PMU – Equivalence
Requirement
1.0 | Use Linux perf interface to collect data about performance events on a per core basis | |
2.0 | Use jevents library (PMU tools) | |
3.0 | Should have a configurable interval | |
4.0 | Should have configurable hardware specific events list | |
5.0 | Provide SNMP support for any collectd values, through an PMU MIB | |
6.0 | Provide support for multi PMU uncore events | |
| 7.0 | Provide option to choose all the events from json event list file |
Overview
Performance counters are CPU hardware registers that count hardware events such as instructions executed, cache-misses suffered, or branches mispredicted. They form a basis for profiling applications to trace dynamic control flow and identify hotspots. Linux perf interface provides rich generalized abstractions over hardware specific capabilities.
PMU Tools
PMU tools is a collection of tools for profiling and performance analysis on Intel CPUs on top of Linux perf. This uses performance counters in the CPU. These tools are developed and maintained on https://github.com/andikleen/pmu-tools. In addition to a number of tools for profiling and performance analysis this package provides jevents library.
jevents library
jevents is a C library to use from C programs to make access to the kernel Linux perf interface easier. It also includes some examples to use the library. This library provides the following features:
- Resolving symbolic event names using downloaded event files
- Reading performance counters from ring 3 in C programs,
- Handling the perf ring buffer (for example to read memory addresses)
For more information on jevents see https://github.com/andikleen/pmu-tools/tree/master/jevents.
Design
intel_pmu plugin
The intel_pmu plugin collects information provided by Linux perf interface. It is not done directly, but through jevents API. Using this interface, the intel_pmu plugin collects the hardware specific metrics defined in event list file which should contain definitions of PMU events. The list of events to monitor is configurable.
Plugin configuration
The following configuration options should be supported by intel_pmu collectd plugin:
Name | Description | Comment |
Interval | The interval within which to retrieve statistics on monitored events in seconds | Interval option is supported by collectd and is defined in <LoadPlugin> block. No additional functionality should be developed in intel_pmu plugin to support this option. |
| EventList | Path to hardware events list file for current CPU. | File can be downloaded by event_download.py script which is part of pmu-tools package. |
HardwareEvents | String containing comma separated list of hardware specific events to monitor | "All" can be used to set all events from Event List. |
| Cores | Core groups definition. Monitored metrics are reported only for configured cores. If this option is omitted all available cores are monitored. If a group is enclosed in square brackets each core is added individually to a separate group (that is statistics are not aggregated). | Allowed formats: |
| DispatchMultiPmu | Enable/disable dispatching of cloned multi PMU for uncore events. If disabled only total sum is dispatched as single event. If enabled separate metric is dispatched for every counter. | Uncore event example: UNC_CHA_DIR_LOOKUP.NO_SNP. If enabled information about event type is added to type_instance, e.g.: "UNC_CHA_DIR_LOOKUP.NO_SNP:type=30". It allows to distinguish between multiple counters for one event. |
Here is an example of the plugin configuration section of collectd.conf file:
<Plugin intel_pmu>
EventList "/var/cache/pmu/GenuineIntel-6-55-core.json"
HWSpecificEvents "L2_RQSTS.CODE_RD_HIT,L2_RQSTS.CODE_RD_MISS" "L2_RQSTS.ALL_CODE_RD"
Cores ""
DispatchMultiPmu false
</Plugin>
Another example with only uncore events set:
<Plugin intel_pmu>
EventList "/var/cache/pmu/GenuineIntel-6-55-uncore.json"
HWSpecificEvents "UNC_CHA_TOR_INSERTS.IA_MISS:config1=0x4043200000000" "UNC_IIO_TXN_REQ_BY_CPU.MEM_WRITE.PART0"
Cores "0" "18"
DispatchMultiPmu false
</Plugin>
Implementation details
intel_pmu plugin does not introduce its own layer of functionality. It just reads configuration provided by user and prepares all needed parameters/data structures for jevents API. This table shows the correspondence between plugin’s API and jevents API that is used to configure Linux perf monitoring.
plugin API | jevents API | Description |
pmu_config | Parse events groups to monitor provided by user in collectd.conf | |
pmu_init | alloc_eventlist | Allocate memory for new eventlist |
resolve_event_extra | Resolve hardware specific events names to perf events (perf_event_attr) | |
| jevent_pmu_uncore | Check if event is uncore event | |
| jevent_next_pmu | Expand event into multiple PMU if neccessary (in use for uncore events) | |
setup_event | Setup perf events for monitoring | |
pmu_read | read_all_events | Read values of all monitored events |
pmu_shutdown | free_eventlist | Free memory allocated for eventlist (recursively including all events) |
For more details on plugin API see collectd plugin implementation guide https://collectd.org/wiki/index.php/Plugin_architecture.
Hardware Specific Events
The intel_pmu plugin allows to monitor hardware specific events. To support this functionality plugin will use feature provided by jevents library – resolving symbolic event names using downloaded event files. To be able to use hardware specific event names in configuration file user will have to download events list file for current CPU before using intel_pmu plugin. This can be done using event_download.py script which is part of pmu-tools package.
Note: For uncore events values can be collected only for first core of every socket e.g. '0' '18' etc.
SNMP Support
All metrics collected by intel_pmu plugin should be available through SNMP. This will be achieved by creating proper configuration for snmp_agent collectd plugin. No additional functionality needed in intel_pmu plugin to support SNMP. See description of SNMP feature for more details on snmp_agent plugin.
Considerations
Configuration Considerations
.
Deployment Considerations
By leveraging the core configuration for the PMU plugin, it’s necessary to taskset and isolate cores for specific applications that you would like to monitor until the process support is implemented.
API/GUI/CLI Considerations
Equivalence Considerations
The SNMP MIB used for this plugin is a newly Defined MIB.
Security Considerations
Alarms, events, statistics considerations
Certain platform generations will not support all the metrics intended to be collected by the plugin. Unsupported metrics will not be reported.
Redundancy Considerations
Performance Considerations
Not part of Telemetry so performance is Not Applicable
Testing Consideration
The timing interval requirement needs to be taken into consideration when conducting tests.
The Tests should be carried out on a system under load as well as a relatively idle system.
Other Considerations
Impact
The following table outlines possible impact(s) the deployment of this deliverable may have on the current system.
Ref | System Impact Description | Recommendation / Comments |
1 | Plugin can easily exceed the default limit of allowed file descriptors. |
|
Key Assumptions
The following assumptions apply to the scope specified in this document.
Ref | Assumption | Status |
1 |
Key Exclusions
The following exclusions apply to the scope discussed in this document.
Ref | Exclusion | Status |
1 |
Key Dependencies
The following table outlines the key dependencies associated with this deliverable.
Ref | Dependency | Status |
1 | libjevents | |
2 | ||
3 | ||
4 |