[Development] Views

Иван Комиссаров abbapoh at gmail.com
Thu Jun 6 11:09:17 CEST 2019


I think, your point is wrong. Despite the fact Qt is a GUI toolkit, it should perform well.
Take a look at Qt Item Views. They really sucks in terms of performance.
QAbstractItemModel can have any number of rows/columns (that fits in MAX_INT), but which view can really handle that? None of them!
I had to use a model with 3kk rows in it. Guess what I used to display it? Custom item view.
AFAIK Qt item views still use vectors internally and simply changing the internal container to an std::deque can (needs profiling, though) improve performance since it stores data in chunks. Imagine prepending a row to a table with 3kk rows... well, good luck reallocating a vector in QHeaderView.

Иван Комиссаров

6 июня 2019 г., в 10:17, Vitaly Fanaskov <vitaly.fanaskov at qt.io> написал(а):

>> As a library implementer, you are simply not _allowed_ the freedom to
>> use a convenient tool over the most efficient one. That is, to put it
>> mildly, a disservice to users and a disgrace to the profession of
>> programmers.
> Well, optimization is probably good, but not always, I would say. If 
> your app takes 0.001% less memory and works 0.001% faster then before in 
> some certain configurations... Well, it's probably worthless, unless 
> you've improved google search engine or something like that.
> 
> Qt is GUI framework. Not only, yes, but this is the main purpose. +/- 
> 10MB is almost nothing for GUI apps. Slightly faster lookup/insertions, 
> cache line, proper alignment... Well, nice to have, but when an app 
> spends most of the time on rendering something, it doesn't matter. 
> Highly unlikely will it be a bottle neck.
> 
> The thing is, that we should keep in mind what kind of library Qt is. 
> And also on what devices it runs.
> 
> I would rather have readable code than tiny bit optimized code, but much 
> less readable. Simple and readable code => easy to maintain => easy to 
> extend => easy to add new features. If you app is something for 
> high-frequency trading or should run on devices with super limited 
> resources, just don't use Qt containers. It's not appropriate tool for 
> these cases.
> 
> I don't want to encourage people to write, for example, O(n^2) code 
> instead of O(n). But if you want to "improve" something which is working 
> more or less acceptable... Probably you should put your effort on 
> something else.
> 
>> On 6/6/19 9:05 AM, Mutz, Marc via Development wrote:
>>> On 2019-06-06 08:24, Joerg Bornemann wrote:
>>>> On 6/5/19 5:49 PM, Mutz, Marc via Development wrote:
>>>> 
>>>> As a library implementer, you are simply not _allowed_ the freedom to
>>>> use a convenient tool over the most efficient one. That is, to put it
>>>> mildly, a disservice to users and a disgrace to the profession of
>>>> programmers. 8KiB just to look up a pointer in a map of {string, int}?
>>>> That's 1/4th of the icache size of many processors!
>>> [...]
>>> 
>>> While I agree with this in general... every time we use a sorted vector
>>> as a dictionary replacement we're scattering implementation details all
>>> over the place, creating code that's much harder to read and easier to
>>> make mistakes in (*).
>>> 
>>> Maybe it's time for a general purpose dictionary class based on a sorted
>>> vector?
>> 
>> FTR: More of the same: 
>> https://codereview.qt-project.org/c/qt/qtbase/+/264128 and 
>> https://codereview.qt-project.org/c/qt/qtbase/+/264129
>> 
>> I very strongly object to the notion that a find_if or lower_bound is 
>> harder to read. More code does _not_ equate to less readable, as Qt 
>> over and over has shown. There are different patterns involved than in 
>> using Qt containers, sure, and by all means, if find_if frightens you, 
>> then use a raw for loop, but this stuff is not rocket science. And 
>> depending on how you define 'mistakes', it's just as easy to make a 
>> mistake by forgetting qAsConst() on a Qt container than it is to, say, 
>> combine iterators from different containers.
>> 
>> As for QSortedVector/QFlatMap. There's a reason there's none in the 
>> std, yet, and it has to do with when to sort. In one of the patches 
>> above, we don't need to sort at all, because there're only ever O(10) 
>> elements in there. Sorting, as performed by a QFlatMap would be 
>> overkill there. In another, we don't even store the key, as it's equal 
>> to the position of the element in the array. Sorting the key would be 
>> nonsense. Oftem, you populate the data structure once and then only 
>> perform lookups. In that case, a QFlatMap would waste time sorting 
>> while you don't need it. So, yes, by all means, let's have a QFlatMap, 
>> but it would just be another over-complicated container that people 
>> misuse. Let's, as a community, learn how to use a raw vector (or 
>> array) first, then introduce convenience.
>> 
>> Don't pick a container by it's API. Pick it by how you use it. No-one 
>> would use a RB tree for O(10) items if he had to implement it himself. 
>> You wouldn't even implement one for O(1M) elements if insertions in 
>> the middle are very infrequent. You are CS engineers. What would Knuth 
>> say if he caught you using a RB tree with a static maximum of 10 
>> entries? There's a reason for this. It's horribly slow, and only used 
>> because in very limited circumstances, evenry other container is 
>> _even_ slower. It's a very, very, complex beast. Just because the 
>> compiler writes it for you at nothing more than a mention of 
>> QMap<T,V>, doesn't mean it's less complex. And that complexity doesn't 
>> go away just because you wrap it in a nice API: the compiler has a 
>> hard time with it, and so does the CPU, as evidenced by the O(KiB) 
>> savings involved in each replacement of a QMap with a vector. Let's 
>> also not forget the memory overhead: a QMap<int, int> uses at least 24 
>> bytes of of storage per element. Plus allocation overhead. Some 
>> platforms (Windows? At some point at least?) didn't hand out heap in 
>> less than 64 bytes. That's 64 bytes of memory for 8 bytes of payload. 
>> A vector uses exactly 8. So a map uses anywhere between 3x and 8x more 
>> memory.
>> 
>> Just ask yourself: if you didn't have QMap/std::map or the hashed 
>> versions, what data structure would you use? If the answer _actually_ 
>> is "a RB tree (because I really need to insert and remove and lookup 
>> roughly the same number of times", then fine, go use QMap. If it 
>> _actually_ is "a hash table", then consider QHash/unorderd_map. Or 
>> maybe an Open Addressing hash table would be better? BTW: with vector, 
>> you can even implement a Heap (std::make/push/pop_heap).
>> 
>> There's no replacement for thinking here. There's no data structure 
>> that will work best in all cases.
>> 
>> Thanks,
>> Marc
>> _______________________________________________
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>> Development at qt-project.org
>> https://lists.qt-project.org/listinfo/development
> 
> -- 
> Best Regards,
> 
> Fanaskov Vitaly
> Senior Software Engineer
> 
> The Qt Company / Qt Quick and Widgets Team
> 
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