Alex Bennée <kernel-hacker@xxxxxxxxxx> writes:
> On 30 May 2013 16:33, Thomas Rast <trast@xxxxxxxxxxx> wrote:
>> Alex Bennée <kernel-hacker@xxxxxxxxxx> writes:
>>
>>> 41.58% git libcrypto.so.1.0.0 [.] sha1_block_data_order_ssse3
>>> 33.62% git libz.so.1.2.3.4 [.] inflate_fast
>>> 10.39% git libz.so.1.2.3.4 [.] adler32
>>> 2.03% git [kernel.kallsyms] [k] clear_page_c
>>
>> Do you have any large blobs in the repo that are referenced directly by
>> a tag?
>
> Most probably. I've certainly done a bunch of releases (which are tagged) were
> the last thing that was updated was an FPGA image.
[...]
>> git-describe should probably be fixed to avoid loading blobs, though I'm
>> not sure off hand if we have any infrastructure to infer the type of a
>> loose object without inflating it. (This could probably be added by
>> inflating only the first block.) We do have this for packed objects, so
>> at least for packed repos there's a speedup to be had.
>
> Will it be loading the blob for every commit it traverses or just ones that hit
> a tag? Why does it need to load the blob at all? Surely the commit
> tree state doesn't
> need to be walked down?
No, my theory is that you tagged *the blobs*. Git supports this.
git-describe needs to look at the commit (if any) obtained by peeling
each tag (i.e. dereferencing tags until it reaches a non-tag). So to do
that, it resolves the tag's referent and loads it. Usually this will be
a commit, in which case it is marked as reached by the tag.
As my example shows, it also resolves tags' referents if they refer to
non-commits, in particular, it will decompress large blobs that are
(directly) referenced by a tag.
Note that while annotated tags provide the type information themselves,
e.g.
$ git cat-file tag junio-gpg-pub
object fe113d3f96636710600c6b02d5fd421fa7e87dd6
type blob
tag junio-gpg-pub
[...]
unannotated tags are simply refs, so it is not enough to just look at
the tag objects' referent type.
I had a brief look around sha1_file.c, in particular sha1_object_info,
and it turns out we lack the "deflate only early part" logic as I
suspected. So that'll have to be fixed first. After that I *think* it
should automatically carry over into the tag readers.
--
Thomas Rast
trast@{inf,student}.ethz.ch
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