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.
> Because this just so happens to exactly reproduce your symptoms:
>
> # in a random git.git
> $ time git describe --debug
> [...]
> real 0m0.390s
> user 0m0.037s
> sys 0m0.011s
> $ git tag big1 $(dd if=/dev/urandom bs=1M count=512 | git hash-object -w --stdin)
> 512+0 records in
> 512+0 records out
> 536870912 bytes (537 MB) copied, 45.5088 s, 11.8 MB/s
> $ time git describe --debug
> [...]
> real 0m1.875s
> user 0m1.738s
> sys 0m0.129s
> $ git tag big2 $(dd if=/dev/urandom bs=1M count=512 | git hash-object -w --stdin)
> 512+0 records in
> 512+0 records out
> 536870912 bytes (537 MB) copied, 44.972 s, 11.9 MB/s
> $ time git describe --debugsuche zur Beschreibung von HEAD
> [...]
> real 0m3.620s
> user 0m3.357s
> sys 0m0.248s
>
> (I actually ran the git-describe invocations more than once to ensure
> that they are again cache-hot.)
That looks pretty promising as a replication.
> 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?
>
> --
> Thomas Rast
> trast@{inf,student}.ethz.ch
--
Alex, homepage: http://www.bennee.com/~alex/
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