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 20^th 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 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 100^th fast query) and the slow query threshold to 200 (profile all queries slower than 200 milliseconds), run the mongod instance as follows:

$ 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:

> db.runCommand( { profile: 2, slowms: 200, ratelimit: 100 } );
{ "was" : 0, "slowms" : 100, "ratelimit" : 1, "ok" : 1 }

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

> 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:

> 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):

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.

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