Re: Path forward for Virtualized Swap?

[Chris Li]

On Mon, Sep 21, 2026 at 8:53 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> On Mon, 2026-09-21 at 08:10 -1000, Chris Li wrote:
> > On Mon, Sep 21, 2026 at 6:40 AM Rik van Riel <riel@xxxxxxxxxxx>
> > wrote:
> > >
> > > On Mon, 2026-09-21 at 06:31 -1000, Chris Li wrote:
> > > >
> > > > Give me some real-world examples of people hitting this kind of
> > > > limit.
> > > > e.g. MAX(%)  > 100% and still have a usesable system. I suspect
> > > > that
> > > > with 100% zswap usage, the system becomes unusable. Please show
> > > > me a
> > > > real world case that proves me wrong.
> > > >
> > >
> > > Here's my desktop system right now. The amount
> > > of Zswapped is more than twice AnonPages, and
> > > the system stays perfectly usable.
> > >
> > > $ grep -E '(Anon|Zswap|GPU|Swap)' /proc/meminfo
> > > SwapCached:       252340 kB
> > > SwapTotal:      41523604 kB
> > > SwapFree:        9802088 kB
> > > Zswap:           8812236 kB
> > > Zswapped:       24927876 kB
> > > AnonPages:      11615760 kB
> > > AnonHugePages:   1402880 kB
> > > GPUActive:      13112428 kB
> > > GPUReclaim:        95980 kB
> > >
> > > This is also an example where I cannot really
> > > predict how much zswap space I need, because
> > > that depends on what AI model is loaded,
> > > which drives the GPUActive number, as well
> > > as the number of browser tabs.
> > >
> > > The system is perfectly responsive.
> >
> > Great, that is getting somewhere. We are talking about actual usage.
> > I
> > really appreciate that.
> >
> > May I ask what the total amount of memory (RAM) in your system is?
> >
> > I am not asking zswap compare to ANA memory. I want to get the % of
> > the total RAM.
> > I assume your desktop has more than 8G of RAM, right?
>
> Right now it has 64GB of memory.

That is exactly my point. You are running 1/8 = 12.5% system ram.

> However, if I were running with 48GB, I would
> need MORE zswap space, while if I were running
> with 96GB, I would need less zswap space.

My claim is that, if you run 64GB zswap out, with 64GB RAM, your
application will have >=50% swap out. It is unusable. The swapped-out
application will likely slow to snail or crash way before that.

>
> How can people be expected to figure out how
> much zswap space their system will need,

My point is that, I haven't seen a case where the application is still
usable with a reasonable SLO when 100% of RAM is zswapped out. The
example you just provided also correlates with my observation. In a
real workload, nobody in their right mind would want to zswap 100% of
RAM out and expect a reasonable SLO. You are running at 12%.

> when the amount needed to fit a workload
> does not have a positive correlation with
> the amount of system memory?

It absolutely has a positive correlation with % of the system memory
based on my experience. The % of memory swap out has a very positive
correlation with the thrashing on the application. You have not shown
any data point for a system with 100% RAM swap out. I am not aware of
anybody using 100% zswap out. So please educate me with the real data
points. I am curious how the system and application behave at that
point. I have done many swap stress tests in the past while develping
the swap stack. At 100% you will run into all kinds of OOM thrashing
issues.

Chris