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

CGEL
2022-03-22T09:09:18Z

On Tue, Mar 22, 2022 at 08:55:15AM +0100, David Hildenbrand wrote:
> On 22.03.22 04:12, CGEL wrote:
> > On Mon, Mar 21, 2022 at 04:45:40PM +0100, David Hildenbrand wrote:
> >> On 20.03.22 07:13, CGEL wrote:
> >>> On Fri, Mar 18, 2022 at 09:24:44AM +0100, David Hildenbrand wrote:
> >>>> On 18.03.22 02:41, CGEL wrote:
> >>>>> 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.
> >>>>>
> >>>>
> >>>> I'm not convinced that we want to make or write-fault handler more
> >>>> complicated for such a corner case with an unclear, eventual use case.
> >>>
> >>> IIRC, KSM is designed for VM. But recently we found KSM works well for
> >>> system with many containers(save about 10%~20% of total memroy), and
> >>> container technology is more popular today, so KSM may be used more.
> >>>
> >>> To reduce the impact for write-fault handler, we may write a new function
> >>> with ifdef CONFIG_KSM inside to do this job?
> >>
> >> Maybe we just want to catch the impact of the write-fault handler when
> >> copying more generally?
> >>
> > We know kernel has different kind of COW, some are transparent for user.
> > For example child process may cause COW, and user should not care this
> > performance impact, because it's kernel inside mechanism, user is hard
> > to do something. But KSM is different, user can do the policy tuning in
> > userspace. If we metric all the COW, it may be noise, doesn't it?
>
> Only to some degree I think. The other delays (e.g., SWAP, RECLAIM) are
> also not completely transparent to the user, no? I mean, user space
> might affect them to some degree with some tunables, but it's not
> completely transparent for the user either.
>
> IIRC, we have these sources of COW that result in a r/w anon page (->
> MAP_PRIVATE):
> (1) R/O-mapped, (possibly) shared anonymous page (fork() or KSM)
> (2) R/O-mapped, shared zeropage (e.g., KSM, read-only access to
> unpopulated page in MAP_ANON)
> (3) R/O-mapped, shared file/device/... page that requires a private copy
> on modifications (e.g., MAP_PRIVATE !MAP_ANON)
>
> Note that your current patch won't catch when KSM placed the shared
> zeropage (use_zero_page).
>
> Tracking the overall overhead might be of value I think, and it would
> still allow for determining how much KSM is involved by measuring with
> and without KSM enabled.
>
> >>>
> >>>> IIRC, whenever using KSM you're already agreeing to eventually pay a
> >>>> performance price, and the price heavily depends on other factors in the
> >>>> system. Simply looking at the number of write-faults might already give
> >>>> an indication what changed with KSM being enabled.
> >>>>
> >>> While saying "you're already agreeing to pay a performance price", I think
> >>> this is the shortcoming of KSM that putting off it being used more widely.
> >>> It's not easy for user/app to decide how to use madvise(, ,MADV_MERGEABLE).
> >>
> >> ... and my point is that the metric you're introducing might absolutely
> >> not be expressive for such users playing with MADV_MERGEABLE. IMHO
> >> people will look at actual application performance to figure out what
> >> "harm" will be done, no?
> >>
> >> But I do see value in capturing how many COW we have in general --
> >> either via a counter or via a delay as proposed by you.
> >>
> > Thanks for your affirmative. As describe above, or we add a vm counter:
> > KSM_COW?
>
> As I'm messing with the COW logic lately (e.g., [1]) I'd welcome vm
> counters for all different kind of COW-related events, especially
>
> (1) COW of an anon, !KSM page
> (2) COW of a KSM page
> (3) COW of the shared zeropage
> (4) Reuse instead of COW
>
> I used some VM counters myself to debug/test some of my latest changes.
>
> >>>
> >>> Is there a more easy way to use KSM, enjoying memory saving while minimum
> >>> the performance price for container? We think it's possible, and are working
> >>> for a new patch: provide a knob for cgroup to enable/disable KSM for all tasks
> >>> in this cgroup, so if your container is delay sensitive just leave it, and if
> >>> not you can easy to enable KSM without modify app code.
> >>>
> >>> Before using the new knob, user might want to know the precise impact of KSM.
> >>> I think write-faults is indirection. If indirection is good enough, why we need
> >>> taskstats and PSI? By the way, getdelays support container statistics.
> >>
> >> Would anything speak against making this more generic and capturing the
> >> delay for any COW, not just for KSM?
> > I think we'd better to export data to userspace that is meaning for user.
> > User may no need kernel inside mechanism'data.
>
> Reading Documentation/accounting/delay-accounting.rst I wonder what we
> best put in there.
>
> "Tasks encounter delays in execution when they wait for some kernel
> resource to become available."
>
> I mean, in any COW event we are waiting for the kernel to create a copy.
>
>
> This could be of value even if we add separate VM counters.
>
I think your statement is good enough. I will modify this patch to support
counting all COW events delay, and submit patch to add new VM counters for
different kinds of COW.

Great thanks!
>
>
> [1]
> https://lore.kernel.org/linux-mm/20220315104741.63071-2-david@redhat.com/T/
>
> --
> Thanks,
>
> David / dhildenb