Analyzing Application Performance with Hardware Event-based Stack Sampling

Configure Analysis

Collect Data

Multitask operating systems execute all software threads in time slices (thread execution quanta). Intel® VTune™ Amplifier profiler handles thread quantum switches and performs all monitoring operations in correlation with the thread quantum layout.

The figure below explains the general idea of per-thread quantum monitoring:

• The profiler gains control whenever a thread gets scheduled on and then off a processor (that is, at thread quantum borders). That enables the profiler to take exact measurements of any hardware performance events or timestamps, as well as collect a call stack to the point where the thread gets activated and inactivated.

• The profiler determines a reason for thread inactivation: it can either be an explicit request for synchronization (thread 0 calls the WaitForSingleObject function in the example above), or a so-called thread quantum expiration, when the operating system scheduler preempts the current thread to run another, higher-priority one instead.

• The time during which a thread remains inactive is also measured directly and differentiated based on the thread inactivation reason: inactivity caused by a request for synchronization is called Wait time, while inactivity caused by preemption is called Inactive time.

While a thread is active on a processor (inside a quantum), the profiler employs event-based sampling to reconstruct the program logic and associate hardware events and other characteristics with the program code. Unlike the traditional event-based sampling, the profiler upon each sampling interrupt also collects:

• call stack information

• branching information (if configured so)

• processor timestamps

All that allows for statistically reconstructing program execution logic (call and control flow graphs) and tracing threading activity over time, as well as collecting virtually any information related to hardware utilization and performance.

Analyze Performance

Select the Hardware Events viewpoint and click the Event Count tab. By default, the data in the grid are sorted by the Clockticks (CPU_CLK_UNHALTED) event count providing primary hotspots on top of the list.

Click the plus sign to expand each hotspot node (a function, by default) into a series of call paths, along which the hotspot was executed. VTune Amplifier decomposes all hardware events per call path based on the frequency of the path execution.

The counts of the hardware events of all execution paths leading to a sampled node sum up to the event count of that node. For example, for the _schedule function, which is the top hotspot of the application, the INST_RETIRED.ANY event count equals the sum of event counts for 7 calling sequences: 22 657 490 276 = 20 377 044 + 5 269 274 +0 + 718 190 701 + 21 857 068 455 + 26 565 167 + 30 019 635.

Such a decomposition is extremely important if a hotspot is in a third-party library function whose code cannot be modified, or whose behavior depends on input parameters. In this case the only way of optimization is analyzing the callers and eliminating excessive invocations of the function, or learning which parameters/conditions cause most of the performance degradation.

Explore Parallelism

When the call stacks collection is enabled, the VTune Amplifier analyzes context switches and displays data on the threads activity using the context switch performance metrics.

Click the Synchronization Context Switches column header to sort the data by this metric. The synchronization hotspots with the highest number of context switches and high Wait time values typically signals a thread contention on this stack.

Select a context switch oriented type of the stack (for example, the Preemption Context Switches type) in the drop-down menu of the Call Stack pane and explore the Timeline pane that shows each separate thread execution quantum. A dark-green bar represents a single thread activity quantum, grey bars and light-green bars - thread inactivity periods (context switches). Hover over a context switch region in the Timeline pane to view details on its duration, start time and the reason of thread inactivity.

When you select a context switch region in the Timeline pane, the Call Stack pane displays a call sequence at which a preceding quantum was interrupted.

You may also select a hardware or software event from the Timeline drop-down menu and see how the event maps to the thread activity quanta (or to the inactivity periods).

Correlate data you obtained during the performance and parallelism analysis. Those execution paths that are listed as the performance hotspots with the highest event count and as the synchronization hotspots are obvious candidates for optimization. Your next step could be analyzing power metrics to understand the cost of such a synchronization scheme in terms of energy.