easy login time win

[Havoc Pennington <hp redhat com>]

Hi,

Some of the "boot poster" results from FC3 showed gconf eating a lot of
time during login.

If you do:

time gconftool-2 --config-source=xml:readonly:/etc/gconf/gconf.xml.defaults --direct -R / > /dev/null

that reports only 3-4 seconds on the second run (disk access out of the
picture), but it's still 3-4 seconds. Disk access at least doubles it.

I've been profiling gconf tonight and I believe that most of this is
coming from loading the XML for all the translations of the short and
long descriptions. Only gconf-editor needs these, but they are making
the XML files large. And gmarkup.c does a fairly alarming amount of work
per-character.

I started micro-optimizing gmarkup.c, but that isn't the best approach
here. To get the big win we should be able to just remove this whole
block of work from the login process.

One thought is to do an mmap cache as we have for icon themes, and in
the mmap format be sure to split the short/long descriptions into their
own half of the file. We could then try to avoid ever touching that
half. Some additional hacking inside gconf to keep it from ever asking
for the full schema object may be required to get that far.[1]

There could be other approaches. Of course Future GConf
as described at http://www.gnome.org/projects/gconf/plans.html does not
have this problem, but I think it's worth fixing in Today GConf.

Maybe a good project for someone... should bring noticeable speedups.

One caveat, I haven't profiled gconf during login; I'm assuming that 
gconftool-2 --config-source=xml:readonly:/etc/gconf/gconf.xml.defaults --direct -R / > /dev/null
is a reasonable approximation of what it does at login time. Conceivably
I'm wrong about that.

Havoc

[1] to implement this I'd add a get_default_from_schema() to the backend
vtable and use that instead of the query_value() to get at the schemas,
or something along those lines.