thin-pack: try harder to use preferred base objects as 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. preferred
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.

The effect of this change can be seen on two simulated upload-pack
workloads. The first is based on 44 reflog entries from my git.git
origin/master reflog, and represents the packs that kernel.org sent me git
updates for the past month or two. The second workload represents much
larger fetches, going from git's v1.0.0 tag to v1.1.0, then v1.1.0 to
v1.2.0, and so on.

The table below shows the average generated pack size and the average CPU
time consumed for each dataset, both before and after the patch:

                  dataset
            | reflog | tags
---------------------------------
     before | 53358  | 2750977
size  after | 32398  | 2668479
     change |   -39% |      -3%
---------------------------------
     before |  0.18  | 1.12
CPU   after |  0.18  | 1.15
     change |    +0% |      +3%

This patch makes a much bigger difference for packs with a shorter slice
of history (since its effect is seen at the boundaries of the pack) though
it has some benefit even for larger packs.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index c6e2d8766b..8bfe3a6ffb 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;