KSyCoca, Thread safety, and Cache invalidation

[Milian Wolff]

On Thursday, July 30, 2015 09:23:00 PM Mark Gaiser wrote:
> On Tue, Jul 28, 2015 at 4:28 PM, David Faure <faure at kde.org> wrote:
> > On Tuesday 14 July 2015 19:28:58 Thomas Lübking wrote:
> > > On Dienstag, 14. Juli 2015 19:00:14 CEST, Milian Wolff wrote:
> > > > On Tuesday, July 14, 2015 11:49:25 AM David Faure wrote:
> > > >> On Friday 26 June 2015 18:03:00 Frank Reininghaus wrote: ...
> > > > 
> > > > 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.
> > > 
> > > Actually, checking currentTime() is already the problem here (causing
> > 
> > the IO for the timezone stuff), see
> > http://marc.info/?l=kde-core-devel&m=143533622526705&w=1 (the 1st
> > paragraph part of my comment somehow turned into a second-level quote)
> > 
> > > Comparing a monotic timer (QElapsedTime) is however - at least on linux
> > 
> > - close to cost free PLUS: one can have an entirely free "do not check
> > again during this event cycle" flag.
> > 
> > I used QElapsedTimer in https://codereview.qt-project.org/122247
> > and the resulting performance improvements are amazing.
> > kbuildsycoca5 is twice faster.
> 
> Ohh, interesting! You're optimizing stuff :D
> 
> I just ran "kbuildsycoca5 --noincremental" through kcachegrind without your
> patch to Qt and i do see one other quite big hot spot you might be
> interested in.
> In my case the profile collected 1.147.572.920 instructions. Nearly half of
> all those instructions (516.700.823) are spend in QMimeType::inherits. The
> cost per QMimeType::inherits call is 52.692 instructions (it's being called
> 9806 times). That function is heavy! I don't know exactly what it's doing
> since Qt is build in release mode without debug symbols (system package) so
> i only see lots of cycles being spend there, not what they are doing.
> 
> You can ignore this message if QMimeType::inherits internally calls the
> method you optimized thus this function would be greatly improved already.
> Internally it's calling this function:
> http://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/mimetypes/qmimedatabas
> e.cpp#n216

It's improved by his patch, see https://codereview.qt-project.org/#/c/122247/ and the benchmark results 
therein.

Bye

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