Re: [PATCH v1 0/4] mm/ksm: Add ksm advisor

[David Hildenbrand]

On 10.10.23 18:02, Stefan Roesch wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
> > On 06.10.23 18:17, Stefan Roesch wrote:
> > > David Hildenbrand <david@xxxxxxxxxx> writes:
> > >
> > > > On 04.10.23 21:02, Stefan Roesch wrote:
> > > > > What is the KSM advisor?
> > > > > =========================
> > > > > The ksm advisor automatically manages the pages_to_scan setting to
> > > > > achieve a target scan time. The target scan time defines how many seconds
> > > > > it should take to scan all the candidate KSM pages. In other words the
> > > > > pages_to_scan rate is changed by the advisor to achieve the target scan
> > > > > time.
> > > > > Why do we need a KSM advisor?
> > > > > ==============================
> > > > > The number of candidate pages for KSM is dynamic. It can often be observed
> > > > > that during the startup of an application more candidate pages need to be
> > > > > processed. Without an advisor the pages_to_scan parameter needs to be
> > > > > sized for the maximum number of candidate pages. With the scan time
> > > > > advisor the pages_to_scan parameter based can be changed based on demand.
> > > > > Algorithm
> > > > > ==========
> > > > > The algorithm calculates the change value based on the target scan time
> > > > > and the previous scan time. To avoid pertubations an exponentially
> > > > > weighted moving average is applied.
> > > > > The algorithm has a max and min
> > > > > value to:
> > > > > - guarantee responsiveness to changes
> > > > > - to avoid to spend too much CPU
> > > > > Parameters to influence the KSM scan advisor
> > > > > =============================================
> > > > > The respective parameters are:
> > > > > - ksm_advisor_mode
> > > > >      0: None (default), 1: scan time advisor
> > > > > - ksm_advisor_target_scan_time
> > > > >      how many seconds a scan should of all candidate pages take
> > > > > - ksm_advisor_min_pages
> > > > >      minimum value for pages_to_scan per batch
> > > > > - ksm_advisor_max_pages
> > > > >      maximum value for pages_to_scan per batch
> > > > > The parameters are exposed as knobs in /sys/kernel/mm/ksm.
> > > > > By default the scan time advisor is disabled.
> > > >
> > > > What would be the main reason to not have this enabled as default?
> > > There might be already exisiting users which directly set pages_to_scan
> > > and tuned the KSM settings accordingly, as the default setting of 100 for
> > > pages_to_scan is too low for typical workloads.
> >
> > Good point.
> >
> > > > IIUC, it is kind-of an auto-tuning of pages_to_scan. Would "auto-tuning"
> > > > describe it better than "advisor" ?
> > > >
> > > > [...]
> > > I'm fine with auto-tune. I was also thinking about that name, but I
> > > chose advisor, its a bit less strong and it needs input from the user.
> >
> > I'm not a native speaker, but "adviser" to me implies that no action is taken,
> > only advises are given :) But again, no native speaker.
> >
> > > > > How is defining a target scan time better?
> > > > > ===========================================
> > > > > For an administrator it is more logical to set a target scan time.. The
> > > > > administrator can determine how many pages are scanned on each scan.
> > > > > Therefore setting a target scan time makes more sense.
> > > > > In addition the administrator might have a good idea about the
> > > > > memory sizing of its respective workloads.
> > > >
> > > > Is there any way you could imagine where we could have this just do something
> > > > reasonable without any user input? IOW, true auto-tuning?
> > > True auto-tuning might be difficult as users might want to be able to
> > > choose how aggressive KSM is. Some might want it to be as aggressive as
> > > possible to get the maximum de-duplication rate. Others might want a
> > > more balanced approach that takes CPU-consumption into consideration.
> > > I guess it depends if you are memory-bound, cpu-bound or both.
> >
> > Agreed, more below.
> >
> > > > I read above:
> > > > > - guarantee responsiveness to changes
> > > > > - to avoid to spend too much CPU
> > > >
> > > > whereby both things are accountable/measurable to use that as the input for
> > > > auto-tuning?
> > > I'm not sure a true auto-tuning can be achieved. I think we need
> > > some input from the user
> > > - How much resources to consume
> > > - How fast memory changes or how stable memory is
> > >     (this we might be able to detect)
> >
> > Setting the pages_to_scan is a bit mystical. Setting upper/lower pages_to_scan
> > bounds is similarly mystical, and highly workload dependent.
> >
> > So I agree that a better abstraction to automatically tune the scanning is
> > reasonable. I wonder if we can let the user give better inputs that are less
> > workload dependent.
> >
> > For example, do we need min/max values for pages_to_scan, or can we replace it
> > by something better to the auto-tuning algorithm?
> >
> > IMHO "target scan time" goes into the right direction, but it can still be
> > fairly workload dependent. Maybe a "max CPU consumption" or sth. like that would
> > similarly help to limit CPU waste, and it could be fairly workload dependent.
>
> I can look into replacing min/max values for pages_to_scan with min/max
> cpu utilization. This might be easier for users to decide on. However I
> still think that we need a target value like scan time to optimize for.

Agreed, it can't be completely automatic.

--
Cheers,

David / dhildenb