Profiling Hardware Without Intel Sampling Drivers

This collection of recipes helps you set up driverless Linux* Perf*-based performance profiling with Intel® VTune™ Amplifier, understand benefits and workarounds for possible limitations.

Content expert: Alexey Budankov, Alexey Bayduraev

Intel processors provide performance monitoring unit (PMU) events that can be used to analyze how effectively your code utilizes hardware resources. VTune Amplifier can collect and analyze PMU events for microarchitecture analysis types, such as HPC Performance Characterization, Memory Access, and Microarchitecture Exploration. If required, you can also configure the Hotspots and Threading analysis types to use the PMU event-based sampling instead of the default user-mode sampling; for example, if your analysis requires a smaller sampling interval.

For PMU event-based analysis, the VTune Amplifier uses Intel sampling drivers that require administrative privileges to install them on a target system. If you do not have administrative privileges or your environment does not allow third-party drivers to be injected to the systems, the VTune Amplifier cannot access PMU events via Intel sampling drivers and determine hardware performance bottlenecks. For such cases, the VTune Amplifier has adopted hardware performance monitoring capabilities through a built-in Linux Perf performance monitoring system.

VTune Amplifier enables the hardware event-based sampling analysis in the Perf driverless mode if:

  • Intel sampling drivers cannot be installed (for example, if installed without root privileges).

  • Collection with stacks is selected with a non-zero stack size and the requirements for driverless collection are satisfied.

  • The option to use driverless collection is enabled and the requirements for driverless collection are satisfied.

The latest versions of the VTune Amplifier (2019 and higher) have extended the Linux Perf (driverless) support providing the profiling functionality, a collection overhead, and trace size competitive with the Intel sampling driver-based solution. However, VTune Amplifier capabilities in the driverless mode depend on your Linux OS configuration and might have some limitations described in some recipes below.

Note

  • To enable the Perf driverless collection to match all the hardware profiling functionality provided with Intel drivers, you will need administrative privileges to configure system options as described below.

  • To check which collector type - Perf or Intel sampling driver (SEP) - was used for you analysis, see the Collection and Platform Info section of the Summary window.

Note

You can run the amplxe-self-checker.sh script provided with the VTune Amplifier to validate product capabilities on your analysis system. The script runs a representative set of analysis types on a stable benchmark and informs you on limitations the VTune Amplifier encountered on the system. The recommendations in this diagnostics may help you properly configure your system for the driverless Perf collection or offer to install the Intel sampling driver if the system configuration cannot help. To run the script, enter:

<vtune_install_dir>/bin64/amplxe-self-checker.sh

Enable System Wide or User Process Profiling

Analysis types: all.

Concepts: System-wide analysis collects performance information about all processes running on the system, including system services and so on.

Driverless mode limitations: Additional configuration is required to enable system-wide or user process profiling.

To enable system-wide analysis in the driverless mode:

  1. Configure a VTune Amplifier project and from the WHAT pane select either the Profile System target or the Launch Application target with the Analyze system-wide option enabled.

  2. Check the /proc/sys/kernel/perf_event_paranoid file value with the following command:

    cat /proc/sys/kernel/perf_event_paranoid

    If the value is less than 1, the VTune Amplifier can proceed with the system-wide collection.

  3. If the perf_event_paranoid value is equal to 1 (which limits the collection to user processes only) or more than 1 (which prevents the VTune Amplifier from using the Perf driverless mode), set the perf_event_paranoid value to 0 for the system-wide collection:

    echo 0 > /proc/sys/kernel/perf_event_paranoid

Note

In some environments, perf_event_paranoid is regulated by the security policy. For more information about Linux Perf security requirements, see https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html.

Intel sampling driver limitations: by default, the Intel sampling driver supports system-wide collection. But if it is built and loaded with the --per-user option, the collection will be limited to user processes only.

Enable Core and Uncore Event Collection

Analysis types: Memory Access, HPC Performance Characterization, and other analysis types based on uncore events

Core events can be collected both system wide and per user processes. To collect uncore events in the Perf driverless mode, enable system-wide analysis.

Driverless mode limitations:

  • Memory Access analysis requires access to uncore events and will not run without ability to collect them. Other analysis types, like HPC Performance Characterization, will run but miss metrics based on uncore events such as DRAM Bandwidth, OPA Interconnect Bandwidth, and Packet Rate.

  • Uncore collection in the driverless mode is not supported on Intel Atom® processors.

To collect uncore events in the driverless mode:

Set the perf_event_paranoid value to 0 to enable system-wide performance monitoring, which is a prerequisite for the uncore event collection.

Intel sampling driver limitations: none.

Enable Multi-Process Profiling

Analysis types: all

By default, the Linux kernel limits the size of the memory available for capturing performance data by 518Kb. To know the current value, enter:

cat /proc/sys/kernel/perf_event_mlock_kb

Driverless mode limitations: For some parallel applications (for example, MPI applications with multiple ranks per node) in the user process collection mode on multi-core systems (>64 logical cores), the limit of 518Kb tends to be reached and the data collection will not be available.

To enable multi-process profiling on a multi-core system:

Set the perf_event_paranoid value to 0 to enable system-wide performance monitoring.

Intel sampling driver limitations: none. Any number of processes with default settings can be profiled.

Profile a Large Number of PMU Events on Multi-Core Systems

Analysis types: Microarchitecture Exploration

Driverless mode limitations: Linux Perf allocates file descriptors for every configured PMU event on each CPU. So, on a multi-core system with a long events list used by such analyses as Microarchitecture Exploration, this limit is easily reachable and can prevent the collection in the driverless mode.

