[Interest] Can a view somehow notify a model of which rows it is going to show on screen?

[Mark Gaiser]

On Fri, Apr 25, 2014 at 2:35 PM, Jan Kundrát <jkt at flaska.net> wrote:
> On Tuesday, 22 April 2014 23:51:07 CEST, Mark Gaiser wrote:
>> There we have it. All visible item indexes accurately known. :)
>
> The model does not know if there's any view attached to it, though.

Now it does so i'm breaking the MVC principle a bit for better
performance. Oops ;)
>
>> Usecase:
>> If you only look at small folders then this definitely is premature
>> optimizations since the "regular sorting" will do just fine there.
>> Bit if you look at insanely massive folders with 500.000 items. All
>> stuffed in one ListView (where you only see ~50 or so). Sorting that
>> by string in a natural way (using QCollator) is very slow. In my case
>> ~6 seconds.
>
> Six seconds to sort 500k items doing a locale-aware string comparison
> sounds like a bug. Check this:
>
> svist ~ # locale | grep CTYPE
> LC_CTYPE=cs_CZ.utf8
> svist ~ # wc -l /var/log/messages
> 763524 /var/log/messages
> svist ~ # time sort /var/log/messages > /dev/null
>
> real    0m0.244s
> user    0m0.555s
> sys     0m0.055s
>
> 763k items just got sorted in a fraction of a second, including (cached) IO
> and printing to stdout. Sure, these items were actually presorted (my log
> was the easiest think to work on, and it's ordered already), but if you
> need six seconds for something so trivial, your program has a bug.
>
> Use a profiler. Take a look at where the time is spent. You mentioned "a
> folder" -- are you doing stuff like directory IO in there, and including
> that in your measurements? Again, profile, measure, do not guess. And do
> not try to fight file IO by doing partial sorts.

Your testcase is wrong :)
I'm testing "natural sorting". Aka, the stuff that turns:
4.txt
2.txt
10.txt
0.txt
8.txt

into
0.txt
2.txt
4.txt
8.txt
10.txt

For that i'm using QCollator with setNumericMode(true). This exact
option is what makes a natural sort slow. There is nothing that can be
done about it other then reducing the number of items to sort.
I promise you, if you run your same test with this then it will be a
lot slower :)
>
>> I know that giving a ListView a task with that many items
>> is insane and in most cases you're probably better of splitting the
>> data in chunks.
>
> That said, introducing deliberate chunking is also ugly from the user's
> POV.
>
> Cheers,
> Jan
>
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