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
