Re: Path forward for Virtualized Swap?

[Klara Modin]

Hi,

On 2026-09-22 21:10:53 -1000, Chris Li wrote:
> On Tue, Sep 22, 2026 at 5:53 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
> >
> > On Tue, 2026-09-22 at 05:43 -1000, Chris Li wrote:
> > > On Tue, Sep 22, 2026 at 5:20 AM Gregory Price <gourry@xxxxxxxxxx>
> > > wrote:
> > > >
> > > > On Tue, Sep 22, 2026 at 03:32:44AM -1000, Chris Li wrote:
> > > > > > Right now it has 64GB of memory.
> > > > >
> > > > > That is exactly my point. You are running 1/8 = 12.5% system ram.
> > > > >
> > > >
> > > > Chris you are missing the point
> > > >
> > > > Zswap:           8812236 kB
> > > > Zswapped:       24927876 kB
> > >
> > > I am well aware of the point. The 7%-10% data I provided is before
> > > compression. After compression, the real saving is about 1.5% - 2%.
> > >
> > Here I'm at 28% without any issues, with room for
> > more.
> 
> 28% is one thing, 100% is a difference beast completely. If you
> haven't tried it, don't assume 100% will behave the same as 28%. There
> is a point where too much zswap makes the system unusable. It is hard
> to pinpoint the exact upper boundary. However, figuring out the upper
> bound larger than that exact boundary is not hard at all. I haven't
> seen any one can use 100% system RAM sized memory swap out to zswap.
> If you have a data point showing what that system with 100% swap to
> zswap looks like, please share it.
> 

Chiming in as more of a user perspective.

On my 4 GiB BPI-F3, I can reach more than 10:1 compression ratio on
zswap during some parts when building GCC 17 snapshots. E.g:

MemTotal:        3966864 kB
SwapCached:        31872 kB
SwapTotal:      16777212 kB
SwapFree:       16638128 kB
Zswap:            280140 kB
Zswapped:        2859840 kB
AnonPages:       2927376 kB
AnonHugePages:   1409024 kB

While this is 72 % rather than the 100 % you asked for, I think this
shows that what size a potential limit on the uncompressed size of zswap
is suitable heavily depends on the workload.

I have been using vswap consistently on all my machines since about
August, and I have also tried one or two versions of xswap (but the
current lack of writeback makes it inconvenient for me). I really
appreciate the work being put in to decouple zswap from needing a
physical swap device to work.

> >
> > > You are not listening. The boundary was set due to feedback from the
> > > application SLO. Those are real deployments not imaginary usage.
> > > Please respect the user.
> > >
> > Different users need different things.
> >
> > The kernel needs to be able to accommodate all of them.
> >
> > The kernel default should accommodate the people who
> > are least capable of configuring their systems.
> >
> > Hyperscalers can set their own defaults across their
> > fleets. They know how to adjust settings.
> 
> They can use a safe value, e.g., 100% of RAM. Let the people who know
> exactly what they want to swap configure the boundary they want.

My personal preference would be for a default uncapped (or very high,
and I don't think 100 % of RAM is high) limit on the uncompressed data
which is backed by zswap, since that would mean one less knob to tune.

> 
> Chris

Regards,
Klara Modin