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.
