The out-of-swap killer makes poor choices

[Mark Millard]

On 2021-Feb-24, at 10:36, Konstantin Belousov <kostikbel T gmail.com> wrote:

> On Wed, Feb 24, 2021 at 10:34:23AM -0700, Alan Somers wrote:
>> There's another silly problem that I didn't mention in my original post.
>> The old rule of thumb is that the swap partition's size should be twice as
>> large as the amount of RAM.  However, that's no longer possible in many
>> cases.  The kernel imposes a hard limit of 64 GiB (on amd64 at least) on
>> the usable size of any swap partition, and many servers now have far more
>> than 64 GiB of RAM.  So the advice needs to change with the times.  I don't
> I do not think so. The usable size of the swap is determined by the
> amount of swap metadata we pre-configure at boot time. Usually it is
> sized proportionally to the available physical memory, but you can
> override swap zones size manually with the knob.

There was a period of time when the 128 GiByte RAM ThreadRipper
had its previous 192 GiByte swap partition use rejected and I
had to split it into 3 64 GiByte ones. Later I saw a checkin that
was a correction to some calculation (vague memory) and I retried
having one 192 GiByte swap partition and it was again allowed.

The ability to dump to a swap partition when there was a
64 GiByte limitation with 128 GiByte of RAM had implications
for the configuration. I actually arranged having a partition
that was only used for dump's potential use. That took some
rearrangement to form a large enough space, making other
tradeoffs to do so.


(I'm not sure if I can find the commit that lead to me switching
back to more than 64 GiByte for a swap file on the large memory
machine. I do not remember details any more.)


>> know what the best size would be for a modern server, but I would guess
>> that it must be at least several times the RSS of your largest process, and
>> also at least one tenth of RAM (for use as a dump device with compressed
>> core dumps).


===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)