Re: [RFC] mm/vmscan: add periodic slab shrinker

[Kent Overstreet <kent.overstreet@gmail.com>]

On Wed, Apr 06, 2022 at 10:01:30AM +1000, Dave Chinner wrote:
> On Tue, Apr 05, 2022 at 10:21:53PM +0100, Matthew Wilcox wrote:
> > On Tue, Apr 05, 2022 at 01:58:59PM -0700, Yang Shi wrote:
> > > Yeah, I agree it actually doesn't make too much sense to return the
> > > number of reclaimed objects. Other part of vmscan returns the number
> > > of base pages, the sizes of slab objects are varied, it may be much
> > > smaller than a page, for example, dentry may be 192 bytes.
> > 
> > From the point of view of vmscan, it only cares about the number of pages
> > freed because it's trying to free pages.  But from the point of view of
> > trying to keep the number of non-useful objects in check, the number of
> > objects freed is more important, and it doesn't matter whether we ended
> > up freeing any pages because we made memory available for this slab cache.
> 
> Yes and no. If the memory pressure is being placed on this cache,
> then freeing any number of objects is a win-win situation - reclaim
> makes progress and new allocations don't need to wait for reclaim.
> 
> However, if there is no pressure on this slab cache, then freeing
> objects but no actual memory pages is largely wasted reclaim effort.
> Freeing those objects does nothing to alleviate the memory shortage,
> and the memory freed is not going to be consumed any time soon so
> all we've done is fragment the slab cache and require the subsystem
> to spend more resources re-populating it. That's a lose-lose.
> 
> We want to select the shrinkers that will result in the former
> occurring, not the latter.

Do we have any existing shrinkers that preferentially free from mostly empty
slab pages though? And do we want them to?

You're talking about memory fragmentation, and I'm not sure that should be the
shrinker's concern (on the other hand, I'm not sure it shouldn't - just freeing
the objects on mostly empty slab pages is pretty reasonable for cached objects).

We could also plumb compaction down to the slab level, and just request the
subsystem move those objects. Might be easier than making that an additional
responsibility of the shrinkers, which really should be more concerned with
implementing cache replacement policy and whatnot - e.g. shrinkers were doing
something more LFU-ish that would also help with the one-off object problem.