[KPhotoAlbum] Fw: Fwd: [Qt bugreports] Updates for QTBUG-75585: Massive performance regression in QML Date object

[Robert Krawitz]

On 9/25/20 3:20 PM, Johannes Zarl-Zierl wrote:
> Am Freitag, 25. September 2020, 03:53:21 CEST schrieb Robert Krawitz:
>> So I've taken a crack at a FastDateTime (wrote the header file and made the
>> changes elsewhere, but haven't actually implemented it).  However, it looks
>> like I don't have ssh keys installed so I can't push now.
>>
>> I've attached the header file for reference.
> 
> Looks good to me...

I've opened an MR for this and assigned it to Johannes:
https://invent.kde.org/graphics/kphotoalbum/-/merge_requests/2

It was quick enough to code up and I did some measurements both with /usr/bin/time and with
kcachegrind.  It sped up startup by about 20-25%, a bit more than I expected based on the
kcachegrind output.  While I was at it, I noticed that the start() and end() methods of ImageDate
returned copied objects rather than const references; the only places in which those were used were
using them in a const manner.

Startup improved from about 12 seconds (with an earlier version of Qt that apparently didn't have
this problem) to 8.75; rebuilding the thumbnail bar took a few seconds with that version of Qt and
about 20 seconds with the current version, now it's probably a few hundred ms (it took about 9
seconds under callgrind, so that's a reasonable estimate).

It would likely be a bit faster yet if Qt were caching this itself, since it apparently (based on
kcachegrind) has to calculate it anyway in the process of generating the QDateTime object.  I
measured QDateTime manipulation as about 6% of the startup time even with this fix; most of that was
calculating the linear time stamp, which is perforce being done twice.  That would not be a
perceptible improvement, but it would be cleaner.

Note that I went simple with it, always calculating the linear timestamp whenever performing a
non-const operation.  It looks like the place where large numbers of date/time objects are created
is reading the database, and the timestamp is always used there to update the date bar, so no sense
getting fancy here with a mutable linear timestamp.