[Devel] Re: memrlimit controller merge to mainline

[KAMEZAWA Hiroyuki]

On Fri, 25 Jul 2008 17:46:45 +0100 (BST)
Hugh Dickins <hugh at veritas.com> wrote:

> IIRC Rik expressed the same by pointing out that a cgroup at its
> swap limit would then be forced to grow in mem (until it hits its
> mem limit): so controlling the less precious resource would increase
> pressure on the more precious resource.  (Actually, that probably
> bears little relation to what he said - sorry, Rik!)  I don't recall
> what answer he got, perhaps I'd be persuaded if I heard it again.
> 
Added Nishimura to CC.

IMHO, from user point of view, both of
 - having 2 controls as mem controller + swap controller
 - mem + swap controller
doesn't have much difference. The users will use as they like.

>From memory controller's point of view, treating mem+swap by the same
controller makes sense. Because memory controller can check wheter we can use
more swap or not, we can avoid hopeless-scanning of Anon at swap-shortage.
(By split-lru, I think we can do this avoidance.)
 
Another-Topic?

In recent servers, memory is big, swap is (relatively) small.
And under memory resource controller, the whole swap is easily occupied
by a group. I want to avoid it.

For users, swap is not precious because it's not fast. 
But for memory reclaiming, swap is precious resource to page out
anonymous/shmem/tmpfs memory. I think usual system-admin considers swap as
some emergency spare of memory. I'd like to allow this "emergency spare" to each
cgroup.
(For example, swap is used even if vm.swappiness==0. This is for avoiding OOM-Killer
 under some situation, this behavior is added by Rik.)


== following is another use case I explained to Rik at 23/May/08 ==

IIRC, a man shown his motivation to controll swap in OLS2007/BOF as following.
Consider following system. (and there is no swap controller.) 
Memory 4G. Swap 1G. with 2 cgroups A, B.

state 1) swap is not used.
  A....memory limit to be 1G  no swap usage memory_usage=0M
  B....memory limit to be 1G  no swap usage memory_usage=0M

state 2) Run a big program on A.
  A....memory limit to be 1G and try to use 1.7G. uses 700MBytes of swap.
       memory_usage=1G swap_usage=700M
  B....memory_usage=0M

state 3) A some of programs ends in 'A'
  A....memory_usage=500M swap_usage=700M
  B....memory_usage=0M.

state 4) Run a big program on B.
  A...memory_usage=500M swap_usage=700M.
  B...memory_usage=1G   swap_usage=300M

Group B can only use 1.3G because of unfair swap use of group A.
But users think why A uses 700M of swap with 500M of free memory....
==



Thanks,
-Kame

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