[Python-ideas] Length hinting and preallocation for container types

Antoine Pitrou solipsis at pitrou.net
Fri Mar 8 11:07:31 CET 2013

Le Fri, 08 Mar 2013 08:16:59 +0900,
"Stephen J. Turnbull" <stephen at xemacs.org>
a écrit :
>  > > But why do you think allocation is slow in the general case?
>  > > Sure, it involves system calls which indeed do slow things down.
>  > 
>  > It depends what one calls a system call. A memory allocation
>  > shouldn't always incur a call to the kernel. Library calls can be
>  > quite fast.
> If the process itself is growing, eventually it does.

I think most allocators would request big chunks of memory from the
kernel, and then carve out the small blocks requested by the user from
that. Therefore, my intuition is that a long-running process, if not
leaky, should end up not stressing mmap / sbrk system calls too much.

> And I suspect that Alex (and Christian!) would consider saving a few
> CPU cycles in an inner loop very important.

They probably would, but that is not a design point for Python.



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