Re: [PATCH 10/10] mm: balance LRU lists based on relative thrashing

From: Johannes Weiner
Date: Thu Jun 16 2016 - 11:12:28 EST

On Wed, Jun 15, 2016 at 11:23:41AM +0900, Minchan Kim wrote:
> On Mon, Jun 13, 2016 at 11:52:31AM -0400, Johannes Weiner wrote:
> > On Fri, Jun 10, 2016 at 11:19:35AM +0900, Minchan Kim wrote:
> > > Other concern about PG_workingset is naming. For file-backed pages, it's
> > > good because file-backed pages started from inactive's head and promoted
> > > active LRU once two touch so it's likely to be workingset. However,
> > > for anonymous page, it starts from active list so every anonymous page
> > > has PG_workingset while mlocked pages cannot have a chance to have it.
> > > It wouldn't matter in eclaim POV but if we would use PG_workingset as
> > > indicator to identify real workingset page, it might be confused.
> > > Maybe, We could mark mlocked pages as workingset unconditionally.
> >
> > Hm I'm not sure it matters. Technically we don't have to set it on
> > anon, but since it's otherwise unused anyway, it's nice to set it to
> > reinforce the notion that anon is currently always workingset.
> When I read your description firstly, I thought the flag for anon page
> is set on only swapin but now I feel you want to set it for all of
> anonymous page but it has several holes like mlocked pages, shmem pages
> and THP and you want to fix it in THP case only.
> Hm, What's the rule?
> It's not consistent and confusing to me. :(

I think you are might be over thinking this a bit ;)

The current LRU code has a notion of workingset pages, which is anon
pages and multi-referenced file pages. shmem are considered file for
this purpose. That's why anon start out active and files/shmem do
not. This patch adds refaulting pages to the mix.

PG_workingset keeps track of pages that were recently workingset, so
we set it when the page enters the workingset (activations and
refaults, and new anon from the start). The only thing we need out of
this flag is to tell us whether reclaim is going after the workingset
because the LRUs have become too small to hold it.

mlocked pages are not really interesting because not only are they not
evictable, they are entirely exempt from aging. Without aging, we can
not say whether they are workingset or not. We'll just leave the flags
alone, like the active flag right now.

> I think it would be better that PageWorkingset function should return
> true in case of PG_swapbacked set if we want to consider all pages of
> anonymous LRU PG_workingset which is more clear, not error-prone, IMHO.

I'm not sure I see the upside, it would be more branches and code.

> Another question:
> Do we want to retain [1]?
> This patch motivates from swap IO could be much faster than file IO
> so that it would be natural if we rely on refaulting feedback rather
> than forcing evicting file cache?
> [1] e9868505987a, mm,vmscan: only evict file pages when we have plenty?

Yes! We don't want to go after the workingset, whether it be cache or
anonymous, while there is single-use page cache lying around that we
can reclaim for free, with no IO and little risk of future IO. Anon
memory doesn't have this equivalent. Only cache is lazy-reclaimed.

Once the cache refaults, we activate it to reflect the fact that it's
workingset. Only when we run out of single-use cache do we want to
reclaim multi-use pages, and *then* we balance workingsets based on
cost of refetching each side from secondary storage.