Re: cygwin, 44k files: how to commit only index?

On 12/8/06, Junio C Hamano <junkio@cox.net> wrote:
> > yes, except that it'll compare the whole trees. Could I make it stop
> > at first mismatch? "-q|--quiet" for git-diff-index perhaps?
> > It's just not only stat, but also, open, read, mmap (yes, I try to use
> > it for packs) and close are really slow here as well.
>
> That sounds like optimizing for a wrong case -- you expect the
> index to match HEAD and trying to catch mistakes by detecting
> a mismatch, right?

I expect the index to differ from HEAD. The test is to avoid the mistake
of doing an empty commit.

> Having said that, I should point out that it is a low hanging
> fruit to optimize "diff-index --cached" for cases where index
> is expected to mostly match HEAD.
>
> The current code for "diff-index --cached" reads the whole tree
> into the index as stage #1 entries (diff-lib.c::run_diff_index),
> and then compares stage #0 (from the original index contents)
> and stage #1 (the tree parameter from the command line).  Even
> if you stop at the first mismatch, you would already have paid
> the overhead to open and read all tree objects before even
> starting the comparison.

But I don't have to pay for the overhead of comparing all
entries, if I can stop at first mismatch and exit with non-0.
I think it'd make a difference (at least some difference).
But, if we could avoid loading of the entries which
will be never compared anyway, the speedup will be
of course more substantial...

> In 'pu' (jc/diff topic), I have a very generic code to walk the
> index, working tree and zero or more trees in parallel, taking
> advantage of cache-tree.  If somebody is interested to learn the
> internals of git, some of the code could be lifted from there
> and simplified to walk just the index and a single tree, and I
> think that would optimize "diff-index --cached" quite a bit.

Will try to look at it.
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