On Fri, Jul 19, 2019 at 04:37:42PM +0200, Oleg Nesterov wrote: > On 07/19, Peter Zijlstra wrote:
> > But I'm still confused, since in the long run, it should still end up > > with a proportionally divided user/system, irrespective of some short > > term wobblies. > > Why? > > Yes, statistically the numbers are proportionally divided. This; due to the loss in precision the distribution is like a step function around the actual s:u ratio line, but on average it still is s:u. Even if it were a perfect function, we'd still see increments in stime even if the current program state never does syscalls, simply because it needs to stay on that s:u line. > but you will (probably) never see the real stime == 1000 && utime == 10000 > numbers if you watch incrementally. See, there are no 'real' stime and utime numbers. What we have are user and system samples -- tick based. If the tick lands in the kernel, we get a system sample, if the tick lands in userspace we get a user sample. What we do have is an accurate (ns) based runtime accounting, and we (re)construct stime and utime from this; we divide the total known runtime in stime and utime pro-rata. Sure, we take a shortcut, it wobbles a bit, but seriously, the samples are inaccurate anyway, so who bloody cares :-) You can construct a program that runs 99% in userspace but has all system samples. All you need to do is make sure you're in a system call when the tick lands. > Just in case... yes I know that these numbers can only "converge" to the > reality, only their sum is correct. But people complain. People always complain, just tell em to go pound sand :-)