KSyCoca, Thread safety, and Cache invalidation

[Milian Wolff]

On Tuesday, July 14, 2015 11:49:25 AM David Faure wrote:
> 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...

It has. Querying the current time repeatedly is quite costly, and often shows 
up in code that extensively uses timers, or QDateTime::currentDateTime() etc.

But it's probably still cheaper than querying on-disk meta data in the worst 
case, esp. on old rotary disks.

Bye

-- 
Milian Wolff
mail at milianw.de
http://milianw.de
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