# Re: \[PATCH RESEND v4\] sched/fair: Add advisory flag for borrowing a timeslice

- From: Peter Zijlstra \<peterz@infradead.org\>
- Date: 2015-01-13 11:25:34 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2015/01/4408651
- Thread: https://ratatoskr.run/linux-api/2014/12/4397446/t.md

```text
On Tue, Dec 23, 2014 at 01:46:58PM -0500, Rik van Riel wrote:
> An uncontended futex is taken without ever going into kernel
> space. Adaptive spinning allows short duration futexes to be
> taken without going into kernel space.

The going into kernel is a red herring afaict, a no-op syscall costs ~180
cycles or something like that. So sure you can do a wee spin (~90 cycles
to try and amortize that), but I really doubt that this is the problem.
```