[Devel] Re: [PATCH 0/4] swapcgroup(v2)

[KAMEZAWA Hiroyuki]

On Thu, 22 May 2008 22:26:55 -0400
Rik van Riel <riel at redhat.com> wrote:

> Even worse is that a cgroup has NO CONTROL over how much
> of its memory is kept in RAM and how much is swapped out.
Could you explain "NO CONTROL" ? cgroup has LRU....
'how mucch memory should be swapped out from memory' is well controlled
in the VM besides LRU logic ?

> This kind of decision is made on a system-wide basis by
> the kernel, dependent on what other processes in the system
> are doing. There also is no easy way for a cgroup to reduce
> its swap use, unlike with other resources.
> 

> In what scenario would you use a resource controller that
> rewards a group for reaching its limit?
> 
> How can the cgroup swap space controller help sysadmins
> achieve performance or fairness goals on a system? 
> 
Perforamnce is not the first goal of this swap controller, I think.
This is for resouce isolation/overcommiting. 

1. Some _crazy_ people considers swap as very-slow-memory resource ;)
   I don't think so but I know there are tons of people....

2. Resource Isolation.
   When a cgroup has memory limitation, it can create tons of swap.
   For example, limit a cgroup's memory to be 128M and malloc 3G bytes.
   2.8Gbytes of swap will be used _easily_. A process can use up all swap. 
   In that case, other process can't use swap.

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....

If we don't have limitation to swap, we'll have to innovate a way to move swap
to memory in some reasonable logic.

Thanks,
-Kame



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