Re: thin packs ending up fat
- From
Junio C Hamano <gitster@pobox.com>
- Date
- Jan 13, 2012, 01:31 UTC
- Message-ID
- <7vwr8wz8u9.fsf@alter.siamese.dyndns.org>
- In-Reply-To
- <20120112223234.GA4949@sigill.intra.peff.net>
From: Jeff King <peff@peff.net> Subject: [PATCH] thin-pack: try harder to create delta against preferred base
When creating a pack using objects that reside in existing packs, we try to avoid recomputing futile delta between an object (trg) and a candidate for its base object (src) if they are stored in the same packfile, and trg is not recorded as a delta already. This heuristics makes sense because it is likely that we tried to express trg as a delta based on src but it did not produce a good delta when we created the existing pack.
As the pack heuristics prefer producing delta to remove data, and Linus's law dictates that the size of a file grows over time, we tend to record the newest version of the file as inflated, and older ones as delta against it.
When creating a thin-pack to transfer recent history, it is likely that we will try to send an object that is recorded in full, as it is newer. But the heuristics to avoid recomputing futile delta effectively forbids us from attempting to express such an object as a delta based on another object. Sending an object in full is often more expensive than sending a suboptimal delta based on other objects, and it is even more so if we could use an object we know the receiving end already has (i.e. referred base object) as the delta base.
Tweak the recomputation avoidance logic, so that we do not punt on computing delta against a preferred base object.
Signed-off-by: Junio C Hamano <gitster@pobox.com> ---
builtin/pack-objects.c | 9 +++++++-- 1 files changed, 7 insertions(+), 2 deletions(-)
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c index c6e2d87..8bfe3a6 100644 --- a/builtin/pack-objects.c +++ b/builtin/pack-objects.c @@ -1248,11 +1248,16 @@ static int try_delta(struct unpacked *trg, struct unpacked *src, return -1; /* - * We do not bother to try a delta that we discarded - * on an earlier try, but only when reusing delta data. + * We do not bother to try a delta that we discarded on an + * earlier try, but only when reusing delta data. Note that + * src_entry that is marked as the preferred_base should always + * be considered, as even if we produce a suboptimal delta against + * it, we will still save the transfer cost, as we already know + * the other side has it and we won't send src_entry at all. */ if (reuse_delta && trg_entry->in_pack && trg_entry->in_pack == src_entry->in_pack && + !src_entry->preferred_base && trg_entry->in_pack_type != OBJ_REF_DELTA && trg_entry->in_pack_type != OBJ_OFS_DELTA) return 0;
-- 1.7.9.rc0.53.gc8bc2