To support profiling a large number of PMU events in the driverless mode:

  1. Check the limit of opened files:

    ulimit -n

  2. If required, increase the limit in the /etc/security/limits.conf file by adding or changing the following lines (particular numbers are chosen as examples):

    • soft nofile 65535

    • hard nofile 65535

    Note

    Administrative privileges are required.

Intel sampling driver limitations: none.

Enable Stack Sampling

Analysis types: Hotspots (Hardware Event-Based Sampling mode), Threading (Hardware Event-Based Sampling and Stack Stitching mode), HPC Performance Characterization (Collect stacks option enabled), GPU Compute/Media Hotspots (Collect stacks option enabled).

Driverless mode limitations:

  • Default 1024 byte stack size may not be enough for a full stack unwinding if a function intensively allocates data on the stack. This may lead to [Skipped stack frame(s)] displayed in the collected data.

  • Linux kernel versions older than 3.7 support only frame-pointer (FP) based stack unwinding. This means that the VTune Amplifier can provide no stacks for binaries built without frame-pointer (-fomit-frame-pointer compiler option), as well as no Glibc stacks since Glibc is built without frame-pointers.

To avoid issues with stack unwinding in the driverless mode:

Increase the stack size. For example:

amplxe-cl -collect hotspots -knob sampling-mode=hw -knob enable-stack-collection=true -knob stack-size=2048 <application>

Otherwise, switch to the Intel sampling driver setting the Stack size option to 0 (unlimited value).

Note

Since stack sampling collection with the Intel sampling driver depends on the kernel implementation, it usually requires an update for a new kernel version, which may bring additional product maintenance cost. To reduce the cost, the VTune Amplifier (2019 Update 4 and later) started using the driverless mode for all analysis types with stacks collection enabled, even when the Intel sampling driver is loaded. If you need to switch to the Intel sampling driver for stack sampling collection, create a custom analysis type and disable the Enable driverless collection option, or use the corresponding command line configuration:

amplxe-cl -collect-with runsa -knob enable-driverless-collection=false -knob event-config=<event-list> <application>

Intel sampling driver limitations: No limitation for the stack unwinding since the Intel sampling driver uses a different algorithm of call stack collection. The driver may require an update if your kernel version is newer than the latest kernel version supported by VTune Amplifier..

Collect Context Switches

Analysis types: Threading

Concepts: Context switch collection helps expose metrics based on thread Inactive Wait time resulted from either synchronization or thread preemption.

Driverless mode limitations: Linux Perf collects context switches from kernel version 4.3 and higher. Identification of the context switch reason (synchronization or preemption) is available from kernel version 4.17. For older kernel versions, the VTune Amplifier switches the collection to the Intel sampling driver if it is available on the system.

Intel sampling driver limitations: none.

Resolve Symbols for Kernel Functions

Analysis types: all

Driverless mode limitations: Additional manual configuration of the kptr_restrict file is required.

To associate performance data with kernel function names:

Set the kptr_restrict configuration file value to 0 as a system administrator:

echo 0 > /proc/sys/kernel/kptr_restrict

Setting the value to 1 limits the file name resolution to user-level modules.

Intel sampling driver limitations: none. The Intel driver resolves kernel symbols if the /boot/System.map-<kernel_version> file is accessible for reading or /proc/sys/kernel/kptr_restrict is set to 0.

Avoid Resource Contention with the NMI Watchdog

Analysis types: all

Driverless mode limitations: NMI watchdog (a hard lockup detector) utilizes one CPU performance counter register that becomes unavailable for Linux Perf. This can increase the number of multiplexing groups and, as a result, impact the accuracy of statistical sampling data.

To improve the accuracy of analysis runs with long events lists in the driverless mode:

Disable the NMI watchdog, using administrative privileges:

echo 0 > /proc/sys/kernel/nmi_watchdog

When the driverless Perf collection is complete, you can re-enable the NMI watchdog (using the administrative privileges):

echo 1 > /proc/sys/kernel/nmi_watchdog

Intel sampling driver limitations: none. Intel driver automatically stops the NMI watchdog for the collection time to avoid this problem with data accuracy.

Reduce Collection Overhead

Analysis types: all

Driverless mode limitations: Linux Perf collection may incur an overhead on CPU intensive applications, since it fully loads all CPUs.

To reduce the collection overhead in the driverless mode:

  • Set a more aggressive affinity optimization as follows:

    amplxe-cl -collect hotspots -knob sampling-mode=hw -knob enable-stack-collection=true -run-pass-thru=--perf-affinity=cpu <application>

  • To reduce the trace size for stack sampling collections, the VTune Amplifier uses a Linux Perf trace compression, which may introduce an additional overhead. To avoid this, disable the trace compression with the -run-pass-thru option:

    amplxe-cl -collect hotspots -knob sampling-mode=hw -knob enable-stack-collection=true -run-pass-thru=--perf-compression=0 -run-pass-thru=-- perf-aio=0 <application>

    This can reduce collector overhead in rare cases, but the trace size increases dramatically.

  • Set the limit of CPU time consumption by Linux Perf collector. For example, for a 10% limit, use the following command (with administrative privileges):

    cat 10 > /proc/sys/kernel/perf_cpu_time_max_percent

    This can drop the sampling frequency and statistical accuracy to reach the limit.

Intel sampling driver limitations: none.

Enable Using Driverless Mode When Required

VTune Amplifier uses the Intel sampling driver if it is loaded in all cases except for the stack sampling collection. To make the VTune Amplifier use the driverless Perf mode for sampling without stacks, create a custom analysis type and select the Enable driverless collection option in the GUI, or set the command line knob value to enable-driverless-collection=true as follows:

amplxe-cl -collect-with runsa -knob enable-driverless-collection=true -knob event-config=<event-list> <application>

The option is available starting with the VTune Amplifier 2019 Update 4.

For more complete information about compiler optimizations, see our Optimization Notice.
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