Can someone explain the 'stalled-cycles' metrics in perf stat output?

Backend
1

Hey folks, I’ve been using the perf stat command to check performance, but I’m stumped by two metrics: stalled-cycles-frontend and stalled-cycles-backend. I’ve googled around but can’t find a clear explanation. What do these numbers mean? Here’s a snippet from my output:

$ perf stat echo 'hello'

Performance stats:

  0.501234 task-clock               # 0.654 CPUs used
         0 context-switches         # 0.000 K/sec
         0 CPU-migrations           # 0.000 K/sec
       189 page-faults              # 0.377 M/sec
    654321 cycles                   # 1.305 GHz
    789012 stalled-cycles-frontend  # 120.58% frontend cycles idle
    456789 stalled-cycles-backend   # 69.81% backend cycles idle
    765432 instructions             # 1.17 insns per cycle
                                    # 1.03 stalled cycles per insn
    145678 branches                 # 290.635 M/sec
      7890 branch-misses            # 5.42% of all branches

0.000766789 seconds time elapsed

Any insights would be super helpful. Thanks!

2

The ‘stalled-cycles’ metrics in perf stat output provide crucial insights into CPU efficiency. Frontend stalls indicate that the CPU is having difficulty fetching and decoding instructions, commonly due to cache misses or branch mispredictions. Backend stalls, on the other hand, occur when there is a delay in instruction execution, often because of data dependencies or limited execution resources.

In your example, the high percentage of frontend stalls (120.58%) highlights significant bottlenecks in the instruction fetch process, while the backend stalls (69.81%) suggest execution delays. Analyzing these metrics can help pinpoint performance issues and guide optimization efforts.

3

hey echo_vibrant, those metrics are about cpu pipeline stalls. frontend stalls happen when the cpu can’t fetch instructions fast enough, like cache misses. backend stalls are when the cpu has instructions but can’t execute em, maybe waiting for data. both slow down ur program. hope that helps!

4

hey there! those stalled-cycles numbers are super intersting, right? they show where the CPU might be hitting snags. have you tried running perf on diffrent types of programs? I wonder how the numbers would change. what kind of optimizations are you thinkin about based on these stats?