Re: [PATCH] revision: trace topo-walk statistics

[Junio C Hamano <gitster@pobox.com>]

Derrick Stolee <stolee@gmail.com> writes:

> On 1/6/2021 8:37 PM, Junio C Hamano wrote:
>> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
>> 
>>> diff --git a/revision.c b/revision.c
>>> index 9dff845bed6..1bb590ece78 100644
>>> --- a/revision.c
>>> +++ b/revision.c
>>> @@ -3308,6 +3308,26 @@ struct topo_walk_info {
>>>  	struct author_date_slab author_date;
>>>  };
>>>  
>>> +static int topo_walk_atexit_registered;
>>> +static unsigned int count_explore_walked;
>>> +static unsigned int count_indegree_walked;
>>> +static unsigned int count_topo_walked;
>> 
>> The revision walk machinery is designed to be callable more than
>> once during the lifetime of a process.  These make readers wonder
>> if they should be defined in "struct rev_info" to allow stats
>> collected per traversal.
>
> That's possible, but the use of an at-exit method means we only
> report one set of statistics and the 'struct rev_info' might
> be defunct.

Ah, sorry for the noise.  Even after making multiple traversal we
want to report the aggregate.

> It does limit how useful the statistics can be when there are
> multiple 'struct rev_info's in use, but we also cannot trust
> that the rev_infos are being cleaned up properly at the end
> of the process to trigger the stats logging.
>
> Of course, maybe that _is_ something we could guarantee, or
> rather _should_ guarantee by patching any leaks. Seems like
> a lot of work when these aggregate statistics will be
> effective enough. But maybe I'm judging the potential work
> too harshly?

But different subsystems would have different "per-invocation"
structure (e.g. "diff" uses "struct diff_options") and some may not
even have an appropriate structure to hang these stats on.  We may
want to design a more general mechanism that can be used to annotate
the subsystems uniformly.  While that could be a worthy longer term
goal, it certainly does not have to be part of this single-patch
topic, I would think.