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

> 
> On Mon, May 20 2019, Jeff Hostetler wrote:
> 
>> On 5/10/2019 5:57 PM, Ævar Arnfjörð Bjarmason wrote:
>>>
>>> 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
>>> facility.
>>>
>>> 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.
>>>
>>
>> Very nice!
>>
>> Yes, there is more work to do to add more regions to get more
>> granular data for interesting/problematic things.  My primary
>> goal in this phase has been to get the basic machinery in place
>> and be vetted with some universally interesting regions, such as
>> reading/writing the index and the phases of status.
>>
>> Going forward, we can trivially (permanently) add new regions as we
>> want.  I tend to use temporary "experimental" regions during my perf
>> investigations so that I don't clutter up the mainline source with
>> uninteresting noise.
> 
> Indeed, a lot more regions are needed.
> 
>> WRT the TODO's in your script:
>>
>> [] I don't think data events will be useful for your usage.  The data
>> values are orthogonal to the time values.
> 
> I haven't done this, so I'm not asserting that it's useful, but from
> some brief grepping a few datapoints are overwhelmingly common, and can
> be faked up into regions of sorts for the purposes of a flamegraph.
> 
> E.g. "git checkout" will reliably have read/version early on, and then
> write/version, in that case mostly/entirely redundant to the
> do_{read,write}_index region, but in general I think we'll be able to
> loosely plot data points say as "given the median runtime, here's the
> median % of time into the command we first encounter this data point".

> 
>> [] I would add the child_start/_exit events to the stack.  This will
>> give you the names of non-builtin/shell commands and hooks.  The
>> various "child_class" and "use_shell" and "hook_name" fields will help
>> you avoid duplicate stack frames (which you'll get for builtin
>> commands).
> 
> Yeah that's very useful. Any reason not to do something like this:
> 
>      diff --git a/git.c b/git.c
>      index 1bf9c94550..6c926ae013 100644
>      --- a/git.c
>      +++ b/git.c
>      @@ -698 +698 @@ static void execv_dashed_external(const char **argv)
>      -       trace2_cmd_name("_run_dashed_");
>      +       trace2_cmd_name(is_builtin(argv[0]) ? argv[0] : "_run_dashed_");
> 
> I haven't tested, but we e.g. report 'git-submodule' as just
> '_run_dashed_', seems we could do better...
>