Column: Embedded design
Aſt er launching Tracealyzer, select
“File->Open Folder” and select the “lttng-traces” directory. Tracealyzer will analyse the directory for the captured trace session and load all included fi les (an LTTng trace consists of multiple fi les). Choose the appropriate view to analyse the driver’s performance. Whilst Tracealyzer has views to provide
Figure 5: “Actor Instances” window
useful insights into the userspace-kernel interaction, my focus is on the kernel space and, thus, the “Trace View”, “Actor Instances”, “CPU Load Graph” and “Context Switch Intensity” views, which can be selected by clicking on the appropriate icons in the main Tracealyzer window. Upon opening the Trace View, there will be
a graph; see Figure 3. T e leſt most column is for “Actor
Names”, which are the userspace and kernel processes/threads executing at any given time. T e leſt most column (in grey) is the kernel “swapper”, which is the idle task in the Linux kernel. Farther on the right is a blue line, which is a kernel thread (“kworker”). As we implement the driver and monitor, we’re going to use the Trace View to verify that my kernel thread is given enough execution resources to manage the DMA transfer. Even farther right are a green line, the
lttng userspace process, and a yellow line, the rcu_sched userspace process. Right-clicking in the window, select “Zoom
Figure 6: If we select another kernel thread, we see anomalies in the graph
Out – Show Full Traces”, which will show a concise view of all the threads and processes executed during the entire duration of the capture; see Figure 4. It can be seen that the swapper occupies
the most execution resources here, which makes sense, since no substantial tasks are being done. When the driver is running and
transferring data from my device to the Phytec development board, we can see sections of the Trace View that are consumed by blue fragments. T is would indicate that my specifi c kernel thread has been given a greater chunk of the execution resources on the platform to perform the transfer. The next important view is the “Actor
Figure 7: Different actors using the CPU
Instances”, where we have selected the “Execution Time” in the drop down box at the top left; see Figure 5.
12 February 2021
www.electronicsworld.co.uk
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