KSyCoca, Thread safety, and Cache invalidation

[David Faure]

On Friday 26 June 2015 18:03:00 Frank Reininghaus wrote:
> checking the mtime frequently can have a bad effect on the performance
> though if it's done incorrectly. There is a bug report about high CPU
> usage in Dolphin if "sort by type" is used:
> 
> https://bugs.kde.org/show_bug.cgi?id=346974
> 
> According to the backtrace, the process is busy inside
> QMimeDataBase::mimeTypeForName(QString) doing time-related things and
> accessing /etc/localtime all the time, probably because of the mtime
> check that you mentioned. I don't know that Qt version was used
> though, so I'm not sure if that still applies to the most recent
> versions.
> 
> (I cannot quite reproduce the problem though because there are other
> parts of Qt's mime type handling which keep my CPU even more busy than
> the time check. I should check if this still happens in recent git
> snapshots of Qt and try to investigate this further...)

The mimetype code in Qt hasn't really changed, you won't get a difference by 
upgrading Qt.

Interesting find, though. Maybe we could check the mtime only if more than N 
seconds have passed since the last check, but I wonder if even checking that 
(e.g. with QElapsedTimer) has a cost in terms of syscalls...

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5