This laptop with 8GiB RAM is running two VMs at the same time: Windows
10 and Fedora Workstation 32. The host is Fedora Workstation 32. And
there is only swap-on-zram sized to 50% RAM. At this compression
ratio, it wouldn't ever end up using more than 25% RAM. I don't expect
to hold a compression ratio of nearly 3:1 over time, the initial
evictions seem to be pretty highly compressible. Not sure why that is.
Anyway 2:1 is a fairly safe bet for the general case.
$ free -m
total used free shared buff/cache available
Mem: 7863 7038 131 30 693 529
Swap: 3930 778 3152
$ swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 3.9G 778M -2
$ zramctl
NAME ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lzo-rle 3.9G 777M 240.4M 249.6M 8 [SWAP]
$
In this example, the system evicts 777M, at a cost of 250M, for a
total gain of ~527M.
Quit all VM's...
$ free -m
total used free shared buff/cache available
Mem: 7863 678 6283 30 901 6892
Swap: 3930 229 3701
$ swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 3.9G 229.5M -2
$ zramctl
NAME ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lzo-rle 3.9G 228.6M 74M 105.1M 8 [SWAP]
$
We see here that it has free'd up swap and memory, having dropped
pages in swap. But there is some zram metadata overhead that is not
dropped for whatever reason. But the net gain is still better than
50%.
--
Chris Murphy
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