[TUHS] VM over-commit (and the OOM killers)

[Warner Losh]

On Fri, Feb 28, 2025 at 9:12 AM segaloco via TUHS <tuhs at tuhs.org> wrote:

> On Friday, February 28th, 2025 at 8:04 AM, Dennis Boone <drb at msu.edu>
> wrote:
>
> > > I’m probably a lost soul on this issue, but swap space is just a way
> >
> > > to turn program bugs into performance problems.
> >
> >
> > You're hardly the only one. Some years ago, running Linux web and
> > database servers, I quit creating swap space. A runaway program would
> > turn the system into an infinite game of shuffle-the-pages well before
> > the OOM killer actually decided to kill something, and in that state,
> > one couldn't even reboot. This expanded the time window of "broken"
> > from tens of seconds, and perhaps a service restart, into tens of
> > minutes and a power button recovery. Every #$%^&* time.
> >
> > De
>
> I've read several bits of guidance lately suggesting avoiding swap due to
> the increasing prevalence of solid-state memories.  The assertion is that
> I/O heavy swapping, especially if you get into a thrashing state, is liable
> to age current storage technologies much more than it would have in the
> platter disk era.  I've heard contrary opinions that it isn't as large of a
> liability in reality.  I haven't settled on one or the other, I keep a swap
> file around on the microSD that runs my RPi, but I've only needed to
> swapon, like OP, when compiling gcc.
>

Swapping to flash can be fine. The days when a "spot" could be worn out are
20 years in the past. If you use it as a "shock absorber" to cope with
transient loads, there's no issues.

However a lot depends on which kind of flash you have, you might have
trouble if the load is constant. Today there's a wide range of flash
drives. Ranging from < .1DWPD drives (you can't write even 10% of the
drive's capacity in one day w/o wearing it out in the warranty period) to
3, 5 or 10 DWPD drives. 0.3 and 1.0 seem popular in the consumer / prosumer
/ low-end-datacenter space. Those can be a problem if you are constantly
swapping (but again are fine for coping with transients).

The MicroSD is likely to be closer to the .1-.3 DWPD (unless you bought an
expensive one).

Warner
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