Re: [PATCH] delayacct: track delays from ksm cow

CGEL
2022-03-18T01:41:26Z

On Thu, Mar 17, 2022 at 11:05:22AM +0100, David Hildenbrand wrote:
> On 17.03.22 10:48, CGEL wrote:
> > On Thu, Mar 17, 2022 at 09:17:13AM +0100, David Hildenbrand wrote:
> >> On 17.03.22 03:03, CGEL wrote:
> >>> On Wed, Mar 16, 2022 at 03:56:23PM +0100, David Hildenbrand wrote:
> >>>> On 16.03.22 14:34, cgel.zte@gmail.com wrote:
> >>>>> From: Yang Yang <yang.yang29@zte.com.cn>
> >>>>>
> >>>>> Delay accounting does not track the delay of ksm cow. When tasks
> >>>>> have many ksm pages, it may spend a amount of time waiting for ksm
> >>>>> cow.
> >>>>>
> >>>>> To get the impact of tasks in ksm cow, measure the delay when ksm
> >>>>> cow happens. This could help users to decide whether to user ksm
> >>>>> or not.
> >>>>>
> >>>>> Also update tools/accounting/getdelays.c:
> >>>>>
> >>>>> / # ./getdelays -dl -p 231
> >>>>> print delayacct stats ON
> >>>>> listen forever
> >>>>> PID 231
> >>>>>
> >>>>> CPU count real total virtual total delay total delay average
> >>>>> 6247 1859000000 2154070021 1674255063 0.268ms
> >>>>> IO count delay total delay average
> >>>>> 0 0 0ms
> >>>>> SWAP count delay total delay average
> >>>>> 0 0 0ms
> >>>>> RECLAIM count delay total delay average
> >>>>> 0 0 0ms
> >>>>> THRASHING count delay total delay average
> >>>>> 0 0 0ms
> >>>>> KSM count delay total delay average
> >>>>> 3635 271567604 0ms
> >>>>>
> >>>>
> >>>> TBH I'm not sure how particularly helpful this is and if we want this.
> >>>>
> >>> Thanks for replying.
> >>>
> >>> Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want
> >>> save memory, it's a tradeoff by suffering delay on ksm cow. Users can
> >>> get to know how much memory ksm saved by reading
> >>> /sys/kernel/mm/ksm/pages_sharing, but they don't know what the costs of
> >>> ksm cow delay, and this is important of some delay sensitive tasks. If
> >>> users know both saved memory and ksm cow delay, they could better use
> >>> madvise(, , MADV_MERGEABLE).
> >>
> >> But that happens after the effects, no?
> >>
> >> IOW a user already called madvise(, , MADV_MERGEABLE) and then gets the
> >> results.
> >>
> > Image user are developing or porting their applications on experiment
> > machine, they could takes those benchmark as feedback to adjust whether
> > to use madvise(, , MADV_MERGEABLE) or it's range.
>
> And why can't they run it with and without and observe performance using
> existing metrics (or even application-specific metrics?)?
>
>
I think the reason why we need this patch, is just like why we need
swap,reclaim,thrashing getdelay information. When system is complex,
it's hard to precise tell which kernel activity impact the observe
performance or application-specific metrics, preempt? cgroup throttle?
swap? reclaim? IO?

So if we could get the factor's precise impact data, when we are tunning
the factor(for this patch it's ksm), it's more efficient.

> --
> Thanks,
>
> David / dhildenb