Re: [PATCH 1/2] mm: zbud: deprecate CONFIG_ZBUD

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Tue, Jan 28, 2025 at 10:04:27AM -0600, Seth Jennings wrote:
> On Tue, Jan 28, 2025 at 9:37 AM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:
> >
> > On Tue, Jan 28, 2025 at 11:21:10AM +0100, Vlastimil Babka wrote:
> > > On 1/28/25 00:58, Yosry Ahmed wrote:
> > > > The zbud compressed pages allocator is rarely used, most users use
> > > > zsmalloc. zbud consumes much more memory (only stores 1 or 2 compressed
> > > > pages per physical page). The only advantage of zbud is a marginal
> > > > performance improvement that by no means justify the memory overhead.
> > > >
> > > > Historically, zsmalloc had significantly worse latency than zbud and
> > > > z3fold but offered better memory savings.  This is no longer the case as
> > > > shown by a simple recent analysis [1].  In a kernel build test on tmpfs
> > > > in a limited cgroup, zbud 2-3% less time than zsmalloc, but at the cost
> > > > of using ~32% more memory (1.5G vs 1.13G). The tradeoff does not make
> > > > sense for zbud in any practical scenario.
> > > >
> > > > The only alleged advantage of zbud is not having the dependency on
> > > > CONFIG_MMU, but CONFIG_SWAP already depends on CONFIG_MMU anyway, and
> > > > zbud is only used by zswap.
> > > >
> > > > Following in the footsteps of [2], which deprecated z3fold, deprecated
> > > > zbud as planned and remove it in a few cycles if no objections are
> > > > raised from active users.
> > > >
> > > > Rename the user-visible config options so that users with CONFIG_ZBUD=y
> > > > get a new prompt with explanation during make oldconfig. Also, remove
> > > > CONFIG_ZBUD from defconfig.
> > > >
> > > > [1]https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@mail.gmail.com/
> > > > [2]https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@google.com/
> > > >
> > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> > >
> > > Seems weird not to Cc the folks listed in MAINTAINERS for ZBUD? Unless their
> > > addresses are known to bounce?
> >
> > Ugh I had them in the CC list, but I played around with it and probably
> > lost them :/
> >
> > Seth, Dan, apologies.
> 
> I haven't contributed in this space for quite some time so I defer to
> more active contributors on this.
> 
> If it is true that zsmalloc has no/negligible deficiencies compared to
> zbud, it seems reasonable.

Zsmalloc has been the default allocator and is widely used, I am unaware
of any zbud users in prod settings.

> > but at the cost of using ~32% more memory
> 
> I do push back here as this % could be highly variable depending on
> the compressibility of the data.

100% agreed, this is just an example from this specific testing
scenario.

> If the average compressibility is ~50%, zbud should be very memory efficient.
> 
> If the average compressibility is very high or very low, it is less
> memory efficient.

That being said, the average compression ratio hovers around 30% for
workloads I am familiar with. I think I have seen similar ratios from
others as well but I will let others chime in.