GPU621/Intel Parallel Studio VTune Amplifier

Group Members

1. Iurii Kondrakov
2. James Mai
3. Stephen Griffis
4. Email All

Introduction

This page will demonstrate the purpose, features, and usage of the Intel Parallel Studio VTune Profiler. In short, it is a performance analyzing tool for 32-bit, 64-bit, and x86 architectures running Linux-based or Microsoft Windows operating systems. It supports both Intel and AMD hardware, but advanced hardware-based features and acceleration require an Intel-manufactured CPU. Furthermore, the application is specifically designed to help with optimization of application and system performance, High-performance computing configurations, and more by detecting performance bottlenecks through run-time environment profiling on serial and multi-threaded software.

Details

Intel Parallel Studio VTune Amplifier is available as a standalone application or as part of the Intel Parallel Studio bundle. The tool analyzes the run-time environment metrics of the target executable such as CPU utilization, Throttling, Thread usage, Memory usage, I/O, and other system-level resources to provide a developer with an extensive and precise report and suggestions for possible improvements. Additionally, the application measures the performance of a program, and each function separately and displays the report in an integrated Visual Studio user-friendly interface with comprehensive graphs and tables. Furthermore, the tool can perform an overview to help a developer identify any bottlenecks to resolve and suggest little tweaks such as increasing the grain size or the scope of the data when using the TBB library that Intel provides. Overall, this is a powerful and reliable tool for scalable parallelization.

Our team will point the usage and importance of the tool by demonstrating a little case study with the code implemented during the semester. Precisely, the Workshop 7 - Threading Building Blocks(parallel_scan) which is about a trivial Prefix Scan Problem. We will compare the performance and resource utilization of Serial, OpenMP and TBB versions each being analyzed by Intel Parallel Studio VTune Amplifier in Visual Studio.

Features & Functionalities

Performance Snapshot

This feature will take all the different types of analysis and provide you a summary of each analysis and show how the performance is for each one, while highlighting the ones with the worst performances. This allows users to easily pinpoint which sections need to be prioritized and which sections may require more time to resolve.

Algorithm

Hotspots

This feature will User-Mode Sampling or Hardware Event Based Sampling to collect data while your application is running. After the data collection is completed, it will display where in the process does the code stall or take the most time running and how well you’re utilizing your CPU threads.

You can also open the source code and display which functions are taking up the most CPU time. Which allows you to pinpoint where you should start on optimizing your code and allows you to focus on the functions that are causing the most run-time delay.

User-Mode Sampling

VTune uses a low overhead (~5%) sampling and tracing collection that works to get the information needed without slowing down the application significantly. The data collector uses the OS timer to profile the application, collects samples of all active instruction addresses in intervals of 10ms, and captures a call sequence. Once everything has been collected, it will display the results of the data collection in the results tab.

Hardware Event-Based Sampling

VTune will analyze not just the application running, but all processes running on your system at the moment of run-time and will provide CPU run time performance on the system as a whole. It will still create a list of functions that run in the current application while timing them, but it won't capture the call sequences as hotspots.

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For more information on Hotspots click here

Anomaly Detection Analysis

This feature will analyze your code and search for different anomalies caught during run-time. The different type of anomalies is:

Context Switch Anomaly

This helps to pinpoint issues with threads idling too long due to synchronization issues.

Kernal-Induced Anomaly

This provides insight on any issues with the connection between the internal kernel and the software.

Frequency Drops

These can be rather concerning since CPU frequency drops are likely due to an issue with your hardware like inefficient cooling or other CPU related issues.

Control Flow Deviation Anomaly

When the Instructions Retired metric is exceptionally large for some threads which could be the cause of a code deviation during execution.

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For more information on Anomaly Detection click here

Microarchitecture

Microarchitecture Exploration

This feature does an analysis to give you information on how well your code is going through the core pipeline and it does some calculations to determine ratios used for identifying the issues that you may have at a hardware-level.

For more information on Microarchitecture Exploration click here

Memory Access

This feature is used to locate memory related issues, like proper memory allocations/de-allocations, high bandwidth issues, and NUMA(Non-Uniform Memory Access) problems

Memory Access analysis type uses hardware event-based sampling to collect data for the following metrics:

• Loads and Stores metrics that show the total number of loads and stores

• LLC Miss Count metric that shows the total number of last-level cache misses

• • Local DRAM Access Count metric that shows the total number of LLC misses serviced by the local memory
  • Remote DRAM Access Count metric that shows the number of accesses to the remote socket memory
  • Remote Cache Access Count metric that shows the number of accesses to the remote socket cache
• Memory Bound metric that shows a fraction of cycles spent waiting due to demand load or store instructions
  • L1 Bound metric that shows how often the machine was stalled without missing the L1 data cache
  • L2 Bound metric that shows how often the machine was stalled on L2 cache
  • L3 Bound metric that shows how often the CPU was stalled on L3 cache, or contended with a sibling core
  • L3 Latency metric that shows a fraction of cycles with demand load accesses that hit the L3 cache under unloaded scenarios (possibly L3 latency limited)
  • NUMA: % of Remote Accesses metric shows percentage of memory requests to remote DRAM. The lower its value is, the better.
  • DRAM Bound metric that shows how often the CPU was stalled on the main memory (DRAM). This metric enables you to identify

