Re: [PATCH v9 07/14] mm: multi-gen LRU: exploit locality in rmap

[Barry Song <21cnbao@gmail.com>]

On Thu, Apr 7, 2022 at 3:04 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Wed, Apr 6, 2022 at 8:29 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Wed, Mar 9, 2022 at 3:48 PM Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > Searching the rmap for PTEs mapping each page on an LRU list (to test
> > > and clear the accessed bit) can be expensive because pages from
> > > different VMAs (PA space) are not cache friendly to the rmap (VA
> > > space). For workloads mostly using mapped pages, the rmap has a high
> > > CPU cost in the reclaim path.
> > >
> > > This patch exploits spatial locality to reduce the trips into the
> > > rmap. When shrink_page_list() walks the rmap and finds a young PTE, a
> > > new function lru_gen_look_around() scans at most BITS_PER_LONG-1
> > > adjacent PTEs. On finding another young PTE, it clears the accessed
> > > bit and updates the gen counter of the page mapped by this PTE to
> > > (max_seq%MAX_NR_GENS)+1.
> >
> > Hi Yu,
> > It seems an interesting feature to save the cost of rmap. but will it lead to
> > possible judging of cold pages as hot pages?
> > In case a page is mapped by 20 processes,  and it has been accessed
> > by 5 of them, when we look around one of the 5 processes, the page
> > will be young and this pte is cleared. but we still have 4 ptes which are not
> > cleared. then we don't access the page for a long time, but the 4 uncleared
> > PTEs will still make the page "hot" since they are not cleared, we will find
> > the page is hot either due to look-arounding the 4 processes or rmapping
> > the page later?
>
> Why are the remaining 4 accessed PTEs skipped? The rmap should check
> all the 20 PTEs.

for example page A is the neighbour of page B in process 1, when we do rmap
for B, we look-around and clear A's pte in process 1. but A's ptes are
still set in
process 2,3,4,5.

>
> Even if they were skipped, it doesn't matter. The same argument could
> be made for the rest of 1 millions minus 1 pages that have been timely
> scanned, on a 4GB laptop. The fundamental principle (assumption) of
> MGLRU is never about making the best choices. Nothing can because it's
> impossible to predict the future that well, given the complexity of
> today's workloads, not on a phone, definitely not on a server that
> runs mixed types of workloads. The primary goal is to avoid the worst
> choices at a minimum (scanning) cost. The second goal is to pick good
> ones at an acceptable cost, which probably are a half of all possible
> choices.

thanks
barry