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0 pointsa_t485y ago0 comments

It's much more difficult to prefetch them, though - if you're traversing a large list it misses the cache every time, rather than just the first element.

(someone please correct me if I'm wrong)

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7 comments · 1 top-level

asdfasgasdgasdg5y ago· 6 in thread

That's right. The reason why you see linked lists so much in old C stuff is (IMO) similar to the reason you see null terminated strings (and the associated bugs if you truncate incorrectly). And that's because of one of C's original sin: the failure to include size information in array objects.

There is also the claim that linked lists are better in certain types of performance sensitive code because you can sometimes avoid allocating. I don't fully understand the logic there myself but I trust that there are cases where this is true.

flohofwoe5y ago

The actual reason why you see linked lists (and similar pointer-chasing data structures) so much in old code is because the entire problem of slow-vs-fast memory accesses didn't exist back then. Memory access always happened in a fixed and very low number of CPU cycles (the entire RAM essentially had L1 cache performance). The cpu-memory-gap only slowly started to widen during the 90's when CPU frequencies started to skyrocket and caches had to be added to CPUs. Unfortunately this problem still isn't as widely recognized as it should be in high-level programming circles even today.

asdfasgasdgasdg5y ago

That's a really good point. Thanks for the context!

pushrax5y ago

If your primary operation is inserting at a random location in the list, linked lists are faster than arrays at large sizes. You avoid having to move all the memory after the index you are modifying (to make space for the inserted element).

A linked list also avoids copying the entire array when you need to insert more elements than you have allocated space for.

reificator5y ago

> If your primary operation is inserting at a random location in the list, linked lists are faster than arrays at large sizes. You avoid having to move all the memory after the index you are modifying (to make space for the inserted element).

This is false. Big O notation says it should be true, you'll get marked wrong if you say arrays are faster in your algorithms & data structures final, but when you're running on actual hardware the array is faster at all sizes of n and as n becomes larger so does the gap in performance.

Here is a talk[0] by Bjarne Stroustrop (Creator of C++) that even includes imaginary graphs demonstrating this phenomenon. If you want a visual for what the missing graph was supposed to look like, here's a similar one.[1]

Here's another video[2] by Scott Meyers (Author of Effective C++) that goes into more detail about why this happens.

[0]: https://www.youtube.com/watch?v=YQs6IC-vgmo [1]: https://airspeedvelocity.files.wordpress.com/2015/08/pasted_... [2]: https://www.youtube.com/watch?v=WDIkqP4JbkE

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cma5y ago

Depends on element size and some other stuff, but if it is a singly linked list, and truely random insertion location, iterating to that location is N/2 on average, where inserting and copying the rest of the array is also N/2. Small elements and the array could still be much faster since they are all prefetched during the copy, vs jumping all over memory during the list iteration and potentially stalling on each for ~200 instructions waiting on main memory.

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whimsicalism5y ago

B-tree would be faster than either

j / k navigate · click thread line to collapse

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