Caching local time zone

Lubos Lunak l.lunak at
Thu Sep 21 13:06:49 BST 2006

On Thursday 21 September 2006 11:04, David Jarvie wrote:
> Using KDateTime for date/times in the local time zone can potentially
> result in very frequent calls to KSystemTimeZones::local() which fetches
> and returns the current local system time zone. Depending on the system,
> that function may open and read files, and compare their contents. Even if
> the files are still cached (as would usually be the case on a UNIX system
> when the function calls are made in quick succession), the overhead in file
> system function calls makes me feel uneasy when the function could be
> called many times a second if arrays of date/time values are being
> converted to or from the local time zone.

 Ah, so that's why the class does such awful amount of unnecessary I/O? Note 
that this does not only affect runtime but also KDE startup (since the I/O 
cannot be cached at that time, kernel has pretty bad performance when it 
comes to many small files and of course everybody has the clock applet). Just 
strace -f startkde to see.

 I'd first suggest rethinking the design. Say, a kded module (with the right 
kded startup phase, see kdebase/ksmserver/README) that does whatever initial 
checks needed, saves the current timezone to some config file and KTimeZone 
uses only that. Next startup the module only checks if there have been any 
changes and does nothing if not. Checking for changes while KDE is running 
can be done using a file watch.

> Plainly, the system time zone is not going to change frequently, so I think
> that it would be better to cache it and only reread it periodically. I
> propose to set a timer to refresh it once a minute. Any
> comments/objections?
> Note that this caching applies to the time *zone* (e.g. "Europe/Paris"). It
> has no impact on the handling of daylight saving time shifts which will
> still be handled without any time lag.

Lubos Lunak
KDE developer
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