On Wed, Nov 22, 2017 at 1:27 PM, Jeff King <peff@xxxxxxxx> wrote:
> On Tue, Nov 21, 2017 at 03:43:36PM -0800, Phil Hord wrote:
>
>> With many thousands of references, a simple `git rev-parse HEAD` may take
>> more than a second to return because it first loads all the refs into
>> memory even though it will never use them.
>
> The overall goal of lazy-loading seems reasonable, but I'm slightly
> confused: how and why does "git rev-parse HEAD" load ref decorations?
>
> Grepping around I find that we mostly load them only when appropriate
> (when the "log" family sees a decorate option, when we see %d/%D in a
> pretty format, or with --simplify-by-decoration in a traversal). And
> poking at "rev-parse HEAD" in gdb seems to confirm that it does not hit
> that function.
Hm. I think I was confused.
I wrote v1 of this patch a few months ago. Clearly I was wrong about
rev-parse being afflicted. We have a script that was suffering and it
uses both "git log --format=%h" and "git rev-parse" to get hashes; I
remember testing both, but I can't find it in my $zsh_history; my
memory and my commit-message must be faulty.
However, "git log" does not need any --decorate option to trigger this lag.
$ git for-each-ref| wc -l
24172
$ time git log --format=%h -1
git log --format=%h -1 0.47s user 0.04s system 99% cpu 0.509 total
I grepped the code just now, too, and I see the same as you, though;
it seems to hold off unless !!decoration_style. Nevertheless, gdb
shows me decoration_style=1 with this command:
GIT_CONFIG=/dev/null cgdb --args git log -1 --format="%h"
Here are timing tests on this repo without this change:
git log --format=%h -1 0.54s user 0.05s system 99% cpu
0.597 total
git log --format=%h -1 --decorate 0.54s user 0.04s system 98% cpu
0.590 total
git log --format=%h%d -1 0.53s user 0.05s system 99% cpu
0.578 total
And the same commands with this change:
git log --format=%h -1 0.01s user 0.01s system 71%
cpu 0.017 total
git log --format=%h -1 --decorate 0.00s user 0.01s system 92%
cpu 0.009 total
git log --format=%h%d -1 0.53s user 0.09s system 88%
cpu 0.699 total
> I have definitely seen "rev-parse HEAD" be O(# of refs), but that is
> mostly attributable to having all the refs packed (and until v2.15.0,
> the packed-refs code would read the whole file into memory).
Hm. Could this be why rev-parse was slow for me? My original problem
showed up on v1.9 (build machine) and I patched it on v2.14.0-rc1.
But, no; testing on 1.9, 2.11 and 2.14 still doesn't show me the lag
in rev-parse. I remain befuddled.
> I've also
> seen unnecessary ref lookups due to replace refs (we load al of the
> packed refs to find out that no, there's nothing in refs/replace).
I haven't seen this in the code, but I have had refs/replace hacks in
the past. Is that enough to wake this up?
Phil
