# Profiling rate limit

Percona Server for MongoDB can limit the number of queries collected by the database profiler
to decrease its impact on performance.
Rate limit is an integer between 1 and 1000
and represents the fraction of queries to be profiled.
For example, if you set it to 20, then every 20th query will be logged.
For compatibility reasons, rate limit of 0 is the same as setting it to 1,
and will effectively disable the feature
meaning that every query will be profiled.

The MongoDB database profiler can operate in one of three modes:


* `0`: Profiling is disabled. This is the default setting.

* `1`: The profiler collects data only for *slow* queries.
By default, queries that take more than 100 milliseconds to execute
are considered *slow*.

* `2`: Collects profiling data for all database operations.

Mode `1` ignores all *fast* queries,
which may be the cause of problems that you are trying to find.
Mode `2` provides a comprehensive picture of database performance,
but may introduce unnecessary overhead.

With rate limiting you can collect profiling data for all database operations
and reduce overhead by sampling queries.
Slow queries ignore rate limiting and are always collected by the profiler.

## Comparing to the `sampleRate` option

The `sampleRate` option (= [slowOpSampleRate](https://docs.mongodb.com/manual/reference/program/mongod/index.html#cmdoption-mongod-slowopsamplerate) config file option) is a similar
concept to `rateLimit`. But it works at different profile level, completely
ignores operations faster than `slowOpsThresholdMs` (a.k.a. `slowMs`), and affects the
log file printing, too.

|                               | sampleRate  | rateLimit  |
| ----------------------------- | ----------- | ---------- | 
| Affects profiling level 1     | yes         | no         |
| Affects profiling level 2     | no          | yes        |
| Discards/filters slow ops     | yes         | no         |
| Discards/filters fast ops     | no          | yes        |
| Affects log file              | yes         | no         |
| Example value of option       | 0.02        | 50         |

`rateLimit` is a better way to have continuous profiling for monitoring or live
analysis purposes. `sampleRate` requires setting slowOpsThresholdMs to zero if
you want to sample all types of operations. `sampleRate` has an effect on the log file which may either decrease or increase the log volume.

## Enabling the rate limit

To enable rate limiting, set the profiler mode to `2`
and specify the value of the rate limit.
Optionally, you can also change the default threshold for slow queries,
which will not be sampled by rate limiting.

For example, to set the rate limit to `100`
(profile every 100th *fast* query)
and the slow query threshold to `200`
(profile all queries slower than 200 milliseconds),
run the `mongod` instance as follows:

```{.bash data-prompt="$"}
$ mongod --profile 2 --slowms 200 --rateLimit 100
```

To do the same at runtime, use the `profile` command.
It returns the *previous* settings
and `"ok" : 1` indicates that the operation was successful:

```{.javascript data-prompt=">"}
> db.runCommand( { profile: 2, slowms: 200, ratelimit: 100 } );
{ "was" : 0, "slowms" : 100, "ratelimit" : 1, "ok" : 1 }
```

To check the current settings, run `profile: -1`:

```{.javascript data-prompt=">"}
> db.runCommand( { profile: -1 } );
{ "was" : 2, "slowms" : 200, "ratelimit" : 100, "ok" : 1 }
```

If you want to set or get just the rate limit value,
use the `profilingRateLimit` parameter on the `admin` database:

```{.javascript data-prompt=">"}
> db.getSiblingDB('admin').runCommand( { setParameter: 1, "profilingRateLimit": 100 } );
{ "was" : 1, "ok" : 1 }
> db.getSiblingDB('admin').runCommand( { getParameter: 1, "profilingRateLimit": 1 } );
{ "profilingRateLimit" : 100, "ok" : 1 }
```

If you want rate limiting to persist when you restart `mongod`,
set the corresponding variables in the MongoDB configuration file
(by default, `/etc/mongod.conf`):

```yaml
operationProfiling:
  mode: all
  slowOpThresholdMs: 200
  rateLimit: 100
```

!!! note
 
    The value of the `operationProfiling.mode` variable is a string, which you can set to either `off`, `slowOp`, or `all`, corresponding to profiling modes `0`, `1`, and `2`.

## Profiler collection extension

Each document in the `system.profile` collection
includes an additional `rateLimit` field.
This field always has the value of `1` for *slow* queries
and the current rate limit value for *fast* queries.