[Unison-hackers] Update detection optimization

[Jerome Vouillon]

On Sun, Jul 26, 2009 at 09:07:09PM -0400, Benjamin Pierce wrote:
> > ...
> > Finally, this is with xferbycopying disabled, otherwise Unison takes
> > 70% of its time filling the hash tables :-(.
> 
> Should we reconsider this preference?  (Did any of your recent changes  
> improve the performance at all?)

I think we should keep it on by default.  My home directory is
probably not a typical replica.  I have large maildir immutable
directories.  For these directories, Unison does nothing but calling
Xferhint.insertEntry.  Thus, it is not surprising that it spends so
much time there.

Also, Xferhing.insertEntry is now more than twice as fast. (I have
kept only one of the two hash tables, and I use a faster hash function.)

On my home directory, Unison still spends about 50% of its time during
update detection on filling the hash table, but update detection now
takes less that one second, which is fast enough for me.  For a long
time, I had xferbycopying disabled, and it is now enabled.

-- Jerome