Re: [PATCH 1/2] mm/damon/sysfs: Implement recording feature

[cuiyangpei <cuiyangpei@gmail.com>]

On Fri, Jan 26, 2024 at 12:04:54AM -0800, SeongJae Park wrote:
> On Fri, 26 Jan 2024 14:57:06 +0800 cuiyangpei <cuiyangpei@gmail.com> wrote:
> 
> > On Mon, Jan 22, 2024 at 09:56:11AM -0800, SeongJae Park wrote:
> > > Hi cuiyangpei,
> > > 
> > > On Mon, 22 Jan 2024 13:46:31 +0800 cuiyangpei <cuiyangpei@gmail.com> wrote:
> > > 
> > > > On Sun, Dec 03, 2023 at 07:37:45PM +0000, SeongJae Park wrote:
> > > > > On 2023-12-03T13:43:13+08:00 cuiyangpei <cuiyangpei@gmail.com> wrote:
> > > > > 
> > > > > > On Fri, Dec 01, 2023 at 05:31:12PM +0000, SeongJae Park wrote:
> > > > > > > Hi Cuiyangpei,
> > > > > > > 
> > > > > > > On Fri, 1 Dec 2023 20:25:07 +0800 cuiyangpei <cuiyangpei@gmail.com> wrote:
> > > > > > > 
> > > > > > > > On Thu, Nov 30, 2023 at 07:44:20PM +0000, SeongJae Park wrote:
> > > > > > > > > Hi Cuiyangpei,
> > > > > > > > > 
> > > > > > > > > On Thu, 30 Nov 2023 17:14:26 +0800 cuiyangpei <cuiyangpei@gmail.com> wrote:
> [...]
> > > > Is there any way to catch sampling result immediately after setting the
> > > > "update_schemes_tried_regions" state?
> > > 
> > > There is no way for exactly doing this.  You would need to proactively collect
> > > snapshots while the app is foreground, and use the latest one that collected
> > > before the app goes background, like recording-based approach would do.
> > > 
> > > I think recent DAMON changes might make an alternative approach available,
> > > though.  From v6.7, DAMON provides pseudo-moving-average monitoring result in
> > > sampling interval granualrity, since patchset "mm/damon: provide pseudo-moving
> > > sum based access rate".  And a followup patchset, namely "mm/damon: implement
> > > DAMOS apply intervals", has made DAMOS works in the sampling interval
> > > granualrity.  Both patchsets are merged into v6.7-rc1.
> > > 
> > > Hence, I think you could use 'update_schemes_tried_regions' after you noticed
> > > the app's state transition, with DAMOS apply interval of one sampling interval.
> > > Then you will get the monitoring results after one sampling interval.  Of
> > > course, the snapshot may contain some of background access pattern, but
> > > wouldn't made it changed significantly, unless you set aggregation interval too
> > > short.
> > 
> > All other actions will apply at one sampling interval except for the
> > `stat` action.
> > 
> > We use 'update_schemes_tried_regions' after switch to the background. The 
> > before_damos_apply callback function will only be set when the next aggregation
> > interval arrives. The `tried_regions` will only be updated after setting the 
> > callback function. 
> > 
> > DAMON is still sampling during setting 'update_schemes_tried_regions' to the next
> > aggregation time, which is not what we expected. The pseudo-moving-average
> > monitoring result can reduce nr_accesees inaccuracy, but age is still being modified
> > during this time, so it can't improve this issue.
> > 
> > Please let me know if my understanding is incorrect. Thank you.
> 
> So, 'update_schemes_tried_regions' command is firstly handled by
> 'damon_sysfs_cmd_request_callback()', which is registered as
> after_wmarks_check() and after_aggregation() callback.  Hence
> 'update_schemes_tried_regions' command is still effectively working in
> aggregation interval granularity.  I think this is what you found, right?
> 
Yes.
> If I'm not wrongly understanding your point, I think the concern is valid.  I
> think we should make it works in sampling interval granularity.  I will try to
> make so.  Would that work for your use case?
> 
It's much better than working in aggregation interval.

I have a question. Why does the 'update_schemes_tried_regions' command need to work
in the sampling time or aggregation time? 'update_schemes_tried_regions' is a
relatively special state that updates the regions that corresponding operation scheme.
Can it be separated from other states and controlled by sysfs node to respond immediately
after being written?

> > > 
> > > > Alternatively, can it return the "last_nr_accesses" and "last_age" values in
> > > > tried_regions/<N> directory?
> > > 
> > > This could also be a good alternative in my think.  Nice idea.  But, because
> > > the previously mentioned alternative is already available while this require a
> > > bit small but additional changes, could we check if the previously one make
> > > sense and works first?  We could revisit this idea if it turns out the previous
> > > alternative is not suffice in my opinion.
> > > 
> > Can you consider adding "last_nr_accesses" and "last_age"  two files in
> > 'tried_regions/<N>' directory?
> 
> Actually we don't have 'last_age' field, right?  And in case of
> 'last_nr_accesses', it is a hidden private field, since it is intended to be
> accessed by only DAMON core code.  Making it exposed to user means exposing
> implementation details, and the mechanism that coupled with an exposed
> interface is hard to be changed, so be unflexible.  Hence I'd prefer making
> 'update_schemes_tried_regions' works in sampling interval granularity, more
> than exposing the two information if it works for your use case.
> 
Ok, I get it. 
> Thanks,
> SJ
> 
> [...]