Projects/Summer of Code/2007/Projects/Icon Cache


To improve startup times of KDE applications, I plan to create an icon cache which would store all icons in a common location. This would eliminate the need to scan tens of directories for icon locations, thus reducing disk seeking. And as the cache would consist of only two files, it would often be wholly cached in the memory by the operating system, eliminating disk access completely. Furthermore, the cache enables loading icons directly from SVGs, using any scale, as the SVGs need to be converted into pixmaps only on the first use. Finally, the cache would make it possible to support simple icon compositing, making it much easier for third-party applications to provide icons which always fit into user's current icon theme.

Author: Rivo Laks

Mentor: Aaron Seigo


Code will be available from KDE's SVN server, under /branches/work/soc-iconcache


The icon cache will consist of two files: one will be index and the other will hold the actual data.


Index will be implemented using AVL- or B-tree

Every entry in the index consists of:

  • Hash (e.g. md5) of the icon, computed using:
    • Icon name
    • Icon size
    • Effects applied to the icon?
    • ...
  • offset of this icon's data in the data file


The data file entries have:

  • Icon name
  • Icon size
  • Effects applied to the icon?
  • Statistics?
    • Last used timestamp
    • Number of uses
  • Path of the original icon file?
  • and of course data, probably compressed with e.g. qCompress()
    • is there any advantage in storing uncompressed data? would it make it possible to use mmap() and share the icon data between processes?

Those entries are written directly one after another. As we needn't support deletion of entries, this will be the most simple & fastest way.


To keep the cache's size within reasonable limits, a daemon similar to kio_http_cache_cleaner would periodically check it's size. If the cache is too big, the daemon would rebuild it from scratch, using most recently and frequently used entries until cache's size is 80% or so of the limit. This would avoid having to support on-the-fly deletion of entries and thus make the cache faster. This would

Additionally the daemon can be used to periodically rebuild the cache, arranging it's contents into a more favourable pattern:

  • move most often used icons to the beginning of the file -> might reduce seeking
  • move icons with similar last-used timestamps together -> icons used by single app would be together
    • makes sense only for icons which aren't very frequently used (frequently used ones go to the beginning of the file)

This page was last edited on 31 May 2007, at 13:52. Content is available under Creative Commons License SA 4.0 unless otherwise noted.