Re: [PATCH] diff-delta: produce optimal pack data

On Fri, 24 Feb 2006, Junio C Hamano wrote:

Nicolas Pitre [off-list ref] writes:

quoted

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.

Indeed.

I had this patch in my personal tree for a while.  I was
wondring why sometimes progress indication during "Deltifying"
stage stops for literally several seconds, or more.

Note that above I'm saying that _keeping_ adler32 for small blocks is 
even longer.  In other words, for small blocks, the version not using 
adler32 is about 3 times faster.  

I also noticed the significant slowdown after I made the 
improved progress patch. The idea now has to do with detecting 
patological cases and breaking out of them early.

In Linux 2.6 repository, these object pairs take forever to
delta.

        blob 9af06ba723df75fed49f7ccae5b6c9c34bc5115f -> 
        blob dfc9cd58dc065d17030d875d3fea6e7862ede143
        size (491102 -> 496045)
        58 seconds

        blob 4917ec509720a42846d513addc11cbd25e0e3c4f -> 
        blob dfc9cd58dc065d17030d875d3fea6e7862ede143
        size (495831 -> 496045)
        64 seconds

Thanks for this.  I'll see what I can do to tweak the code to better 
cope with those.  Just keep my fourth delta patch in the pu branch for 
now.


Nicolas