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>
