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    SubjectRe: [PATCH] remove memory.limit v.s. memsw.limit comparison.
    On Fri, 5 Jun 2009 00:45:03 +0900 (JST), "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > Balbir Singh wrote:
    > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-06-04
    > > 14:10:43]:
    > >
    > >> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > >>
    > >> Removes memory.limit < memsw.limit at setting limit check completely.
    > >>
    > >> The limitation "memory.limit <= memsw.limit" was added just because
    > >> it seems sane ...if memory.limit > memsw.limit, only memsw.limit works.
    > >>
    > >> But To implement this limitation, we needed to use private mutex and
    > >> make
    > >> the code a bit complated.
    > >> As Nishimura pointed out, in real world, there are people who only want
    > >> to use memsw.limit.
    > >>
    > >> Then, this patch removes the check. user-land library or middleware can
    > >> check
    > >> this in userland easily if this really concerns.
    > >>
    > >> And this is a good change to charge-and-reclaim.
    > >>
    > >> Now, memory.limit is always checked before memsw.limit
    > >> and it may do swap-out. But, if memory.limit == memsw.limit, swap-out is
    > >> finally no help and hits memsw.limit again. So, let's allow the
    > >> condition
    > >> memory.limit > memsw.limit. Then we can skip unnecesary swap-out.
    > >>
    > >> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > >> ---
    > >
    > > There is one other option, we could set memory.limit_in_bytes ==
    > > memory.memsw.limit_in_bytes provided it is set to LONG_LONG_MAX. I am
    > > not convinced that we should allow memsw.limit_in_bytes to be less
    > > that limit_in_bytes, it will create confusion and the API is already
    > > exposed.
    > >
    > Ahhhh, I get your point.
    > if memory.memsw.limit_in_bytes < memory.limit_in_bytes, no swap will
    > be used bacause currnet try_to_free_pages() for memcg skips swap-out.
    > Then, only global-LRU will use swap.
    > This behavior is not easy to understand.
    >
    > Sorry, I don't push this patch as this is. But adding documentation about
    > "What happens when you set memory.limit == memsw.limit" will be necessary.
    >
    I agree.

    > ...maybe give all jobs to user-land and keep the kernel as it is now
    > is a good choice.
    >
    > BTW, I'd like to avoid useless swap-out in memory.limit == memsw.limit case.
    > If someone has good idea, please :(
    >
    I think so too.

    From my simple thoughts, how about changing __mem_cgroup_try_charge() like:

    1. initialize "noswap" as "bool noswap = !!(mem->res.limit == mem->memsw.limit)".
    2. add check "if (mem->res.limit == mem->memsw.limit)" on charge failure to mem->res
    and set "noswap" to true if needed.
    3. charge mem->memsw before mem->res.

    There would be other ideas, but I prefer 1 among these choices.


    Thanks,
    Daisuke Nishimura.


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