[this is a resend of some comments I made deep in another thread on rename limiting; I wanted to get your comments, Junio, but I was afraid you didn't see it buried in the mess. If you have already read it and have nothing to say, just tell me to shut up. I was able to get 1000% speedup on a (perhaps pathological) diff, and I suspect there may be more speedups possible in the spanhash lookups. So I think it's worth pursuing.] Hmm. Actually, doing some profiling, it looks like about 75% of the time is spent not in the O(n^2) comparison, but in generating the hash fingerprints of each file. There seems to be a serious performance problem in diffcore-rename. There is infrastructure to cache the "cnt_data" member of each filespec, but it never gets used because we immediately free the filespec data after use. Oops. With this patch: diff --git a/diffcore-rename.c b/diffcore-rename.c index 6bde439..531a844 100644 --- a/diffcore-rename.c +++ b/diffcore-rename.c @@ -362,10 +362,7 @@ void diffcore_rename(struct diff_options *options) m->score = estimate_similarity(one, two, minimum_score); m->name_score = basename_same(one, two); - diff_free_filespec_data(one); } - /* We do not need the text anymore */ - diff_free_filespec_data(two); dst_cnt++; } /* cost matrix sorted by most to least similar pair */ My 20-minute diff becomes a 2-minute diff. The downside is that the memory usage is much increased (for obvious reasons, it should increase by the dataset size, since we are keeping pointers to the data around -- in my case, around 1G extra). However, keeping around _just_ the cnt_data caused only about 100M of extra memory consumption (and gave the same performance boost). Of course, 2 minutes for a git-status is still way too slow, so there we might still need a limiter. It also looks like 57% of my time is spent in spanhash_find, and another 29% in diffcore_count_changes. The spanhash data structure is a bit confusing. At first, it looked like we were doing a linear search for a matching hash, but it's not quite, since we seem to start at some magic spot based on the hashval we're looking up. But it seems to be an array of (hash, count) pairs for each file. Is there a reason not to use a hash table mapping hash -> count? That would make insertion and lookup O(1), presumably at the cost of a bit more memory (since each filespec would have the full table). Junio, can you shed some light on that decision? -Peff - To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
