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upload-pack: use peel_ref for ref advertisements
When upload-pack advertises refs, we attempt to peel tags and advertise the peeled version. We currently hand-roll the tag dereferencing, and use as many optimizations as we can to avoid loading non-tag objects into memory. Not only has peel_ref recently learned these optimizations, too, but it also contains an even more important one: it has access to the "peeled" data from the pack-refs file. That means we can avoid not only loading annotated tags entirely, but also avoid doing any kind of object lookup at all. This cut the CPU time to advertise refs by 50% in the linux-2.6 repo, as measured by: echo 0000 | git-upload-pack . >/dev/null best-of-five, warm cache, objects and refs fully packed: [before] [after] real 0m0.026s real 0m0.013s user 0m0.024s user 0m0.008s sys 0m0.000s sys 0m0.000s Those numbers are irrelevantly small compared to an actual fetch. Here's a larger repo (400K refs, of which 12K are unique, and of which only 107 are unique annotated tags): [before] [after] real 0m0.704s real 0m0.596s user 0m0.600s user 0m0.496s sys 0m0.096s sys 0m0.092s This shows only a 15% speedup (mostly because it has fewer actual tags to parse), but a larger absolute value (100ms, which isn't a lot compared to a real fetch, but this advertisement happens on every fetch, even if the client is just finding out they are completely up to date). In truly pathological cases, where you have a large number of unique annotated tags, it can make an even bigger difference. Here are the numbers for a linux-2.6 repository that has had every seventh commit tagged (so about 50K tags): [before] [after] real 0m0.443s real 0m0.097s user 0m0.416s user 0m0.080s sys 0m0.024s sys 0m0.012s Signed-off-by: Jeff King <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>
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