Lab 3: Finding Parallelism Issues
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Copyright © 2010, Intel Corporation. All Rights Reserved.Table of Contents
Lab 3: Finding Parallelism Issues i
Lab 3: Finding Parallelism Issues 1
Activity 1 – Collect Locks-And-Waits Data 2
Activity 2 – Find Causes of Poor Parallelism 3
Lab 3: Finding Parallelism Issues
Time Required | Thirty minutes |
Objective | In this lab session, you will use Intel® VTune™ Amplifier XE to determine the synchronization objects or APIs that are limiting parallelism in an application. After successfully completing this lab's activities, you will be able to:
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Activity 1 – Collect Locks-And-Waits Data
Time Required | Ten minutes |
Objective |
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- Right-click on analyzing_locks in the Solution Explorer window and select "Set As Startup Project"
- Click on the "New Analysis" button
- Select "Algorithm Analysis->Locks And Waits" in the analysis type pane
- Click "Start" – The tachyon application will run. Note that as the application runs it draws and image of several different silver balls on the screen. Make a note of the execution time displayed in the application window as before.
- After the application completes the Intel® VTune™ Amplifier XE will spend some time analyzing the data. When it is finished analyzing, the summary pane for Locks And Waits appears. Note the analysis explanation pane comes up. Read it and then clear the pane.
At this point the application has run to completion and the Intel® VTune™ Analyzer is ready to display the analyzed results.
- What is the result screen that appears after clearing the analysis explanation pane?
- What useful data is in this first screen?
Activity 2 – Find Causes of Poor Parallelism
Time Required | Ten minutes |
Objective |
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Codes Description |
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- Click "Bottom-Up" tab. Notice at or near the top is a Sync Object labeled "critical section 0xnnnnnnnn". This is a critical section that is in the user code and is limiting parallelism as shown by the large amount of wait time indicated and the amount of "Poor" time (fewer than the available number of CPUs used) shown.
- Click on the arrow to the left of the term "Critical Section" to expand the list of callers to that Critical Section. Notice that the Critical Section is referenced by the function named draw_task.
- Double click on the function name "draw_task". The source and assembly view are now displayed. Note that there is a Critical Section that is used in the draw_task function (you may need to scroll down the source code to see it).
This Critical Section is actually not needed - the "for" loop is already thread safe. It was accidentally put in as an extra precaution. This is a source of some unneeded serial, "poor CPU usage" time. - To improve the speed and parallelism of the application comment out the calls to "EnterCriticalSection" and "LeaveCriticalSection" inside draw_task, then rebuild. Then run the app and notice the execution time in the title bar when the app completes.
- Rerun the Concurrency analysis again as you did in Lab 2 and see if the app is now faster. Also, see if the offending Critical Section is no longer on the list of sync objects.