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As part of the .eap to .desktop conversion, raster wants me to redo the
relevant caching to suit.  He has made many "eap caching needs
fixing" noises in the recent past, so maybe a fresh start is needed
at this point.  

This is a discussion and design document.  Major parts of it where sent
to the devel list but got no response.  Raster said to put it in here,
and keep it up to date with whatever information and design is relevant.


The problem -

It's really the E_App structure as defined in e_apps.h that may need
caching, no matter what their source.  I say "no matter what their
source" because at the moment the sources are .eap files and .desktop
files, and at some unspecified date in the future .eap files might go
away.  I say "may need caching" because this is a fresh start, do they
really need caching?

To answer that last question we need to look at all the places that use
E_Apps and check to see if those uses are performance critical.  Then
we need to see how much time it takes to load an E_App from it's source.

Grepping through the source code shows that E_App is used in the ibar,
engage, and mbar modules, plus in E itself.  Devilhorns tells me that
mbar will be replaced, so I will ignore that for now.  HandyAndE tells
me that the stand alone version of engage is deprecated, so I can
ignore that.  Both ibar and engage essentially do the same things with
E_App, so I'll speak about those in generic terms.  I'll also try to
avoid spending time fully groking all the code that uses E_Apps, as I
have some time pressure here.  That's why I'm thinking out loud, if I
miss something, other devs will point it out.

Interesting that neither entangle, nor the stand alone eapp_edit came up
in the grep results.  There may be other things that deal with
application icons in their own way.  I'll ignore them for now.  I know
raster will say "engage entangle and such ar not part of corr e, ignre
them".  Entangle and the stand alone eapp_edit are now obsolete, as they
where not included in the .desktop update.

Ibar and engage both display a handful of application icons, allow the
user to click on those icons to start the application.  Ibar runs a
"starting application" animation, and engage keeps track of open
windows that belong to all applications, but tries to keep the open
window icons next to the application icon if it has one for that
application.  I don't know if ibar uses the freedesktop.org startup
notification spec, some other method, or just runs the animation for an
arbitrary time.  Either way, there are three operations in these
modules that use E_Apps - show the icon, start the application, and
match newly created windows with the E_App.

E also needs to do those three operations, and more.  Menus, winlist,
pager, exebuf, and borders all need to show the icon.  Menus and exebuf
need to start the application.  When a window is opened, it needs to be
matched to it's E_App to show it's icon and other things.  Exebuf needs
to search through all the E_Apps looking for matches to the partially
typed in command.  EFM needs to show an icon for any .eap or .desktop it
is currently displaying, maybe needs to run the application.  The
built in eap editor needs to read, display, and save all the
application icon information.

These operations fall into two distinct categories - dealing with one
E_App at a time, or searching through all E_Apps with arbitrary search
criteria.  One at a time is often done in user time (the user only
operates one icon at a time) so caching is not so important there.
Searching through all E_Apps is an expensive operation that should be
optimised as much as possible.  Note that I count the showing of menus
as the later category, as the user may want to quickly access many sub
menus full of E_Apps.

Since the source of the image data used for the icon can be something
that needs time to decode, at the times when lots of icons are being
used at once (exebuf and menus mostly) we probably want that image data
cached for speed as well.  The searching through all E_Apps can occur
on data that is part of the original file that is the source, so an in
memory search will be a lot faster than any file searching.

So the problem is - as fast as possible search through all the E_Apps,
no matter what their source, and give access to all the information
they have ,including image data.  Since this is useful in the "one at a
time" category, might as well use the same method there to.


The issues with possible solutions -

An in memory cache of all E_App information is needed.  Since the
information that could be used to search this cache can be quite
arbitrary, hashes and other such structures may not be useful.  On the
other hand, some sort of index mechanism may be useful for the most
common searches, and those that need to be quickest.

Lets try to come up with some numbers.  The eap torture test involves
over two thousand .eaps, and that's from a full install of SuSE 9.3
Pro.  Certain distros have just as many applications, or more, and they
are growing all the time.  120x120 pixels is probably the largest
commonly used icon size, 32 bits per pixel to include the alpha
channel.  I have seen .desktop files up to 25 KB in size, but that
includes all the translations for name, comment, and keywords,
something that is not currently stored in E_App.  A much more common
size is 2 - 4 KB, but that still mostly is for the name translations.
Note, .desktop files only include the name of the icon, not the image
data itself.  I use the .desktop files for this part of the discussion
because they include all the non image data that is typically in an
E_App.  1 KB is probably maximum storage requirement for non image,
single translation E_App data.  That could be a maximum of 56 KB per
E_App, and possibly 100 MB for the entire cache including image data.
Ouch, no wonder raster suggested mmaping the cache file.

