On Wed, Aug 19, 2020 at 03:55:47PM +0000, David Laight wrote:
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.
You'd be surprised. We replaced the custom PID allocator with the
generic IDR allocator a few years ago and got a pretty decent speedup.
If you think you can do better, then submit patches. You have to support
all the existing use cases, of course.
