iaevents

iaevents

iaevents is a golang library that makes accessing the Linux kernel's perf interface easier. It provides tools to load, parse and read Intel CPU events specified in JSON format available from https://github.com/intel/perfmon. It is based on Andi Kleen's pmu-tools/jevents library that provides such functionality in C language .

Library exports main features as implementation of interfaces:

• Reader - provide events definition from JSON file(s)
• Matcher - match selected events for Transformer
• Transformer - transform event from event definition to proper perf event attributes
• Activator - set up event and start measuring
• MultiActivator - set up multi PMU event (for uncore events) and start measuring on all units
• ActiveEventMonitor - read current value of perf event
• EventGroupMonitor - read current values for perf events aggregated in one group
• PlacementFactory - provide valid placement provider for selected unit and CPUs
• PlacementProvider - provide placement of particular event

Library contains default implementation for all the listed interfaces. It handles both core and uncore type events. To transform the event from JSON definition to corresponding perf attributes the library read the perf specific data from sysfs files. The measurements are obtained with the use of perf_event_open syscall.

System requirements

The iaevents library is intended to be used only on linux 64-bit systems. To make system wide measurements for all processes/threads with "PID = -1" the process using the library requires CAP_SYS_ADMIN capability or /proc/sys/kernel/perf_event_paranoid has to be set to value less than 1.

Note: Library access all the required files in read mode only. According to principle of least privilege the user should restrict the files to read-only mode.

Note on number of open file descriptors

The library opens file descriptors whose quantity depends on number of monitored CPUs and number of monitored counters. It can easily exceed the default per process limit of allowed file descriptors. Depending on configuration, it might be required to increase the limit on the number of open file descriptors allowed. This can be done for example by using ulimit -n command.

Gather JSON files

User has to acquire json events file for specific system architecture. It can be done automatically by using the pmu-tools event_download.py script, or files can be obtained directly from this link: https://github.com/intel/perfmon

The library supports two JSON file formats: https://github.com/intel/perfmon/issues/22

Create events reader

The library requires to provide event definitions for Transfomer. It is fulfilled by Reader interface. To instantiate the default Reader implementation create new JSONFilesReader with NewFilesReader method.

reader := iaevents.NewFilesReader()

Next run AddFiles method with a path(s) to desired JSON file(s). If at least one file is in the old format, the method will add all paths, but will return DeprecatedFormatError error

err := reader.AddFiles(pathToFile1, pathToFile2)

It's possbile to get headers info from perfmon events (only for the new JSON perfmon format). GetHeaders() will return map[string]JSONHeader object, where the key of the map is path to the file

events, err := iaevents.Read()
if err != nil {
  return
}
headers := iavents.GetHeaders()
for path, header := ranger headers {
  fmt.Println(path)
  fmt.Println(header)
}

It is possible to look for default JSON file names for local CPU in provided path.

cpuid, err := iaevents.CheckCPUid()
err = iaevents.AddDefaultFiles(reader, pathToDir, cpuid)

Transform selected event(s) into perf attributes

To select the events to transform, user must use Matcher interface. The iaevents provide default matcher that matches the events by names.

matcher := iaevents.NewNameMatcher(EventName1, EventName2, MoreEventNames...)

To match all events provided by Reader, create new matcher without arguments.

matcher := iaevents.NewNameMatcher()

To resolve perf attributes of selected events call Transform method from PerfTransfomer. It returns slice of events with resolved perf attributes.

transformer := iaevents.NewPerfTransformer()
perfEvents, err := transformer.Transform(reader, matcher)

Activate events

To start measuring on PMU counters perf events need to be activated. During activation library makes call to perf_event_open syscall and obtain specific file descriptors for given event and cpu(s). The required function to activate the event is different for core and uncore events. To check if event belongs to uncore category user can check Uncore flag for perf event.

if perfEvent.Uncore {
    // Handle uncore event
}

Target process and extra perf options

TargetProcess can be used to monitor event for selected process or cgroup. To enable system wide monitoring of event use -1 as PID.

process := iaevents.NewEventTargetProcess(-1, 0)

Options gives user the possibility to modify the event with custom modifiers and perf attributes. The iaevents library provides builder to simplify creation of Options object.

builder := iaevents.NewOptions()

Use SetEnableOnExec to set perf event attribute bits with PerfBitDisabled and PerfBitEnableOnExec.

builder.SetEnableOnExec(true)

Use SetAttrModifiers to set list of custom perf event attribute modifiers. More info can be found on Perf Wiki.

builder.SetAttrModifiers(modifiers)

Build finds the correct setting for perf attributes and returns options to use with Activate methods.

options, err := builder.Build()

More about perf event attributes on linux MAN page.

