Re: Fedora 34 Change: Scale ZRAM to Full Memory Size (Self-Contained Change proposal)

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On Tue, Jan 12, 2021 at 4:06 AM Chris Murphy <lists@xxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Jan 11, 2021 at 1:55 PM ElXreno <elxreno@xxxxxxxxx> wrote:
>
> > I think it's a bad idea. It is better to make `zram-fraction` equal to
> > 0.8 or 0.9, but not 1.0.
>
> Just to clarify. zram-fraction 1.0 means the zram *device size* will
> be equal to RAM, capped at 8G. The actual amount of memory used is
> zero, until the kernel starts to evict dirty pages to swap, where they
> are then compressed. And the maximum amount of RAM due to compression
> would be predicated on compression ratio. If we assume a conservative
> 2:1 ratio (compression results in 50% of original size), the max ram
> would be the lesser of 4G or 50% of RAM.

For reference this is the config we ran on Fedora arm with the zram
package for a number of releases before F-33 and the new tool tool
over and we had few reports of issues with this factor even with
devices with a small amount of real memeory so I have no concerns, in
fact it just gets us back to what we had there by default in F-32.

> Does this reduce your concern?
>
> 80% might still be reasonable, and still pretty conservative.
>
> > For example, if you run Darktable, load a floating-point TIFF image into
> > it, and try to export it, on systems with little RAM, it will fill both
> > RAM itself and zram, and cause the system to freeze completely because
> > the memory is almost incompressible.
> > Also, other data (video editors, for example) which are not compressible
> > can get into RAM.
>
> Are you sure that this data is compressed in memory as well as the
> on-disk encoding? The editing operations themselves surely operate on
> uncompressed data, and then it's compressed upon being written to
> disk.
>
> --
> Chris Murphy
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