Re: [PATCH] psi: reduce min window size to 50ms

[Suren Baghdasaryan]

On Fri, Feb 10, 2023 at 4:45 PM Sudarshan Rajagopalan
<quic_sudaraja@xxxxxxxxxxx> wrote:
>
>
> On 2/10/2023 3:03 PM, Suren Baghdasaryan wrote:
> > On Fri, Feb 10, 2023 at 2:31 PM Sudarshan Rajagopalan
> > <quic_sudaraja@xxxxxxxxxxx> wrote:
> >> The PSI mechanism is useful tool to monitor pressure stall
> >> information in the system. Currently, the minimum window size
> >> is set to 500ms. May we know what is the rationale for this?
> > The limit was set to avoid regressions in performance and power
> > consumption if the window is set too small and the system ends up
> > polling too frequently. That said, the limit was chosen based on
> > results of specific experiments which might not represent all
>
> Rightly as you said, the effect on power and performance depends on type
> of the system - embedded systems, or Android mobile, or commercial VMs
> or servers. With higher PSI sampling, it may not be much of power impact
> to embedded systems with low-tier chipsets or performance impact to
> powerful servers.
>
> > usecases. If you want to change this limit, you would need to describe
> > why the new limit is inherently better than the current one (why not
> > higher, why not lower).
>
> This is in regards to the userspace daemon [1] that we are working on,
> that dynamically resizes the VM memory based on PSI memory pressure
> events. With current min window size of 500ms, the PSI monitor sampling
> period would be 50ms. So to detect increase in memory demand in system
> and plug-in memory into VM when pressure goes up, the minimum time the
> process needs to stall for is 50ms before a event can be generated and
> sent out to userspace and the daemon can do actions.
>
> This again I'm talking w.r.t. lightweight embedded systems, where even
> background kswapd/kcompd (which I'm calling it as natural memory
> pressure) in the system would be less than 5-10ms stall. So any stall
> more than 5-10ms would "hint" us that a memory consuming usecase has
> ranB  and memory may need to be plugged in.
>
> So in these cases, having as low as 5ms psimon sampling time would give
> us faster reaction time and daemon can be responsive more quickly. In
> general, this will reduce the malloc latencies significantly.
>
> Pasting here the same excerpt I mentioned in [1].

My question is: why do you think 5ms is the optimal limit here? I want
to avoid a race to the bottom where next time someone can argue that
they would like to detect a stall within a lower period than 5ms.
Technically the limit can be as small as one wants but at some point I
think we should consider the possibility of this being used for a DoS
attack.


>
> "
>
> 4. Detecting increase in memory demand b   when a certain usecase starts
> in VM that does memory allocations, it will stall causing PSI mechanism
> to generate a memory pressure event to userspace. To simply put, when
> pressure increases certain set threshold, it can make educated guess
> that a memory requiring usecase has ran and VM system needs memory to be
> added.
>
> "
>
> [1]
> https://lore.kernel.org/linux-arm-kernel/1bf30145-22a5-cc46-e583-25053460b105@xxxxxxxxxx/T/#m95ccf038c568271e759a277a08b8e44e51e8f90b
>
> > Thanks,
> > Suren.
> >
> >> For lightweight systems such as Linux Embedded Systems, PSI
> >> can be used to monitor and track memory pressure building up
> >> in the system and respond quickly to such memory demands.
> >> Example, the Linux Embedded Systems could be a secondary VM
> >> system which requests for memory from Primary host. With 500ms
> >> window size, the sampling period is 50ms (one-tenth of windwo
> >> size). So the minimum amount of time the process needs to stall,
> >> so that a PSI event can be generated and actions can be done
> >> is 50ms. This reaction time can be much reduced by reducing the
> >> sampling time (by reducing window size), so that responses to
> >> such memory pressures in system can be serviced much quicker.
> >>
> >> Please let us know your thoughts on reducing window size to 50ms.
> >>
> >> Sudarshan Rajagopalan (1):
> >>    psi: reduce min window size to 50ms
> >>
> >>   kernel/sched/psi.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> --
> >> 2.7.4
> >>