Note, this is a likely maximum for those people that install two
thousand applications and like really big icons.  I have over two
thousand applications installed, and I like the icons in engage to zoom
out really big.  B-)  Typical users will require a much smaller amount
of data.

The original caching strategy uses multiple caches, on a per directory
basis, but still has a single cache for the all directory, the one with
all the .eaps in it.  raster has mentioned that a single cache might be
a better solution.  With .desktop files scattered all over the place, a
single cache is more desirable for searching purposes.

This does raise some interesting issues.  While .eap files have in the
past been per user, the majority of .desktop files are likely to be
system files, mostly unchanging, but with new ones added whenever new
software is installed.  The user can create their own, and override
system ones.  There is also the translations to deal with.  Do we
include only one translation in the cache?  Do we have a system cache
and per user caches that allow overrides?  Do we really need image data
in the cache?  Although the freedesktop.org spec says that the user
chooses a single icon size, in practise the various things that render
icons render them at different sizes.

There is the cache update strategy to consider.  The current one causes
problems.  Raster has thought about changing to no auto update, but with
a manually triggered update.  I would prefer automatic updates that
work smoothly and without great pauses in the operation of the wm, as
that gives the users less to complain about.  Maybe we can make use of
the thumb nailing stuff?

Answers to these questions will help choose a caching mechanism.


-----------------------------------------------------------------------

ecore_desktop_paths

USED BY:
     All of the ecore_desktop code.
WHEN:
     At startup.  They hardly ever change.
ISSUES:
    The use of GNOME and KDE apps to figure out some of the paths. 
    They are curently commented out, and guesses are made.
SOLUTION:
    Go with the guesses, run the apps later to correct the guesses.
    Cache the results to disk.

-----------------------------------------------------------------------

ecore_desktop_menu

USED BY:
    "Applications" menu and "Applications" config dialog.
WHEN:
    The menus are parsed at first startup, then manually by the dialog.
ISSUES:
    When new applications are installed, will need to update.
SOLUTION:
    Parse them during idle time.  Monitor relevant directories.
    ~/.e/e/applications/menu/all is the results cached to disk.

-----------------------------------------------------------------------

icon themes

USED BY:
    The icon theme config dialog.
WHEN:
    When the icon theme config dialog is started.
ISSUES:
    The actual icon theme currently in use is a small amount of info that
    can always be in ram, and is quick to get at startup or theme change.
    It's the complete list of themes that are currently installed that can 
    take ages to find.  The list itself should also be small.
SOLUTION:
    Find them during idle time, and finish the job async when the dialog is
    started.  Monitor the icon paths for the installation of new themes.

-----------------------------------------------------------------------

icons

USED BY:
    Whenever an E_App is displayed and others.
WHEN:
    On demand.
ISSUES:
    Combined .edj and FDO searching. FDO searching can take a long time.
    Things that display lots of icons wont want to wait for all those 
    complex searches.
SOLUTION:
    e_app_icon_add() should be used everywhere, and it should register a 
    rectangle for use by the icon.  The caller then shows an empty icon.
    A thumbnailing style process then does all the searching, and does 
    the fm2 bouncy icon thing when the icon is found.
    
    The results of the search should be cached somewhere on disk for 
    future reference.  That needs to be nuked when the user changes 
    icon theme.  Changing the icon theme currently does not update all 
    displayed icons.

-----------------------------------------------------------------------

.desktop files

USED BY:
    E_Apps and others.
WHEN:
    Whenever the E_App needs to be read from disk.
ISSUES:
    Currently a fairly simple in memory hash of them is kept.  Since 
    each Ecore_Desktop holds a hash of the fields, this can be 
    expensive in memory.  Also, we would like a pre parsed binary on
    disk cache for the sheer speed.  ~/.e/e/applications/all is a 
    directory with all of them that are for E_Apps, some are just links 
    to system files. 
SOLUTION:

-----------------------------------------------------------------------

E_Apps

USED BY:
    Menus, bars, borders, exebuf, lots of other places.
WHEN:
    Currently all read in at startup time.  Should change this to an 
    idle time task.  Still need to read in them all near startup time 
    though.  WE need all of them for searching.
ISSUES:
    The current eap caching code is not up to scratch, and the .desktop 
    conversion didn't help.  It does hide some other bugs though, so that 
    needs to be cleaned up first.
    
    We need quick sorted access to all Apps on the system.  Things like 
    menus and bars need quick access to the ones in their .order files.
    As mentioned above, we need quick searching of them all.
SOLUTION:
    Nuke the existing cache code.  Replace with...