Re: [PATCH v3 0/4] mm/ksm: Add ksm advisor

Andrew Morton
2023-12-05T20:49:06Z

On Mon, 4 Dec 2023 15:49:02 -0800 Stefan Roesch <shr@devkernel.io> 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.

Dumb question time. Can this be done in userspace? Presumably this
will require exposing some additional kernel state to userspace.

> 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 limit CPU resource consumption
>
> 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_max_cpu
> upper limit for the cpu usage in percent of the ksmd background thread
>
> The initial value and the max value for the pages_to_scan parameter can
> be limited with:
> - ksm_advisor_min_pages
> minimum value for pages_to_scan per batch
> - ksm_advisor_max_pages
> maximum value for pages_to_scan per batch

Should these be called ksm_advisor_min_pages_to_scan?

> The default settings for the above two parameters should be suitable for
> most workloads.
>
> The parameters are exposed as knobs in /sys/kernel/mm/ksm. By default the
> scan time advisor is disabled.

Disabling it will reduce the effectiveness of testing. What are the
risks of defaulting to "on"?

> Currently there are two advisors:
> - none and
> - scan-time.
>
> Resource savings
> =================
> Tests with various workloads have shown considerable CPU savings. Most
> of the workloads I have investigated have more candidate pages during
> startup. Once the workload is stable in terms of memory, the number of
> candidate pages is reduced. Without the advisor, the pages_to_scan needs
> to be sized for the maximum number of candidate pages. So having this
> advisor definitely helps in reducing CPU consumption.
>
> For the instagram workload, the advisor achieves a 25% CPU reduction.

25% of what? What is the overall affect on machine resource
consumption?

> Once the memory is stable, the pages_to_scan parameter gets reduced to
> about 40% of its max value.
>
> The new advisor works especially well if the smart scan feature is also
> enabled.
>
> 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.
>
> Setting cpu limits is easier than setting The pages_to_scan parameter. The
> pages_to_scan parameter is per batch. For the administrator it is difficult
> to set the pages_to_scan parameter.
>
> Tracing
> =======
> A new tracing event has been added for the scan time advisor. The new
> trace event is called ksm_advisor. It reports the scan time, the new
> pages_to_scan setting and the cpu usage of the ksmd background thread.
>
> Other approaches
> =================
>
> Approach 1: Adapt pages_to_scan after processing each batch. If KSM
> merges pages, increase the scan rate, if less KSM pages, reduce the
> the pages_to_scan rate. This doesn't work too well. While it increases
> the pages_to_scan for a short period, but generally it ends up with a
> too low pages_to_scan rate.
>
> Approach 2: Adapt pages_to_scan after each scan. The problem with that
> approach is that the calculated scan rate tends to be high. The more
> aggressive KSM scans, the more pages it can de-duplicate.
>
> There have been earlier attempts at an advisor:
> propose auto-run mode of ksm and its tests
> (https://marc.info/?l=linux-mm&m=166029880214485&w=2)