[GTALUG] Fedora 33 uses RAM for swap?

[Mauro Souza]

I have a lot of experience running Linux on IBM mainframes, and using RAM
as swap is a good thing, we do that all the time on our mainframes. And
even if it sounds strange, it decreases latency by A LOT. We call it VDISK
(virtual disk). Usually we use a VDISK only as an indicator that the guest
needs more RAM, so as soon as any virtual machine uses the VDISK, we are
notified and take a look at it to see if some process is using more memory
than it should, or there is real need for more RAM.

Imagine you fire up Firefox, open a new tab, and there's no RAM available.
Linux will have to swap something, write pages to disk (with the very slow
disk IO compared to RAM), and after that allocate more memory to Firefox,
that was suspended all this time. If you do this to RAM instead, it is way
faster. And zswap has a process for offloading pages from zswap to disk, so
it really is a cache for swap.

So it's useful.

Mauro
https://www.maurosouza.com - registered Linux User: 294521
Scripture is both history, and a love letter from God.


Em qua., 25 de nov. de 2020 às 11:35, D. Hugh Redelmeier via talk <
talk at gtalug.org> escreveu:

> | From: Giles Orr via talk <talk at gtalug.org>
>
> | I didn't set that up, and I don't think it was there on F32.  So the
> | OS has, without asking, co-opted 1/4 of my 16G of RAM to use as swap
> | space.  This system has an SSD, so when I initially set it up (Fedora
> | 27), I made a conscious decision to go without swap space.  I rarely
> | push the limits of 16G.
>
> [I'm aware of zswap but haven't looked into it.  I'm too pressed for
> time to do the proper research.  So I'll hypothesize, using common
> sense.]
>
> This doesn't surprise me.  Except for the "without asking" part.
>
> I hypothesize that you got this with a fresh install where you opted
> for the installer to decide on partitioning.  If you tell it how to
> partition, I hope it would not give you zswap without your direction.
>
> The zswap is compressed.  So things moved there take less RAM than they
> would without swapping.  (I hope that it notices when things are
> incompressable and hence would save no space.)
>
> I have no idea whether it is truly pre-allocated RAM or it is just a
> limit and that the actual RAM tied up reflects usage.  I would hope
> that it shrinks and grows as needed.
>
> It may be that, in terms of RAM, this is a pure win.  In terms of CPU,
> this is another matter.
>
> | Can someone please explain
> | A) if I'm correct about this behaviour in the first place, and
>
> Check the F33 Release Notes?  (I have not.)
>
> I've certainly seeen discussion about ZSwap.  Unfortunately, I've
> mostly ignored it.  Other distros have it too.  I don't know whether
> they automatically configure it on installation.
>
> | B) why it's useful?  Thanks.
>
> See above.
>
> - for sure, some running processes are almost never actual run.  ZSwap
>   is a win for those.  Unless latency maters.
>
> - I assume that when zswap fills up, old-fashioned disk swap is used.
>   In this case, zswap is a win over having only swap.
>
>   You could think of zswap as a kind of cache or intermediate stage
>   for disk swap.
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