The diff-delta code can exhibit O(m*n) behavior with some patological
data set where most hash entries end up in the same hash bucket.
The latest code rework reduced the block size making it particularly
vulnerable to this issue, but the issue was always there and can be
triggered regardless of the block size.
This patch does two things:
1) the hashing has been reworked to offer a better distribution to
atenuate the problem a bit, and
2) a limit is imposed to the number of entries that can exist in the
same hash bucket.
Because of the above the code is a bit more expensive on average, but
the problematic samples used to diagnoze the issue are now orders of
magnitude less expensive to process with only a slight loss in
compression.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
This reverts 6b7d25d97b commit.
It turns out that the new algorithm has a really bad corner
case, that literally spends minutes for inputs that takes less
than a quater seconds to delta with the old algorithm. The
resulting delta is 50% smaller which is admirable, but the
performance degradation is simply unacceptable for unconditional
use.
Some example cases are these blobs in Linux 2.6 repository:
4917ec509720a42846d513addc11cbd25e0e3c4f
9af06ba723df75fed49f7ccae5b6c9c34bc5115f
dfc9cd58dc065d17030d875d3fea6e7862ede143
Signed-off-by: Junio C Hamano <junkio@cox.net>
Indexing based on adler32 has a match precision based on the block size
(currently 16). Lowering the block size would produce smaller deltas
but the indexing memory and computing cost increases significantly.
For optimal delta result the indexing block size should be 3 with an
increment of 1 (instead of 16 and 16). With such low params the adler32
becomes a clear overhead increasing the time for git-repack by a factor
of 3. And with such small blocks the adler 32 is not very useful as the
whole of the block bits can be used directly.
This patch replaces the adler32 with an open coded index value based on
3 characters directly. This gives sufficient bits for hashing and
allows for optimal delta with reasonable CPU cycles.
The resulting packs are 6% smaller on average. The increase in CPU time
is about 25%. But this cost is now hidden by the delta reuse patch
while the saving on data transfers is always there.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Testing for realloc and size limit can be done with only one test per
loop. Make it so and fix a theoretical off-by-one comparison error in
the process.
The output buffer memory allocation is also bounded by max_size when
specified.
Finally make some variable unsigned to allow the handling of files up to
4GB in size instead of 2GB.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Since we already depend on zlib, we don't need to define our
own adler32(). Spotted by oprofile.
Signed-off-by: Peter Eriksen <s022018@student.dtu.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>
This patch removes unused remnants of the original xdiff source.
No functional change. Possible tiny speed improvement.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Delta computation with an empty blob used to punt and returned NULL.
This commit allows creation with empty blob; all combination of
empty->empty, empty->something, and something->empty are allowed.
Signed-off-by: Junio C Hamano <junkio@cox.net>
This is a wrap-up patch including all the cleanups I've done to the
delta code and its usage. The most important change is the
factorization of the delta header handling code.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Since the delta data format is not tied to any actual git object
anymore, now is the time to add a small improvement to the delta data
header as it is been done for packed object header. This patch allows
for reducing the delta header of about 2 bytes and makes for simpler
code.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Anything that generates a delta to see if two objects are close usually
isn't interested in the delta ends up being bigger than some specified
size, and this allows us to stop delta generation early when that
happens.
This patch adds the basic library functions to create and replay delta
information. Also included is a test-delta utility to validate the
code.
diff-delta was based on LibXDiff written by Davide Libenzi
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
