Re: I made a flame graph renderer for git's trace2 output

From: Jeff King <peff@peff.net>
Date: May 10, 2019, 21:03 UTC
Message-ID: <20190510210324.GA30947@sigill.intra.peff.net>
In-Reply-To: <87zhnuwdkp.fsf@evledraar.gmail.com>

On Fri, May 10, 2019 at 05:09:58PM +0200, Ævar Arnfjörð Bjarmason wrote:

> As noted in TODOs in the script there's various stuff I'd like to do
> better, and this also shows how we need a lot more trace regions to get
> granular data.

Hmm. My gut reaction was: doesn't "perf record -g make test" already give us that granular data? I know "perf" isn't available everywhere, but the idea of the FlameGraph repo is that it takes input from a lot of sources (though I don't know if it supports any Windows-specific formats yet, which is presumably a point of interesting to trace-2 authors).

But having generated such a flamegraph, it's not all that helpful. It mainly tells us that we spend a lot of time on fork/exec. Which is no surprise, since the test suite is geared not towards heavy workloads, but lots of tiny functionality tests.

TBH, I'm not sure that flame-graphing the test suite is going to be all that useful in the long run. It's going to be heavily weighted by the types of things the test suite does. Flamegraphs are good for understanding where your time is going for a particular workload, but the workload of the test suite is not that interesting.

And once you do have a particular workload of interest that you can replay, then I think the granular "perf" results really can be helpful.

I think the trace2 flamegraph would be most useful if you were collecting across a broad spectrum of workloads done by a user. You _can_ do that with perf or similar tools, but it can be a bit awkward. I do wonder how painful it would be to alias "git" to "perf record git" for a day or something.

-Peff