This "size"/factor/percentage represents the total amount of
**uncompressed** data that is allowed to be written to the zram-backed
swap device. It is **not** the amount of RAM to use for zram swap (as I
had previously and erroneously thought.) What this means is that if we
set the percent to 50% on a device with 3GB of RAM then up to 1.5GB of
*uncompressed* data can be written to it, BUT since compression algos
usually do better than a ratio of 1:1... less than 1.5GB of RAM will
actually be occupied by this data when it is compressed.
A 1.5:1 compression ratio seems relatively safe to assume, even if we
use lz4 (which is less "good" at compressing than zstd), so I propose we
set the size to 150%. Some distros (PureOS) set this higher (200%), but
I think we should be somewhat conservative since we want to avoid any
thrashing in case the compression ratio at runtime ends up being less
than 2:1 for some reason.
This does set a limit of 50% for devices with >32GB of RAM, to
reduce overhead with using a larger amount of zram swap. The kernel
documentation for zram suggests there's a 0.1% overhead, so if we let
this amount get large then it could be tens or hundreds of MB of RAM
"wasted." The amount I chose is kinda arbitrary, I think this can be
improved later. At the very least, users can change this amount in the
deviceinfo variable.
Fixes#4250
Signed-off-by: Clayton Craft <craftyguy@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7754>
Clayton Craft
49b9e02ddf