# An informal survey of Linux dynamic tracers

Sat 09 January 2016

I've gotten some questions about the choice of perf over all the other available Linux tracers. This blog post is a quick overview of my personal experiencing trying several tracers; it is not intended to be authoritative.

## My requirements

Since I use dynamic tracers to iteratively answer questions about production systems, I have several requirements. I'll order them here based on their priority to me:

1. A good tracing tool is stable. I do not want my production systems crashing.
2. A good tracing tool is low overhead. I want to use it in production environments, so it cannot substantially affect system performance. This usually means I want some sort of selective filtering or early output aggregation.
3. A good tracing tool can collect userspace stacks. The most frequent question I ask is "Why is my application X", and userspace stacks are the number one way I answer this question.
4. A good tracing tool has visibility into the kernel. I am frequently asking questions about the system (e.g. "Why am I descheduled?" or "Why am I doing disk IO?") that can only be answered effectively with kernel support.
5. A good tracing tool is easily usable on old (e.g. pre-3.2 kernel) systems. For many of my customers, upgrading (especially to mainline kernel) is a frightening proposition.
6. A good tracing tool already exists on the system. A great tracing tool doesn't require special packages to be installed. For many of my customers, installing new software (especially on a production server) is a challenging or painful process.

## Tracers I have tried

• ftrace: Powerful building block that doesn't have a satisfactory front end yet. Brendan Gregg has some good tools in his perf-tools package. Enabled by default even on old kernels and usable with no external packages, but requires root. Has some quirks: for example, I had difficulty getting userspace stacks to work reliably across all events when I tried it.
• perf: Rapidly growing frontend for many other kernel tracing subsystems, including parts of ftrace. Has a huge surface area in the kernel and in userspace. Kernel support for perf is very common, although the userspace frontent requires a package to be installed. Provides little to no support1 for in-kernel aggregation (some support for event filtering) so all data must be post-processed in userspace -- this can have a large performance effect for very frequent events (e.g. scheduler events). For this reason, I find the interface pretty clunky.

perf is my current favorite tracer because of the support, surface area, and because it can be used in production environments without custom kernel modules.

• systemtap: I can't really find evidence of people using this legitimately except to do kernel development. Was hard to install (required massive download of debug symbols and a custom kernel??). I have concerns about its viability for production environments.

• sysdig: A glorious user interface and easy to install, but definitely the new kid on the block. Requires a custom kernel module to be installed. Only traces at the syscall boundary, which is good enough for some use cases, but I generally prefer more visibility into the kernel (for example, to see that we're getting descheduled due to page faults, etc). Doesn't have a way to filter or aggregate in-kernel events (by design) and cannot collect stack traces. These developers seemed very open to outside contributors and upstream their work pretty quickly, so maybe contributions here could actually be used customers in my lifetime.

## Tracers I have investigated

• lttng: Incredible docs, but requires a kernel module to trace kernel events. Appears to have strong support for a variety of userspace applications. Does not appear to do in-kernel aggregation. Can this collect stack traces?
• ktap: Haven't played around with this but the interface is really pretty. I have some concerns about its stability.
• eBPF: Looks flexible and will be mainline in the kernel, but isn't there on old kernels and doesn't have a good frontend yet. I'm watching iovisor for that.

Feel free to chime in with any other tracers or commentary.

1. perf stat can do some aggregation, but unfortunately cannot aggregate on very complicated things (e.g. stacks).