Optimize IOLoginData::saveItems by reducing number of allocations

[marksamman]

Note that this is untested, just an idea I got by quickly looking at the function. std::list would allocate for each push, std::vector tends to have a growth strategy where it only allocates when it reaches capacity

[nekiro]

This is interesting. I saw a lot of places in source where we use std::list or pop_front, if I understand correctly if you pop_front, all items in list have to be relocated to index - 1, which does not sound like a good performance thing, so this code should be faster right?

Why reserving only 32 though?

[marksamman]

pop_front on std::list is fine, a similar operation on std::vector would have to relocate all items. This is faster because std::list allocates memory for each push while std::vector allocates in bulk. Iterating over a std::vector and accessing the items is also faster than over a std::list because the items will be in CPU cache, but most of the gain in this PR should be from the reduced amount of memory allocation calls

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On Sat, 24 Oct 2020 at 12:26, nekiro ***@***.***> wrote: This is interesting. I saw a lot of places in the source where we use std::list or pop_front, if I understand correctly if you pop_front all items in list have to be relocated to index - 1, which does not sound like a good performance thing, so this code should be faster right? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3176 (comment)>, or unsubscribe </~https://github.com/notifications/unsubscribe-auth/AAE3NNPX2JCWJDH2Q4DMHNTSMKTVBANCNFSM4SA36AUQ> .

[ranisalt]

I think the biggest gains are moving to a container with contiguous allocation, it's just faster to access. The STL implementation of vector doubles in size when it hits the capacity, so it only avoids 5 small reallocations at best.

Another good point is that a list is just wastes a lot of memory with pointers.

This might be the perfect use case of a std::deque, which is a compromise between a list and a vector. Or even a stack, if ordering is not important.

if I understand correctly if you pop_front, all items in list have to be relocated to index - 1

Wrong, a std::list is a linked list and insertions in any position are constant (provided that you have the iterator, and you always have for front and back).

[DSpeichert]

Looks like this started an avalanche of converting things to use vectors, so we should probably revisit this. LGTM, since there are no better proposals.