Re: [PATCH 0/1] Add swappiness argument to memory.reclaim

[Shakeel Butt <shakeelb@google.com>]

On Thu, Nov 30, 2023 at 11:56:42AM -0500, Johannes Weiner wrote:
> On Thu, Nov 30, 2023 at 04:57:41PM +0100, Michal Hocko wrote:
> > On Thu 30-11-23 07:36:53, Dan Schatzberg wrote:
> > [...]
> > > In contrast, I argue in favor of a swappiness setting not as a way to implement
> > > custom reclaim algorithms but rather to bias the balance of anon vs file due to
> > > differences of proactive vs reactive reclaim. In this context, swappiness is the
> > > existing interface for controlling this balance and this patch simply allows for
> > > it to be configured differently for proactive vs reactive reclaim.
> > 
> > I do agree that swappiness is a better interface than explicit anon/file
> > but the problem with swappiness is that it is more of a hint for the reclaim
> > rather than a real control. Just look at get_scan_count and its history.
> > Not only its range has been extended also the extent when it is actually
> > used has been changing all the time and I think it is not a stretch to
> > assume that trend to continue.
> 
> Right, we did tweak the edge behavior of e.g. swappiness=0. And we
> extended the range to express "anon is cheaper than file", which
> wasn't possible before, to support the compressed memory case.
> 
> However, its meaning and impact has been remarkably stable over the
> years: it allows userspace to specify the relative cost of paging IO
> between file and anon pages. This comment is from 2.6.28:
> 
>         /*
>          * With swappiness at 100, anonymous and file have the same priority.
>          * This scanning priority is essentially the inverse of IO cost.
>          */
>         anon_prio = sc->swappiness;
>         file_prio = 200 - sc->swappiness;
> 
> And this is it today:
> 
> 	/*
> 	 * Calculate the pressure balance between anon and file pages.
> 	 *
> 	 * The amount of pressure we put on each LRU is inversely
> 	 * proportional to the cost of reclaiming each list, as
> 	 * determined by the share of pages that are refaulting, times
> 	 * the relative IO cost of bringing back a swapped out
> 	 * anonymous page vs reloading a filesystem page (swappiness).
> 	 *
> 	 * Although we limit that influence to ensure no list gets
> 	 * left behind completely: at least a third of the pressure is
> 	 * applied, before swappiness.
> 	 *
> 	 * With swappiness at 100, anon and file have equal IO cost.
> 	 */
> 	total_cost = sc->anon_cost + sc->file_cost;
> 	anon_cost = total_cost + sc->anon_cost;
> 	file_cost = total_cost + sc->file_cost;
> 	total_cost = anon_cost + file_cost;
> 
> 	ap = swappiness * (total_cost + 1);
> 	ap /= anon_cost + 1;
> 
> 	fp = (200 - swappiness) * (total_cost + 1);
> 	fp /= file_cost + 1;
> 
> So swappiness still means the same it did 15 years ago. We haven't
> changed the default swappiness setting, and we haven't broken any
> existing swappiness configurations through VM changes in that time.
> 
> There are a few scenarios where swappiness doesn't apply:
> 
> - No swap. Oh well, that seems reasonable.
> 
> - Priority=0. This applies to near-OOM situations where the MM system
>   tries to save itself. This isn't a range in which proactive
>   reclaimers (should) operate.
> 
> - sc->file_is_tiny. This doesn't apply to cgroup reclaim and thus
>   proactive reclaim.
> 
> - sc->cache_trim_mode. This implements clean cache dropbehind, and
>   applies in the presence of large, non-refaulting inactive cache. The
>   assumption there is that this data is reclaimable without involving
>   IO to evict, and without the expectation of refault IO in the
>   future. Without IO involvement, the relative IO cost isn't a
>   factor. This will back off when refaults are observed, and the IO
>   cost setting is then taken into account again as expected.
> 
>   If you consider swappiness to mean "reclaim what I ask you to", then
>   this would override that, yes. But in the definition of relative IO
>   cost, this decision making is permissible.
> 
>   Note that this applies to the global swappiness setting as well, and
>   nobody has complained about it.
> 
> So I wouldn't say it's merely a reclaim hint. It controls a very
> concrete and influential factor in VM decision making. And since the
> global swappiness is long-established ABI, I don't expect its meaning
> to change significantly any time soon.

Are you saying the edge case behavior of global swappiness and the user
provided swappiness through memory.reclaim should remain same?