[db48x]

No. Their reputation for poor performance mostly arises because the default data structure is a singly–linked list. Although the language makes these lists extremely convenient for the programmer to use, linked lists invariably result in terrible cache locality. High–performance systems that are written in lisp (and they do exist), achieve that performance by avoiding lists and using vectors and arenas instead.

Of course, it must be said that this performance penalty rarely matters in practice. For most programs, the limiting factor is not performance but the cost of implementation and maintenance. Lisp has has many powerful and convenient features that make implementation faster and surer than in any other language, and which usually greatly lower the cost of maintenance as well. Thus by writing a new program in Lisp you can get it off the ground and earning you money faster than you could in almost any other language. You can also add features more cheaply as time goes on, allowing you to keep up with your competitors. It is only when your system is nearing completion, and all necessary features have been invented and implemented, that you should think about rewriting select parts of that system in a systems language where more efficiency is possible. Not only will you only understand the system after it is written, but you can measure the actual performance of the system to figure out which parts you need to rewrite, and which parts have acceptable performance already and thus do not need to be rewritten.

[pfdietz]

Cache locality is improved if the Lisp has a compacting garbage collector, but you still need extra memory for the cdr links. Lisp Machines had cdr-coding that allowed those to be avoided but I don't think that's practical on stock architectures.

[db48x]

True, but don’t forget that one of the _other_ causes of performance problems in lisp programs is the creation and subsequent collection of garbage. If you know ahead of time which of your data needs to be accessed or created by the most performance–sensitive parts of your program, then you can put that data into vectors to start with and avoid all of the extra work.

[pfdietz]

Long-lived things end up in older generations and aren't traversed much during GC, which is the moral equivalent of being statically allocated.

There's overhead if you mutate old objects in a GCed system due to card marking.

Lisp vectors are vectors of pointers, so there's still an overhead of dereferencing through it. Presumably the objects being pointed to end up compacted together, eventually.

[db48x]

I should have been more clear; I meant using vectors as arena allocators, so that all of your data ends up stored contiguously with no extra indirection.

[mik1998]

> linked lists invariably result in terrible cache locality

With a compacting GC (ie most of the relevant ones) lists often provide better memory locality than arrays. They only lose at a large scale due to having +8 bytes of memory, which makes arrays occasionally cheaper for large sequences.

[gpderetta]

Even with perfect cache locality, a linked list will be significantly slower to traverse than an array because of worse pipelining and parallelization. Partially unrolled lists can help, but at the cost of additional complexity.

Of course not all container accesses require traversal.

[nequo]

Why do lists provide better memory locality than arrays? The array already stores its elements next to each other in memory. The compacting GC helps arrange the elements of the list close to each other too, right? Then wouldn’t the compacting GC at best even out the performance difference for sequential traversal?

[jxf]

> For most programs, the limiting factor is not performance but the cost of implementation and maintenance.

The limiting factor for what? Their commercial success, or something else?

[db48x]

Success, yes. It need not be direct commercial success; lots of programs are written purely for internal use within some larger organization. And even when there are known performance requirements, Lisp is often still a good choice because the performance is handled by specialized hardware. Consider games, for example. Time to market and rapid iteration are both paramount, while performance is taken care of not by writing a fast renderer but by sending everything to the GPU to be rendered on specialized hardware.