Re: memo: mem+swap controller

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 31 Jul 2008 15:25:33 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> Hi, Kamezawa-san.
> 
> On Thu, 31 Jul 2008 10:15:33 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > Hi, mem+swap controller is suggested by Hugh Dickins and I think it's a great
> > idea. Its concept is having 2 limits. (please point out if I misunderstand.)
> > 
> >  - memory.limit_in_bytes       .... limit memory usage.
> >  - memory.total_limit_in_bytes .... limit memory+swap usage.
> > 
> When I've considered more, I wonder how we can accomplish
> "do not use swap in this group".
> 
you can't do that. This direction is to overcommit swap usage by
counting pages which is kicked out.


> Setting "limit_in_bytes == total_limit_in_bytes" doesn't meet it, I think.
> "limit_in_bytes = total_limit_in_bytes = 1G" cannot
> avoid "memory.usage = 700M swap.usage = 300M" under memory pressure
> outside of the group(and I think this behavior is the diffrence
> of "memory controller + swap controller" and "mem+swap controller").
> 
> I think total_limit_in_bytes and swappiness(or some flag to indicate
> "do not swap out"?) for each group would make more sense.
> 
"do not swap out" is bad direction. please use mlock().

(*) I have no objection to add a control file to move a page from swap
    to on-memory.

> > By this, we can avoid excessive use of swap under a cgroup without any bad effect
> > to global LRU. (in page selection algorithm...overhead will be added, of course)
> > 
> Sorry, I cannot understand this part.
> 
>From global LRU's view, anonymous page can be swapped out everytime.
Because it never hits limit.
==
                      no_swap  swap_cache  disk_swap  on_memory  total
no_swap->swap_cache    -1        +1           -         -         -
==
no changes in total.

> > Following is state transition and counter handling design memo.
> > This uses "3" counters to handle above conrrectly. If you have other logic,
> > please teach me. (and blame me if my diagram is broken.)
> > 
> I don't think counting "disk swap" is good idea(global linux
> dosen't count it).
> Instead, I prefer counting "total swap"(that is swap entry).
> 
Maybe my illustration is bad. 

total_swap = swap_cache + disk_swap. Yes, I count swp_entry.
But just divides it to on-memory or not.

This is just a state transition problem. When we counting only total_swap,
we cannot avoid double counting of a swap_cache as memory and as swap.


> > A point is how to handle swap-cache, I think.
> > (Maybe we need a _big_ change in memcg.)
> > 
> I think swap cache should be counted as both memory and swap,
> as global linux does.

No. If we allow double counting, we'll see OOM-Killer very soon. 

If what you say is
==
                      no_swap  swap   on_memory  total
no_swap->swap_cache             +1       -         +1
==
What happens when global lru's swap_out hits limit ?

If what you say is
==
                      no_swap  swap   on_memory  total
no_swap->swap_cache      -1      +1       -        -
==
What happens when SwapCache is mapped ? 

Thanks,
-Kame

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