summaryrefslogtreecommitdiff
path: root/Documentation/git-status.txt
diff options
context:
space:
mode:
authorLibravatar Jeff King <peff@peff.net>2015-12-21 01:19:49 -0500
committerLibravatar Junio C Hamano <gitster@pobox.com>2015-12-21 14:36:11 -0800
commitf4015337daf130944393296550dd7d8a9c172325 (patch)
tree03bd226bfc000248e27103960a65121e579d4669 /Documentation/git-status.txt
parentGit 2.0 (diff)
downloadtgif-f4015337daf130944393296550dd7d8a9c172325.tar.xz
pack-revindex: drop hash table
The main entry point to the pack-revindex code is find_pack_revindex(). This calls revindex_for_pack(), which lazily computes and caches the revindex for the pack. We store the cache in a very simple hash table. It's created by init_pack_revindex(), which inserts an entry for every packfile we know about, and we never grow or shrink the hash. If we ever need the revindex for a pack that isn't in the hash, we die() with an internal error. This can lead to a race, because we may load more packs after having called init_pack_revindex(). For example, imagine we have one process which needs to look at the revindex for a variety of objects (e.g., cat-file's "%(objectsize:disk)" format). Simultaneously, git-gc is running, which is doing a `git repack -ad`. We might hit a sequence like: 1. We need the revidx for some packed object. We call find_pack_revindex() and end up in init_pack_revindex() to create the hash table for all packs we know about. 2. We look up another object and can't find it, because the repack has removed the pack it's in. We re-scan the pack directory and find a new pack containing the object. It gets added to our packed_git list. 3. We call find_pack_revindex() for the new object, which hits revindex_for_pack() for our new pack. It can't find the packed_git in the revindex hash, and dies. You could also replace the `repack` above with a push or fetch to create a new pack, though these are less likely (you would have to somehow learn about the new objects to look them up). Prior to 1a6d8b9 (do not discard revindex when re-preparing packfiles, 2014-01-15), this was safe, as we threw away the revindex whenever we re-scanned the pack directory (and thus re-created the revindex hash on the fly). However, we don't want to simply revert that commit, as it was solving a different race. So we have a few options: - We can fix the race in 1a6d8b9 differently, by having the bitmap code look in the revindex hash instead of caching the pointer. But this would introduce a lot of extra hash lookups for common bitmap operations. - We could teach the revindex to dynamically add new packs to the hash table. This would perform the same, but would mean adding extra code to the revindex hash (which currently cannot be resized at all). - We can get rid of the hash table entirely. There is exactly one revindex per pack, so we can just store it in the packed_git struct. Since it's initialized lazily, it does not add to the startup cost. This is the best of both worlds: less code and fewer hash table lookups. The original code likely avoided this in the name of encapsulation. But the packed_git and reverse_index code are fairly intimate already, so it's not much of a loss. This patch implements the final option. It's a minimal conversion that retains the pack_revindex struct. No callers need to change, and we can do further cleanup in a follow-on patch. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'Documentation/git-status.txt')
0 files changed, 0 insertions, 0 deletions