• DRAM Bandwidth Bound, UPI Utilization Bound issues, as well as Memory Latency issues with the following metrics:
  • Remote / Local DRAM Ratio metric that is defined by the ratio of remote DRAM loads to local DRAM loads
  • Local DRAM metric that shows how often the CPU was stalled on loads from the local memory
  • Remote DRAM metric that shows how often the CPU was stalled on loads from the remote memory
  • Remote Cache metric that shows how often the CPU was stalled on loads from the remote cache in other sockets

• Average Latency metric that shows an average load latency in cycles

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For more information on Memory Access click here

Parallelism

Threading

This feature will analyze your program and provide you with results explaining how well you’re utilizing your cores, how many threads you use throughout your program, shows how much load each thread takes on, and even more in-depth information like wait-time and spin and overhead time.

Like Hotspots there are two modes of data collection User-Mode Sampling and Tracing and Hardware Event-Based Sampling and Context Switches.

User-Mode Sampling and Tracing

This mode recognizes synchronization objects and collect thread wait time by objects. The data can help the user understand the thread interactions and pinpointing where optimization can be performed within the code.

Hardware Event-Based Sampling and Context Switches

This mode collects thread idle wait-time and even though there aren’t any object definition, the problematic synchronization functions and be identified based on the wait time attributed with call stacks.

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For more information on Threading click here

Platform Analyses

System Overview

Versions of the software:

• Standalone VTune Profiler Graphical Interface
• Web Server Interface
• Microsoft Visual Studio Integration
• Eclipse IDE Integration
• Intel System Studio IDE Integration

How to configure and Start Analysis

Intel Parallel Studio VTune Profiler has a couple of versions and a couple of IDE integrations, but for the sake of simplicity and relevance to the course material, we are going to use Visual Studio Integration of the Profiler. To begin, the IDE must be run with root privileges for the Profiler to have access to the hardware information and resources, otherwise, the analysis and collected data will be limited.

To open Vtune press the button in the upper toolkit as shown below:

Next, Configure Analysis button needed to be pressed to get to the VTune Main Menu:

Where a target program and host where it will be executed can be chosen. Also, program parameters and working directories can be configured. Finally, the method of analysis and detection with different options can be enabled from the "How" menu:

Finally, the start button, as shown above, can be pressed to start the actual Vtune configured analysis.

Demonstration

Single-thread Prefix Scan

Starting with the Serial approach to the Prefix Scan Problem.

Code

1. include <iostream>
2. include <chrono>

template <typename T, typename C> int excl_scan( const T* in, // source data T* out, // output data int size, // size of data sets C combine, // combine operation T initial // initial value ) {

if (size > 0) { for (int i = 0; i < size - 1; i++) { out[i] = initial; initial = combine(initial, in[i]); } out[size - 1] = initial; } return 1; // 1 thread }

void reportTime(const char* msg, std::chrono::steady_clock::duration span) { auto ms = std::chrono::duration_cast<std::chrono::microseconds>(span); std::cout << msg << " - took - " << ms.count() << " microseconds" << std::endl; }

int main(int argc, char** argv) { if (argc > 2) { std::cerr << argv[0] << ": invalid number of arguments\n"; std::cerr << "Usage: " << argv[0] << "\n"; std::cerr << "Usage: " << argv[0] << " power_of_2\n"; return 1; } std::cout << "Serial Prefix Scan" << std::endl;

// initial values for testing const int N = 9; const int in_[N]{ 3, 1, 7, 0, 1, 4, 5, 9, 2 };

// command line arguments - none for testing, 1 for large arrays int n, nt{ 1 }; if (argc == 1) { n = N; } else { n = 1 << std::atoi(argv[1]); if (n < N) n = N; } int* in = new int[n]; int* out = new int[n];

// initialize for (int i = 0; i < N; i++) in[i] = in_[i]; for (int i = N; i < n; i++) in[i] = 1; auto add = [](int a, int b) { return a + b; };

std::chrono::steady_clock::time_point ts, te;

// Exclusive Prefix Scan ts = std::chrono::steady_clock::now(); nt = excl_scan<int, decltype(add)>(in, out, n, add, (int)(0)); te = std::chrono::steady_clock::now();

std::cout << nt << " thread" << (nt > 1 ? "s" : "") << std::endl; for (int i = 0; i < N; i++) std::cout << out[i] << ' '; std::cout << out[n - 1] << std::endl; reportTime("Exclusive Scan", te - ts);

delete[] in; delete[] out; } </source>

Sources

1. Intel VTune Profile
2. Installation and Features
3. GPU621 Workshop 7
4. Github repository of the code used