zram-config repost

sd superduper3000 at outlook.com
Thu Nov 7 16:36:41 UTC 2013

repost - sorry posted this in wrong thread before


On 07/11/13 17:28, sd wrote:

Some details about zram-config:

[ppp] 1. I have somehow the impression the system is not fully aware 
about the memory assigned to zram.
[leszek] To 1. You are wrong. Its a normal swap and all apps which are 
aware of swap are aware of zrams swap.

@leszek - you are right, but that is not what I meant. I meant the 
system is somehow not aware about the RAM amount being in use by zram 
(the system sure knows about zram swap memory).

Below are some tests I did (with a live Lubuntu 13.04 cd in virtual box):

(fresh start)

$sudo swapoff -a
$sudo service zram-config stop
$free -h
      total used free
Mem: 994M  866M 127M
Swap:0B   0B 0B

$sudo service zram-config start
$free -h
      total used free
Mem: 994M  870M 123M
Swap:497M    0B 497M

$cat /proc/swaps
...            Size
/dev/zram0 ... 254508
/dev/zram0 ... 254508

So ~ 250M per each zram swap.

Sure the system is aware of the zram swap, free command shows it - this 
is obvious to me.
The system is, however, somehow not aware of the RAM amount zram is 
using, atm it looks like it is using like 0B - may be zram allocates the 
memory dynamically as needed (and the given amount is just a max limit)?

(install memtester)

$sudo memtester 512M

works ok

free -h (on new terminal tab) shows 80M free, no swap used

stop memterster (Ctrl+C)

$sudo memtester 1024M

want 1024, got 1024 trying mlock ...

System freezes, no chance to recover. You have to power the machine down.

(Reset VM with live CD) and next test:

$sudo swapoff -a
$sudo service zram-config stop
$free -h
      total used free
Mem: 994M  734M 259M
Swap:0B    0B 0B

$sudo memtester 1024M
want 1024M, got 604M trying mlock ...

The live CD shows next the lightdm login prompt here, not perfect, but 
much better than the freeze with zram active.
The system behaves better in this case on low memory. At least you have 
a chance to do a propert shutdown, start a new desktop session, etc.

After restart of VM with live CD next test:

$sudo swapoff -a
$sudo service zram-config stop
$sudo service zram-config start
$free -h
      total used free
Mem: 994M  864M 129M
Swap:497M    0B 497M

$sudo memtester 512M
want 512, got 512 locked

$free -h (new terninal tab, while memtester command above active)
      total used free
Mem: 994M  929M 64M
Swap:497M    4K 497M

memtester RES 512m VIRT 516m %MEM 51.5%

Now in a new terminal tab:

$sudo memtester 128M
want 128MB got 128 MB locked

$free -h
similar to above 4M swap used

$sudo memtester 512M (new terminal tab)
Same system freeze as before.

The behavior what happens in a Lubuntu box when there is no more free 
memory seems to be deterministic:

-With zram swap active - it just freezes - no warning, no chance, you 
have to reset power of the system.
-Without zram active - it kills the desktop user session, shows lighdm 
login - much better.

Hope this helps,

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