Post on 22-May-2020
Bhanu Shankar, Ph.D.
Architect, 3D XPoint™ Performance Analysis
Intel Corporation
May 17, 2016
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The products described may contain design defects or errors known as errata which may cause the product to deviate from publishedspecifications. Current characterized errata are available on request.
No license (express or implied, by estoppel or otherwise) to any intellectual property rights is granted by this document.
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© 2016 Intel Corporation.
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Notice revision #20110804
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Copyright © 2016, Intel Corporation. All rights reserved. *Other names and brands may be claimed as the property of others.
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In a single phrase “VTune is the best oscillator for Intel® Platforms”
If there is something to measure on the platform, VTune can do it
Learn a single tool
Use it on multiple Operating Systems
– Windows / Linux / FreeBSD / Android / VxWorks
Use it on Multiple Platforms
– Quark, Atom Family, Core Family, Xeon family, Xeon Phi family
Updated often with new Analyses modes for better insight
Intel® VTune™ AmplifierGet Faster Code Faster
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Familiarity with the basics of Intel® VTune™ Amplifier
Create Projects
Starting a profiling run
– Choose Target and Analysis Type
Types of Analyses available in VTune™ Amplifier
VTune Panes
– Role of the Grid
– Timeline Views
– Grouping Toolbar
Familiarity with the basics
Parallel programming using OpenMP
Intel x86 assembly language
Basics of compiler optimizations
Cache and Memory hierarchies
Audience Knowledge
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I will start with an application and work through the process analyzing its performance.
The focus of this process is to allow you, the user, to be able to find out if your application is memory bound.
If so, is the memory boundedness caused due to NUMA behavior
The application is a modified version of the stream benchmark
Freely available at: http://www.cs.virginia.edu/stream
A simple, synthetic benchmark designed to measure sustainable memory bandwidth
Synopsis of this webinar
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First - Run Advanced Hotspots Analysis
Identify the hotspots
Characterize the application behavior
Secondly - Run General Exploration Analysis
Identify areas to explore after the basic algorithm / hotspot
Lastly – Run Specialized Analysis
For this example - Memory Analysis
– Memory Analysis without objects
– Memory Analysis with objects (Linux only)
General Methodology for using VTune
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Let’s get started
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First - Run Advanced Hotspots
Identify the hotspots
Characterize the application behavior
Secondly - Run General Exploration
Identify areas to explore after the basic algorithm / hotspot
Lastly – Run Specialized Analysis
For Instance - Memory Analysis
– Memory Analysis without objects
– Memory Analysis with objects (Linux only)
Step 1:
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Application – Hotspot – Bottom Up Tab
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Application Source/Object
Source code is simple
object code is straight forward
Why the large CPI?
Not caused by algorithm
Must be machine specific
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First - Run Advanced Hotspots
Identify the hotspots
Characterize the application behavior
Secondly - Run General Exploration
Identify areas to explore after the basic algorithm / hotspot
Lastly – Run Specialized Analysis
For Instance - Memory Analysis
– Memory Analysis without objects
– Memory Analysis with objects (Linux only)
Step 2
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Summary Page
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General Exploration – Bottom Up Tab
Same Loops as earlier
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Let’s Explore – Source level
Yes, Indeed – We have a bottleneck in the memory hierarchy
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First - Run Advanced Hotspots
Identify the hotspots
Characterize the application behavior
Secondly - Run General Exploration
Identify areas to explore after the basic algorithm / hotspot
Lastly – Run Specialized Analysis
For Instance - Memory Analysis
– Memory Analysis without objects – Do we have a bandwidth problem?
– Memory Analysis with objects (Linux only)
Step 3: Find the memory bandwidth
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How do I run Memory Access Analysis?
Make sure this box is unchecked.
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Memory Access - Summary
Looks like a problem accessing remote DRAM
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Memory Access – Bottom-Up View
Imbalance in memory access across both sockets
Average latency is fairly large
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First run Advanced Hotspots
Identify the hotspots
Characterize the application behavior
Secondly run General Exploration
Identify areas to explore after the basic algorithm / hotspot
Lastly – Run Specialized Analysis
For Instance - Memory Analysis
– Memory Analysis without objects
– Memory Analysis with objects (Linux only)
Step 4: Identify the memory object(s)
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Identify the Memory Objects - Configuration
Make sure this box is checked.Minimal size of object to track.
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Identify the Memory Objects
Location of the heap object
Average Latency is large
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Dive into an object
Access to the object in a parallel region - Good
Access to the object in a serial region –Hmmm…Investigate
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Serial Access to memory object
This is where memory is first touched.BINGO!!! Linux stripes memory to local memory of socket!!!
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Did it work? Analyze the fixed applicationRun Memory Access on fixed code
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Fixed Code: Summary Page
Effects of NUMA completely disappearedRemote DRAM access are minimal
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The stream benchmark has 5 loops that are parallelized
Locate the loops by tagged with “#pragma omp parallel for”
Remove the “#pragma omp parallel for” for each or multiple loops
Run Intel® VTune™ Amplifier
See the effects of memory placement and parallel execution
Try the compare results feature on your runs of VTune using the icon
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Lab exerciseTry out what you just learned
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Memory and Inter-socket Bandwidth
Memory Latency
Memory Hierarchy
False Sharing
True Sharing
Effectiveness of Lockless Algorithms
What other problems can I diagnose this way?
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Intel® VTune™ Amplifier continues to add tools to the toolbox to diagnose system performance problems
Memory Access Analysis is one such powerful tool
Stay tuned for more such tools in the future
Summary
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Download and Evaluate Intel® VTune™ Amplifier
https://software.intel.com/en-us/intel-vtune-amplifier-xe
Intel® VTune™ Amplifier Support
https://software.intel.com/en-us/intel-vtune-amplifier-xe-support
Get Help: Ask the Community
https://software.intel.com/en-us/forums/intel-vtune-amplifier-xe
NUMA Architecture
https://software.intel.com/en-us/articles/a-brief-survey-of-numa-non-uniform-memory-architecture-literature
Stream Benchmark
http://www.cs.Virginia.edu/stream
or type “stream benchmark” into your favorite search engine
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Call to Action
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Questions?