Re: [PATCH 1/2] mm: use slab size in the slab shrinking ratio calculation

[Dave Chinner <david@fromorbit.com>]

On Tue, Jul 04, 2017 at 09:21:37AM -0400, Josef Bacik wrote:
> On Tue, Jul 04, 2017 at 12:01:00PM +0900, Minchan Kim wrote:
> > 1. slab *page* reclaim
> > 
> > Your claim is that it's hard to reclaim a page by slab fragmentation so need to
> > reclaim objects more aggressively.
> > 
> > Basically, aggressive scanning doesn't guarantee to reclaim a page but it just
> > increases the possibility. Even, if we think slab works with merging feature(i.e.,
> > it mixes same size several type objects in a slab), the possibility will be huge
> > dropped if you try to bail out on a certain shrinker. So for working well,
> > we should increase aggressiveness too much to sweep every objects from all shrinker.
> > I guess that's why your patch makes the logic very aggressive.
> > In here, my concern with that aggressive is to reclaim all objects too early
> > and it ends up making void caching scheme. I'm not sure it's gain in the end.
> >
> 
> Well the fact is what we have doesn't work, and I've been staring at this
> problem for a few months and I don't have a better solution.
> 
> And keep in mind we're talking about a purely slab workload, something that
> isn't likely to be a common case.  And even if our scan target is 2x, we aren't
> going to reclaim the entire cache before we bail out.  We only scan in
> 'batch_size' chunks, which generally is 1024.  In the worst case that we have
> one in use object on every slab page we own then yes we're fucked, but we're
> still fucked with the current code, only with the current code it'll take us 20
> minutes of looping in the vm vs. seconds scanning the whole list twice.

Right - this is where growth/allocation rate based aging scans
come into play, rather than waiting for the VM to hit some unknown
ceiling and do an unpredictable amount of scanning.

> > 2. stream-workload
> > 
> > Your claim is that every objects can have INUSE flag in that workload so they
> > need to scan full-cycle with removing the flag and finally, next cycle,
> > objects can be reclaimed. On the situation, static incremental scanning would
> > make deep prorioty drop which causes unncessary CPU cycle waste.
> > 
> > Actually, there isn't nice solution for that at the moment. Page cache try
> > to solve it with multi-level LRU and as you said, it would solve the
> > problem. However, it would be too complicated so you could be okay with
> > Dave's suggestion which periodic aging(i.e., LFU) but it's not free so that
> > it could increase runtime latency.
> > 
> > The point is that such workload is hard to solve in general and just
> > general agreessive scanning is not a good solution because it can sweep
> > other shrinkers which don't have such problem so I hope it should be
> > solved by a specific shrinker itself rather than general VM level.
> 
> The only problem I see here is our shrinker list is just a list, there's no
> order or anything and we just walk through one at a time.

That's because we don't really need an ordered list - all shrinkable
caches needed to have the same amount of work done on them for a
given memory pressure. That's how we maintain balance between
caches.

> We could mitigate
> this problem by ordering the list based on objects, but this isn't necessarily a
> good indication of overall size.

shrinkers don't just run on caches, and some of the this they run
against have variable object size. Some of them report reclaimable
memory in bytes rather than object counts to the shrinker
infrastructure.  i.e. the shrinker infrastrcture is abstracted
sufficiently that the accounting of memory used/reclaimed is defined
by the individual subsystems, not the shrinker infrastructure....

> Consider xfs_buf, where each slab object is also hiding 1 page, so
> for every slab object we free we also free 1 page.

Well, that's a very simplistic view - there are objects that hold a
page, but it's a variable size object cache. an xfs-buf can point to
heap memory or multiple pages.

IOWs, the xfs-buf is not a *slab cache*. It's a *buffer cache*, but
we control it's size via a shrinker because that's the only
mechanism we have that provides subsystems with memory pressure
callbacks. This is a clear example of what I said above about
shrinkers being much more than just a mechanism to control the size
of a slab...

> This may
> appear to be a smaller slab by object measures, but may actually
> be larger.

Right, but that's for the subsystem to sort out the working set
balance against all the other caches - the shrinker infrastructure
cannot determine how important different subsystems are relative to
each other, so memory reclaim must try to do the same percentage of
reclaim work across all of them so that everything remains globally
balanced.

My original plan years ago was to make the shrinker infrastructure
API work on "bytes" rather than "subsystem defined objects", but
there were so many broken shrinkers I burnt out before I got that
far....

My suggestion of allocation based aging callbacks is something for
specific caches to be able to run based on their own or the users
size/growth/performance constraints. It's independent of memory
reclaim behaviour and so can be a strongly biased as the user wants.
Memory reclaim will just maintain whatever balance that exists
between the different caches as a result of the subsystem specific
aging callbacks.

> We could definitely make this aspect of the shrinker
> smarter, but these patches here need to still be in place in
> general to solve the problem of us not being aggressive enough
> currently.  Thanks,

Remember that the shrinker callback into a subsystem is just a
mechanism for scanning a subsystem's reclaim list and performing
reclaim. We're not limited to only calling them from
do_shrink_slab() - a historic name that doesn't reflect the reality
of shrinkers these days - if we have a superblock, we can call the
shrinker....

FWIW, we have per-object init callbacks in the slab infrastructure -
ever thought of maybe using them for controlling cache aging
behaviour? e.g. accounting to trigger background aging scans...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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