Re: memo: mem+swap controller

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

> > > 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.
> 
I see. Thanks.

> > > 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.
> 
I agree.
My intention was not counting only total_swap, but counting both
total_swap and swap_cache.

> > > 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. 
> 
on_memory = no_swap + swap_cache (your difinition)
swap = swap_cache + disk_swap
total = no_swap + swap_cache + disk_swap (your difinition)

total is NOT "on_memory + swap"(swap_cache is not doble counted).

so, in the sense of limitting, this is the same as yours.

> 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 ? 
> 
The latter, and I think there would be no defference
about no_swap/swap_cache/disk_swap counters even when the swap cache is mapped.

(current memory controller charges swap caches only when it is mapped,
but I'm not talking about it.)


Thanks,
Daisuke Nishimura.

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