Jeff King <peff@xxxxxxxx> 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
