On 08/16/2012 02:00 PM, Thomas Rast wrote:
Michael Haggerty <mhag...@alum.mit.edu> writes:

On 08/16/2012 10:51 AM, Thomas Rast wrote:
I suppose the real problem is that the "true" ordering
is completely obvious as the one ordering that does not require
preprocessing, but ugly to specify in words.  Perhaps we can bikeshed a
little?  How about

    By default, commits are shown in an order that coincides with
    `--date-order` on well-behaved history, but is faster to compute.

Maybe the problem is not the description of the options, but the
options themselves.  Why does the behavior default to some mysterious
order that we don't even want to document?  Only for the sake of
computational efficiency.  This is the tail wagging the dog.

Why not turn the behavior on its head:

* Change the default behavior to be something well-defined, easy to
document, and convenient for humans, such as "topological order with
ties broken by timestamp" or "approximate timestamp order, but
respecting dependencies".

* Add a new option, --arbitrary-order, that explicitly chooses
efficiency instead of a defined order.

I think that would be a rather bad decision, largely because (taking my
git.git as an example):

   $ time git log | head -1
   commit e5e6172f9060c958e3f0d679cd7049d4007eed2c

   real    0m0.033s
   user    0m0.026s
   sys     0m0.007s

   $ time git log --date-order | head -1
   commit e5e6172f9060c958e3f0d679cd7049d4007eed2c

   real    0m0.429s
   user    0m0.359s
   sys     0m0.031s

That is, even in medium-sized projects like git.git, any -order option
incurs a significant preprocessing time until git-log can show the first
commit.  It scales linearly with the number of commits in the range, and
in a linux.git lying around here is already around 3.9s for the same

Thanks for timing this; I didn't realize how costly this would be. Just to make it even more obvious that this performance regression would bite in daily life, consider

    $ time git log -1

    real    0m0.013s
    user    0m0.000s
    sys     0m0.004s

    $ time git log -1 --topo-order

    real    0m0.334s
    user    0m0.316s
    sys     0m0.012s



Michael Haggerty
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