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authorJeff King <peff@peff.net>2017-02-08 15:53:03 -0500
committerJunio C Hamano <gitster@pobox.com>2017-02-08 15:39:55 -0800
commit41a078c60b82bad4edf9d1bd8e826aae5f020ee5 (patch)
treea9ce65909dbb7521a2776570663fa1e1f60227e3 /object.h
parenta10a17877bcce27b5677b5ac9f1a4e58d5bd8023 (diff)
downloadgit-41a078c60b82bad4edf9d1bd8e826aae5f020ee5.tar.gz
fetch-pack: cache results of for_each_alternate_ref
We may run for_each_alternate_ref() twice, once in find_common() and once in everything_local(). This operation can be expensive, because it involves running a sub-process which must freshly load all of the alternate's refs from disk. Let's cache and reuse the results between the two calls. We can make some optimizations based on the particular use pattern in fetch-pack to keep our memory usage down. The first is that we only care about the sha1s, not the refs themselves. So it's OK to store only the sha1s, and to suppress duplicates. The natural fit would therefore be a sha1_array. However, sha1_array's de-duplication happens only after it has read and sorted all entries. It still stores each duplicate. For an alternate with a large number of refs pointing to the same commits, this is a needless expense. Instead, we'd prefer to eliminate duplicates before putting them in the cache, which implies using a hash. We can further note that fetch-pack will call parse_object() on each alternate sha1. We can therefore keep our cache as a set of pointers to "struct object". That gives us a place to put our "already seen" bit with an optimized hash lookup. And as a bonus, the object stores the sha1 for us, so pointer-to-object is all we need. There are two extra optimizations I didn't do here: - we actually store an array of pointer-to-object. Technically we could just walk the obj_hash table looking for entries with the ALTERNATE flag set (because our use case doesn't care about the order here). But that hash table may be mostly composed of non-ALTERNATE entries, so we'd waste time walking over them. So it would be a slight win in memory use, but a loss in CPU. - the items we pull out of the cache are actual "struct object"s, but then we feed "obj->sha1" to our sub-functions, which promptly call parse_object(). This second parse is cheap, because it starts with lookup_object() and will bail immediately when it sees we've already parsed the object. We could save the extra hash lookup, but it would involve refactoring the functions we call. It may or may not be worth the trouble. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'object.h')
-rw-r--r--object.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/object.h b/object.h
index 614a006756..f52957dcb3 100644
--- a/object.h
+++ b/object.h
@@ -29,7 +29,7 @@ struct object_array {
/*
* object flag allocation:
* revision.h: 0---------10 26
- * fetch-pack.c: 0---4
+ * fetch-pack.c: 0---5
* walker.c: 0-2
* upload-pack.c: 4 11----------------19
* builtin/blame.c: 12-13