TL;DR
An engineer revisits using Ctrl-C in a debugger as a quick, low-effort sampling profiler. It can rapidly reveal simple hot spots and environment issues, but it has clear limits compared with dedicated profilers.
What happened
A veteran developer and profiler author describes leaning on the Ctrl-C technique—interrupting a running program with a debugger to inspect call stacks—as a pragmatic way to find straightforward performance problems. He recounts multiple cases: a debug build that paused for a minute at startup where repeated Ctrl-Cs pointed repeatedly to heavy use of a JSON parsing library, prompting a fix; and a core dump investigation where gdb hung while decoding DWARF macro information, which led to discovering an interaction between compiler debug flags, the LLD linker, and gdb’s DWARF handling. In that case removing the highest-detail debug flag (-ggdb3) eliminated the slowdown. The author contrasts this quick-and-dirty approach with full profilers, arguing Ctrl-C is easy to apply in hostile or constrained environments, requires no special builds, and often suffices for "stupid problems" encountered in practice, while acknowledging it cannot replace specialized tools for subtle or distributed regressions.
Why it matters
- Ctrl-C in a debugger is a low-cost, broadly applicable first step that can surface obvious hotspots without configuring profilers.
- It works in environments where traditional profiling tools are unavailable or hard to set up, including stripped or embedded contexts.
- Relying solely on Ctrl-C can miss small, widespread regressions, tail-latency events, and issues across many threads or machines.
- Interactions between compiler flags, linker behavior and debugger DWARF handling can create surprising slowdowns that are detectable with quick debugger sampling.
Key facts
- Ctrl-C profiling samples call stacks by interrupting a running process and inspecting its stack traces with a debugger.
- The author reports a slow debug-build startup traced repeatedly to heavy use of a JSON parser via Ctrl-C sampling; the exact remediation was unknown but the issue was fixed after reporting it.
- gdb can become slow when processing certain DWARF macro data; the author observed gdb spending time in dwarf_decode_macro_bytes.
- Using -ggdb3 produced richer debug info that in the author’s case caused gdb slowdown when combined with LLD-produced binaries; removing -ggdb3 removed that slowdown.
- LLD in some versions lacked an optimization called relocation relaxation on MIPS, which can affect generated branch instructions and performance compared with other linkers.
- Ctrl-C behaves like a sampling profiler with a very low sampling frequency and thus may miss short or infrequent slow events.
- Tracing profilers (e.g., ftrace / KernelShark) are better for investigating tail latency but may not directly point to the code consuming most CPU time.
- Simulation- or instruction-trace-based profilers can show instruction-level differences but are usually too slow for production use.
What to watch next
- Test whether -ggdb3 or other high-detail debug flags cause slowdowns with your chosen linker/debugger combination.
- Check linker versions and whether relocation relaxation is implemented for your architecture; older linkers may omit this optimization.
- For intermittent tail-latency or distributed-system slowdowns, consider dedicated tracing or sampling profilers rather than relying on Ctrl-C.
Quick glossary
- Sampling profiler: A tool that periodically samples a program’s call stacks to identify where time is spent without instrumenting every function.
- DWARF: A widely used debug data format that stores information for debuggers, including source locations and macro data.
- Relocation relaxation: A linker optimization that shortens or simplifies generated code when it can determine that a target is close enough for a shorter branch instruction.
- Tracing profiler: A profiler that records detailed execution events (e.g., function entry/exit or kernel events), useful for latency and ordering analysis.
Reader FAQ
Can Ctrl-C replace full-featured profilers?
No. Ctrl-C is useful for quick, obvious hotspots but misses small, distributed, or infrequent regressions that dedicated profilers can reveal.
Do you need a special build to use Ctrl-C profiling?
Not usually. A debugger can produce call stacks even without frame pointers or special instrumentation flags.
Why was gdb slow with some binaries?
In the author’s example, gdb spent time decoding DWARF macro data combined with LLD output and -ggdb3 debug info; removing -ggdb3 eliminated the slowdown.
Should I always prefer LLD or another linker for speed?
Not confirmed in the source
Profiling with Ctrl-C June 25th, 2024 I once wrote about how profiler output can be misleading. Someone commented that you don’t need profilers – just Ctrl-C your program in a…
Sources
- Profiling with Ctrl-C (2024)
- Profiling with Ctrl-C
- C and C++ Profiling Tools: What You Need to Know
- Effectiveness of Trace Sampling for Performance …
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