[scaled-font] Lean and mean global glyph cache.

Jeff Muizelaar pointed out that the severe overallocation implicit in the
current version of the glyph cache is obnoxious and prevents him from
accepting the trunk into Mozilla. Jeff captured a trace of scaled font
and glyph usage during a tp run and presented his analysis in
http://lists.cairographics.org/archives/cairo/2009-March/016706.html

Using that data, the design was changed to allocate pages of glyphs from a
capped global pool but with per-font hash tables. This should allow the
glyph cache to have tight memory bounds with fair allocation according to
usage. Note that both the old design and the 1.8 glyph cache had
essentially unbounded memory constraints, since each scaled font could
cache up to 256 glyphs (1.8) or had a reserved page (old), with no limit
on the number of active fonts. Currently the eviction policy is a simple
random strategy, this gives a 'fair' allotment of the cache, but a LRU
variant might perform better.

On a sample run of firefox-3.0.7 perusing BBC news in 32 languages:
1.8:  cache allocation 8190x, ~1.2 MiB; elapsed 88.2s
old:  cache allocation 771x, ~13.8 MiB; elapsed 81.7s
lean: cache allocation 433x,  ~1.8 MiB; elapsed 82.4s

1 files changed, 4 insertions, 2 deletions

diff --git a/src/cairo-scaled-font-private.h b/src/cairo-scaled-font-private.h
index 89820c8f..f6c97488 100644
--- a/src/cairo-scaled-font-private.h
+++ b/src/cairo-scaled-font-private.h
@@ -106,8 +106,10 @@ struct _cairo_scaled_font {
     /* The mutex protects modification to all subsequent fields. */
     cairo_mutex_t mutex;
 
-    int cache_frozen;
-    cairo_scaled_glyph_page_t *mru_page;
+    cairo_hash_table_t *glyphs;
+    cairo_scaled_glyph_page_t *glyph_pages;
+    cairo_bool_t cache_frozen;
+    cairo_bool_t global_cache_frozen;
 
     /*
      * One surface backend may store data in each glyph.