Re: [PATCH] mm: zsmalloc: share slab caches for all zsmalloc zpools

[Minchan Kim <minchan@kernel.org>]

On Thu, Jun 06, 2024 at 04:03:55PM -0700, Yosry Ahmed wrote:
> On Thu, Jun 6, 2024 at 3:36 PM Minchan Kim <minchan@kernel.org> wrote:
> >
> > On Tue, Jun 04, 2024 at 05:53:40PM +0000, Yosry Ahmed wrote:
> > > Zswap creates multiple zpools to improve concurrency. Each zsmalloc
> > > zpool creates its own 'zs_handle' and 'zspage' slab caches. Currently we
> > > end up with 32 slab caches of each type.
> > >
> > > Since each slab cache holds some free objects, we end up with a lot of
> > > free objects distributed among the separate zpool caches. Slab caches
> > > are designed to handle concurrent allocations by using percpu
> > > structures, so having a single instance of each cache should be enough,
> > > and avoids wasting more memory than needed due to fragmentation.
> > >
> > > Additionally, having more slab caches than needed unnecessarily slows
> > > down code paths that iterate slab_caches.
> > >
> > > In the results reported by Eric in [1], the amount of unused slab memory
> > > in these caches goes down from 242808 bytes to 29216 bytes (-88%). This
> > > is calculated by (num_objs - active_objs) * objsize for each 'zs_handle'
> > > and 'zspage' cache. Although this patch did not help with the allocation
> > > failure reported by Eric with zswap + zsmalloc, I think it is still
> > > worth merging on its own.
> > >
> > > [1]https://lore.kernel.org/lkml/20240604134458.3ae4396a@yea/
> >
> > I doubt this is the right direction.
> >
> > Zsmalloc is used for various purposes, each with different object
> > lifecycles. For example, swap operations relatively involve short-lived
> > objects, while filesystem use cases might have longer-lived objects.
> > This mix of lifecycles could lead to fragmentation with this approach.
> 
> Even in a swapfile, some objects can be short-lived and some objects
> can be long-lived, and the line between swap and file systems both
> becomes blurry with shmem/tmpfs. I don't think having separate caches


Many allocators differentiate object lifecycles to minimize
fragmentation. While this isn't a new concept, you argue it's irrelevant
without a clearcut use case.

> here is vital, but I am not generally familiar with the file system
> use cases and I don't have data to prove/disprove it.

The use case I had in mind was build output directories (e.g., Android).
These consume object files in zram until the next build.

Other potential scenarios involve separate zrams: one for foreground
apps (short-term) and another for cached apps (long-term). Even
zswap and zram could have different object lifecycles, as zswap might
write back more aggressively.

While you see no clear use cases, I disagree with dismissing this
concept without strong justification.

> 
> >
> > I believe the original problem arose when zsmalloc reduced its lock
> > granularity from the class level to a global level. And then, Zswap went
> > to mitigate the issue with multiple zpools, but it's essentially another
> > bandaid on top of the existing problem, IMO.
> 
> IIRC we reduced the granularity when we added writeback support to
> zsmalloc, which was relatively recent. I think we have seen lock
> contention with zsmalloc long before that. We have had a similar patch
> internally to use multiple zpools in zswap for many years now.
> 
> +Yu Zhao
> 
> Yu has more historical context about this, I am hoping he will shed
> more light about this.
> 
> >
> > The correct approach would be to further reduce the zsmalloc lock
> > granularity.
> 
> I definitely agree that the correct approach should be to fix the lock
> contention at the source and drop zswap's usage of multiple zpools.
> Nonetheless, I think this patch provides value in the meantime. The
> fragmentation within the slab caches is real with zswap's use case.
> OTOH, sharing a cache between swap and file system use cases leading
> to fragmentation within the same slab cache is a less severe problem
> in my opinion.
> 
> That being said, I don't feel strongly. If you really don't like this
> patch I am fine with dropping it.

How about introducing a flag like "bool slab_merge" in zs_create_pool?
This would allow zswap to unify slabs while others don't.