Re: I made a flame graph renderer for git's trace2 output
- Date: Fri, 10 May 2019 23:57:58 +0200
- From: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
- Subject: Re: I made a flame graph renderer for git's trace2 output
On Fri, May 10 2019, Jeff King wrote:
> 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.
Yeah I should have mentioned that I'm mainly linking to the test suite
rendering as a demo.
My actual use-case for this is to see what production nodes are spending
their time on, similar to what Microsoft is doing with their use of this
The test suite serves as a really good test-case for the output, and to
stress-test my aggregation script, since we're pretty much guaranteed to
run all our commands, and cover a lot of unusual cases.
It also shows that we've got a long way to go in improving the trace2
facility, i.e. adding region enter/leave for some of the things we spend
the most time on.