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Show HN: Perforator – cluster-wide profiling tool for large data centers

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Re: Show HN: Perforator – cluster-wide profiling tool for large data centers

#11

Post by menaerus »

BigRedEye wrote: post #10
menaerus wrote: post #8

Earlier quoted context omitted.

Blog is packed with information, thanks! Isn't it the case that from stack traces it is rather impossible to read that function foo() is burning CPU cycles because it is memory-bound? And the reason could be rather somewhere else and not in that particular function - e.g. multiple other threads creating contention on the memory bus? If so, doesn't this make the profile somewhat an invalid candidate for PGO?

It depends on the event that was sampled to generate the profiles. For example, if you sample instructions by collecting a stack trace every N instructions, you won't actually see foo() burning the CPU. However, if you look at CPU cycles, foo() will be very noticeable. Internally, we use sPGO profiles from sampling CPU cycles, not instructions.

Right, perhaps I was a little bit too vague but what I was trying to say is that by merely sampling the CPU cycles we cannot infer that the foo() was burning CPU because it was memory-bound and which in itself is not an artifact of foo() implementation but rather application-wide threads that happen to saturate the memory bus more quickly.

Or is my doubt incorrect?

Re: Show HN: Perforator – cluster-wide profiling tool for large data centers

#15

Post by BigRedEye »

be-hase wrote: post #14

I'm curious about the differences from Pyroscope. https://github.com/grafana/pyroscope

Great question! Perforator indeed looks similar to Pyroscope. However, we think that the closest existing solutions are https://parca.dev, closed-source Google Wide Profiling, and, speaking of the agent, the beautiful OpenTelemetry eBPF profiler. The main technical differences with Pyroscope we see are:

- Pyroscope's Java support is superior as of now because Pyroscope offloads it to the amazing async-profiler.

- Pyroscope expects native binaries to be compiled with frame pointers: https://grafana.com/docs/pyroscope/latest/configure-client/g.... This is often not the case, and that's the problem we've tried to solve with Perforator. Perforator uses .eh_frame, which is nearly universal and does not impose additional requirements on compiled binaries.

- Pyroscope symbolizes using symtab: https://grafana.com/docs/pyroscope/latest/configure-client/g.... We use DWARF/GSYM to get as correct and verbose stacks as possible (we benchmark our stacks against stacks from gdb).

- Pyroscope symbolizes profiles on an agent, while Perforator symbolizes profiles offline, greatly reducing symbolization costs and agent's overhead. It seems Pyroscope is heading toward the same architecture we use: https://github.com/grafana/pyroscope/pull/3799.

- Perforator can be (and should be!) run as a standalone replacement for perf record.

- Perforator supports sPGO profiles.

In summary, we try to implement native profiling almost perfectly. It's worth noting that Pyroscope is a mature, well-established product that integrates excellently with the Grafana ecosystem. We have just focused on different things: our focus has been on optimizing native code profiling and making it as accurate and low-overhead as possible.

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