform to associated the
      proto extension field with the SQL which needs to be run to generate it.

### Step 4

For this step, invoke trace processor shell to run the metrics (see the
[Quickstart](/docs/quickstart/trace-analysis.md) for downloading instructions):

```shell
$TRACE_PROCESSOR --run-metrics $WORKSPACE/top_five_processes.sql $TRACE 2> /dev/null
```

(For an example trace to test this on, see the Notes section below.)

By passing the SQL file for the metric to be computed, trace processor uses the name of this file to find the proto and to figure out the name of the output table for the proto and the name of the extension field for `TraceMetrics`; this is the reason it was important to choose the names of these other objects carefully.

_Notes:_

- If something doesn't work as intended, check that the workspace looks the same as the contents of this [GitHub gist](https://gist.github.com/tilal6991/c221cf0cae17e298dfa82b118edf9080).
- A good example trace for this metric is the Android example trace used by the Perfetto UI found [here](https://storage.googleapis.com/perfetto-misc/example_android_trace_30s_1).
- stderr is redirected to remove any noise from parsing the trace that trace processor generates.

If everything went successfully, the following output should be visible (specifically this is the output for the Android example trace linked above):

```
[perfetto.protos.top_five_processes] {
  process_info {
    process_name: "com.google.android.GoogleCamera"
    cpu_time_ms: 15154
    num_threads: 125
  }
  process_info {
    process_name: "sugov:4"
    cpu_time_ms: 6846
    num_threads: 1
  }
  process_info {
    process_name: "system_server"
    cpu_time_ms: 6809
    num_threads: 66
  }
  process_info {
    process_name: "cds_ol_rx_threa"
    cpu_time_ms: 6684
    num_threads: 1
  }
  process_info {
    process_name: "com.android.chrome"
    cpu_time_ms: 5125
    num_threads: 49
  }
}
```

### Next steps

* The [common tasks](/docs/contributing/common-tasks.md) page gives a list of
  steps on how new metrics can be added to the trace processor.