Re: ftx font driver [Re: Low redisplay performance (23 regression)]

[YAMAMOTO Mitsuharu]

>>>>> On Thu, 23 Apr 2009 10:01:12 +0900, Kenichi Handa <address@hidden> said:

>>> I haven't compared Emacs 22 and 23, but from a subjective point of
>>> view I have noticed that latest Emacs 23 feels slow and updates
>>> too much for a modern computer. Some particular problems I noticed
>>> randomly:

>> Which font backend driver are you using?  You can check it with
>> (cdr (assq 'font-backend (frame-parameters))).  I found the ftx
>> font driver was much slower than the xft one partly because of
>> repeated calculations of font metrics.  (So I added some cache code
>> to ftcrfont.c in my latest cairo patch posted here).

> Please exlain why it calculates font metrics repeatedly.

> As ftx font driver is not used by default on any platforms, it is
> not tested well and I myself don't remember the code well.

There might be the case that the Xft library is not installed, does
not have a sufficient version, or not found by configure for some
reasons (e.g., PKG_CONFIG_PATH is not set appropriately).  In such
cases, the ftx font driver is selected as a default.  Of course, you
can reproduce such a situation with --without-xft.

The result of time profiling using Shark.app indicates that
FT_Load_Glyph called from ftfont_text_extents takes much time and
calculates font metrics repeatedly without caching.  In particular,
it's really slow if the font is actually in a gzipped PCF format.

                                     YAMAMOTO Mitsuharu
                                address@hidden

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