Re: Tiered swap for zram.

[Mike Mestnik <cheako@mikemestnik.net>]

On 01/25/12 15:56, Mike Mestnik wrote:
> zram is great, but it can fill up easily and when it does new/fresh
> pages get pushed out to disk swap.  I've written a small bash script
> that will flush zram swap to disk every five min.  However this is
> better done with the VM subsystem and I suggest that swap mounts have a
> tier as well as the current priority.
>
> ( while sleep 300; do for ech in /dev/zram?; do swapoff $ech& done; for
> ech in /dev/zram?; do wait; done; for ech in /dev/zram?; do swapon -p 5
> $ech& done; done; )& disown
After running this for five days I've concluded that it does indeed
help.  It clearly flushes the swap every five minutes and the system can
go unavailable(mouse is unable to track/move) during that time.  However
the system is responsive most all of the rest of the time, so I call
that a win.  Vary poor performance some of the time over sluggish
performance practically all the time.

> The idea is that after a page has been in swap tier X for, say five min,
> it will graduate to swap tier Y.  This keeps swap tier X free from any
> long standing pages that are just taking up valuable realestate.
>
> This script seams to do what I'm asking for, but at a heavy cost.
>
>
>
> My second suggestion may take a while longer to implement.  It involves
> adding a new bit-field to the record for each page.  This bitfield would
> indicate the compression level/type of the contents of a page.  For
> example zram would set the bit corresponding to the compresion it's
> configured to use.
>
> This would allow zram to refuse requests to swap pages that are already
> compressed.  Thus further allow it to swap it's self.  When a swap mount
> refuses a page this would be equivalent to that swap space being full
> and the next swap mount would be used.
>
> Future advancements of this bit-field may indicate the contents of
> userspace pages, zram and others can mark a page as not vary well
> compressible.  As such it's clear that this bit-field should be cleared
> or marked dirty if the page is ever written to.
>
> Thank you for a few moments of your time.  I hope that these could be
> implemented.
>