Re: GSoC Git Proposal Draft - ZheNing Hu

[ZheNing Hu <adlternative@gmail.com>]

Jeff King <peff@peff.net> 于2021年4月13日周二 下午2:40写道：
>
> On Sun, Apr 11, 2021 at 11:34:35PM +0800, ZheNing Hu wrote:
>
> > > Why is Olga’s solution rejected?
> > > 1. Olga's solution is to let `git cat-file` use the `ref-filter` interface,
> > > the performance of `cat-file` appears to be degraded due "very eager to
> > > allocate lots of separate strings" in `ref-filter` and other reasons.
> >
> > I am thinking today whether we can directly append some object information
> > directly to `&state->stack->output`, Instead of assigning to `v->s` firstly.
>
> Yes, that's the direction I think we'd want to go.
>
> > But in `cmp_ref_sorting()` we will use `get_ref_atom_value()`, It is possible
> > to compare `v->s` of two different refs, I must goto fill object info in `v->s`.
> >
> > So I think this is one of the reasons why `ref-filter` desires to
> > allocate a large
> > number of strings, right?
>
> Yeah, I think sorting in general is a bit tricky, because it inherently
> requires collecting the value for each item. Just thinking about what
> properties an ideal solution would have (which we might not be able to
> get all of):
>
>   - if we're sorting by something numeric (e.g., an committer
>     timestamp), we should avoid forming it into a string at all
>
>   - if the sort item requires work to extract that overlaps with the
>     output format (e.g., sorting by authordate and showing author name
>     in the format, both of which require parsing the author ident line
>     of a commit), ideally we'd just do that work once per ref/object.
>

Yes i can understand.

>   - if we are sorting, obviously we have to hold some amount of data for
>     each item in memory all at once (since we have to get data on the
>     sort properties for each, and then sort the result). So we'd
>     probably need at least some allocation per ref anyway, and an extra
>     string isn't too bad. But if we're not sorting, then it would be
>     nice to consider one ref/object at a time, which lets us keep our
>     peak memory usage lower, reuse output buffers, etc.
>

Yes, storing these strings in memory is beneficial for sorting.

> I think some of those are in competition with each other. Minimizing
> work shared between the sorting and format steps means keeping more data
> in memory. So it might be sensible to just treat them totally
> independently, and not worry about sharing work (I haven't looked at how
> ref-filter does this now).  TBH, I care a lot less about making the
> "sorting" case fast than I do about making sure that if we _aren't_
> sorting, we go as fast as possible.
>

Okay, so we just focus on the "nosort" case.
I am thinking about finding those cases that git do not need sort and we can
make a flag like "nosort = 1", and then use this "nosort" flag in
`ref-filter` to do the
string copy optimization what we want.
But the problem now is that `git for-each-ref` itself does not support
`--un-sort`,
and it have a default sort order by `refname`. I suspect that there are no
unsorted situation here for us to improve (Any other command call
`ref_array_sort()`
will also have similar situation, and it seem cause a little memory leak, the
ref_sorting entries in sorting_tail aren't free, right?)

> -Peff

--
ZheNing Hu