Activate core event

To activate core event user need to provide on which CPU cores the event should be measured. For example to create placement providers for cores 0, 2 and 4 use NewCorePlacements function.

placements, err := iaevents.NewCorePlacements(perfEvent, 0, 2, 4)

The Activate method starts the underlying PMU counter and returns ActiveEvent for single placement.

activeEvent, err := perfEvent.Activate(placement, process, options)

Activate uncore event

Single uncore event can use several counters for every socket. The example are SBO events.

/sys/devices/uncore_sbox_0
/sys/devices/uncore_sbox_1
/sys/devices/uncore_sbox_2
/sys/devices/uncore_sbox_3

To activate uncore event a special ActivateMulti method should be used. To get all placements for event on given socket use NewUncoreAllPlacements function. The below example shows how to start uncore event on socket 1.

placements, err := iaevents.NewUncoreAllPlacements(perfEvent, 1)
activeMultiEvent, err := perfEvent.ActivateMulti(placements, process, options)

Events grouping

An event group has one event which is the group leader. The leader is created first, and the rest of the group members are created with subsequent perf_event_open() calls with group_fd being set to the file descriptor of the group leader. An event group is scheduled onto the CPU as a unit: it will be put onto the CPU only if all of the events in the group can be put onto the CPU. This means that the values of the member events can be meaningfully compared (added, divided (to get ratios), and so on) with each other, since they have counted events for the same set of executed instructions. To activate set of events in single group user needs to use ActivateGroup function on selected perf events.

activeEventGroup, err := iaevents.ActivateGroup(placement, process, perfEvents)

Monitor events values

While configured events stay active, user can read measurements from them. Depending on type of event, different monitors are returned by activators. It can be ActiveEvent for core event, ActiveMultiEvent for uncore event or ActiveEventGroup for group of events.

Read core event value

value, err := activeEvent.ReadValue()

Read uncore event value

For uncore event the ReadValues method returns slice of values for all placements on given multi PMU.

values, err := activeMultiEvent.ReadValues()

Read events group

For group of events user has to iterate through every event in the group and read the value separately.

for _, activeEvent := range activeEventGroup.Events() {
    value, err := activeEvent.ReadValue()
}

Counter value

Obtained measurements are stored in CounterValue structure as three values:

• Raw
• Enabled
• Running

Raw is a total count of event. Enabled and running are total time the event was enabled and running. Normally these are the same. If more events are started than available counter slots on the PMU, then multiplexing occurs and events only run part of the time. In that case the enabled and running values can be used to scale an estimated value for the counter.

For that case there is also an EventScaledValue method which calculates scaled value as: scaled = raw * enabled / running and returns the result as big.Int.

To aggregate slice of values the library provides function AggregateValues, which helps to aggregate the results across PMU units or multiple cores.

Deactivate events

To finish work with library all active events should be deactivated to close corresponding file descriptors. All types of active events have method Deactivate. After deactivation PMU counters are stopped and user can not read new values.

err := activeEvent.Deactivate()
activeMultiEvent.Deactivate()
activeEventGroup.Deactivate()

Example application

Along with the library there is provided example application iastat to show usage of iaevents package. It has features to list the events read from file and gather measurements with configured options like interval or aggregation.

Build application:

make example

More info:

./iastat -help

Usage:

./iastat [-file <file>] -listevents
./iastat [-file <file>] -events <events> -cpu <CPUs> [-interval <interval>] [-loop <N>] [-no-aggr] [-raw] [-merge]

Licensing

iaevents is distributed under the Apache License, Version 2.0.

Changelog