Re: [PATCH 2/5] Add per cgroup reclaim watermarks.

[Ying Han <yinghan@google.com>]

On Tue, Jan 18, 2011 at 12:36 PM, David Rientjes <rientjes@google.com> wrote:
> On Tue, 18 Jan 2011, Ying Han wrote:
>
>> I agree that "min_free_kbytes" concept doesn't apply well since there
>> is no notion of "reserved pool" in memcg. I borrowed it at the
>> beginning is to add a tunable to the per-memcg watermarks besides the
>> hard_limit.
>
> You may want to add a small amount of memory that a memcg may allocate
> from in oom conditions, however: memory reserves are allocated per-zone
> and if the entire system is oom and that includes several dozen memcgs,
> for example, they could all be contending for the same memory reserves.
> It would be much easier to deplete all reserves since you would have
> several tasks allowed to allocate from this pool: that's not possible
> without memcg since the oom killer is serialized on zones and does not
> kill a task if another oom killed task is already detected in the
> tasklist.

so something like per-memcg min_wmark which also needs to be reserved upfront?

> I think it would be very trivial to DoS the entire machine in this way:
> set up a thousand memcgs with tasks that have core_state, for example, and
> trigger them to all allocate anonymous memory up to their hard limit so
> they oom at the same time.  The machine should livelock with all zones
> having 0 pages free.
>
>> I read the
>> patch posted from Satoru Moriya "Tunable watermarks", and introducing
>> the per-memcg-per-watermark tunable
>> sounds good to me. Might consider adding it to the next post.
>>
>
> Those tunable watermarks were nacked for a reason: they are internal to
> the VM and should be set to sane values by the kernel with no intevention
> needed by userspace.  You'd need to show why a memcg would need a user to
> tune its watermarks to trigger background reclaim and why that's not
> possible by the kernel and how this is a special case in comparsion to the
> per-zone watermarks used by the VM.

KAMEZAWA gave an example on his early post, which some enterprise user
like to keep fixed amount of free pages
regardless of the hard_limit.

Since setting the wmarks has impact on the reclaim behavior of each
memcg,  adding this flexibility helps the system where it like to
treat memcg differently based on the priority.

--Ying


>

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