fast-import: treat cat-blob as a delta base hint for next blob
Delta base for blobs is chosen as a previously saved blob. If we treat cat-blob's blob as a delta base for the next blob, nothing is likely to become worse. For fast-import stream producer like svn-fe cat-blob is used like following: - svn-fe reads file delta in svn format - to apply it, svn-fe asks cat-blob 'svn delta base' - applies 'svn delta' to the response - produces a blob command to store the result Currently there is no way for svn-fe to give fast-import a hint on object delta base. While what's requested in cat-blob is most of the time a best delta base possible. Of course, it could be not a good delta base, but we don't know any better one anyway. So do treat cat-blob's result as a delta base for next blob. The profit is nice: 2x to 7x reduction in pack size AND 1.2x to 3x time speedup due to diff_delta being faster on good deltas. git gc --aggressive can compress it even more, by 10% to 70%, utilizing more cpu time, real time and 3 cpu cores. Tested on 213M and 2.7G fast-import streams, resulting packs are 22M and 113M, import time is 7s and 60s, both streams are produced by svn-fe, sniffed and then used as raw input for fast-import. For git-fast-export produced streams there is no change as it doesn't use cat-blob and doesn't try to reorder blobs in some smart way to make successive deltas small. Signed-off-by: NDmitry Ivankov <divanorama@gmail.com> Acked-by: NDavid Barr <davidbarr@google.com> Acked-by: NJonathan Nieder <jrnieder@gmail.com> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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