RE: [PATCH 00/11] Introduce kernel_clone(), kill _do_fork()

David Laight
2020-08-19T15:55:58Z

From: Matthew Wilcox
> Sent: 19 August 2020 16:45
>
> On Wed, Aug 19, 2020 at 03:41:48PM +0000, David Laight wrote:
> > Does linux have an O(1) (or do I mean o(1)) pid allocator?
> > Or does it have to do a linear scan to find a gap??
>
> O(log(n)). It uses the IDR allocator, so 'n' in this case is the
> number of PIDs currently allocated, and it's log_64 rather than log_2
> (which makes no difference to O() but does make a bit of a difference
> to performance)

Still worse that O(1) - when that is just replacing a variable
with a value read out of an array.
Made pid lookup a trivial O(1) as well.

David

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