Re: [PATCH v13 04/15] mm/damon: Implement region based sampling

[Leonard Foerster]

On 2020-05-25T11:15:01+02:00 SeongJae Park <sjpark@xxxxxxxxxx> wrote:

> From: SeongJae Park <sjpark@xxxxxxxxx>
> 
> This commit implements DAMON's basic access check and region based
> sampling mechanisms.  This change would seems make no sense, mainly
> because it is only a part of the DAMON's logics.  Following two commits
> will make more sense.
> 
> Basic Access Check
> ------------------
> 
> DAMON basically reports what pages are how frequently accessed.  Note
> that the frequency is not an absolute number of accesses, but a relative
> frequency among the pages of the target workloads.
> 
> Users can control the resolution of the reports by setting two time
> intervals, ``sampling interval`` and ``aggregation interval``.  In
> detail, DAMON checks access to each page per ``sampling interval``,
> aggregates the results (counts the number of the accesses to each page),
> and reports the aggregated results per ``aggregation interval``.  For
> the access check of each page, DAMON uses the Accessed bits of PTEs.
> 
> This is thus similar to common periodic access checks based access
> tracking mechanisms, which overhead is increasing as the size of the
> target process grows.
> 
> Region Based Sampling
> ---------------------
> 
> To avoid the unbounded increase of the overhead, DAMON groups a number
> of adjacent pages that assumed to have same access frequencies into a
> region.  As long as the assumption (pages in a region have same access
> frequencies) is kept, only one page in the region is required to be
> checked.  Thus, for each ``sampling interval``, DAMON randomly picks one
> page in each region and clears its Accessed bit.  After one more
> ``sampling interval``, DAMON reads the Accessed bit of the page and
> increases the access frequency of the region if the bit has set
> meanwhile.  Therefore, the monitoring overhead is controllable by
> setting the number of regions.
> 
> Nonetheless, this scheme cannot preserve the quality of the output if
> the assumption is not kept.  Following commit will introduce how we can
> make the guarantee with best effort.
> 
> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>

Reviewed-by: Leonard Foerster <foersleo@xxxxxxxxx>