Note that the timing of events in the first frame is clearly different from the rest of the frames. We see a periodic sequence of events-four fragments of similar struc- ture correspond to processing four sequential frames. To address this problem, Figure 2 displays a real trace from MPlayer showing functions called directly from main and a few other relevant details. Yet, a printed snapshot of the data may be hard to understand because of significant volume of irrelevant details. Nupshot proved powerful in navigating through collected traces interactively, zooming in on relevant fragments. When MPlayer terminates, we save the trace to disk to visualize with the Nupshot 17 tool later. It collects the trace for 200 milliseconds (several frames of the trailer), pulls out the instrumentation and lets the test run to completion. The handler in the tracer library catches it and initiates instrumentation. The signal arrives when MPlayer is in the middle of playback. When the library initializes, it requests an alarm signal to be delivered to it 10 seconds later. When the player starts, we pre-load the tracer library into it using the LD PRELOAD mechanism. To find the cause of the problem, we subject it to self-propelled tracing as follows. At the same time, playback under Linux was choppy and MPlayer reported frames being dropped. 14 In fact, we verified that the hardware capabilities of the machine were adequate for the task: the trailer displayed accurately under two commercial DVD players in Microsoft Windows. The system satisfies typical hardware requirements for DVD playback. To investigate the problem, we used MPlayer to display a short DVD trailer on a low-end Pentium II 450 MHz desktop computer running Linux. In the end, the ease of tracing with self-propelled instrumentation allowed us not only to fix the problem, but also to identify its root cause. Yet, we decided to proceed autonomously, to model real world conditions where many bugs are too specific to be found with community help. As the symptoms are relatively common, the MPlayer community has accumulated a knowledge base of potential fixes. However, in our experience, its quality of DVD playback is not always adequate: the player often drops frames, resulting in user-visible freezes in the playback even on modern systems. It supports a wide variety of video and audio formats and works well even on low-end consumer systems. MPlayer is a popular multimedia player for Linux and several other platforms. At the end of the section, we compare the overhead of self-propelled tracing to that of existing approaches. The problems required different techniques for trace collection, so we describe them separately. To evaluate our new approach, we applied it to analysis of two performance problem (choppy DVD playback in MPlayer and non-responsive GUI in Microsoft Word running under the Wine emulator) and a system crash. The collected data are later returned to the user space via another ioctl. When ktracedrv sees the command, it pulls out all inserted instrumentation, restoring the kernel image to the original state. Similarly, to stop kernel tracing, the application needs to issue the KTRACE STOP ioctl command. From that moment, if one of these events happens, we will collect all incurred function calls into the module’s trace buffer.
#Mplayer downloads driver
The command makes the driver pre- instrument two kernel entry points: do IRQ for following hardware interrupts and system call for following system calls.
Typically, our framework issues this command together with starting user-level tracing. To activate kernel tracing, the application issues the KTRACE START ioctl command.
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