Re: [PATCH v4] vmevent: Implement greater-than attribute state and one-shot mode

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(5/1/12 8:20 PM), Anton Vorontsov wrote:
> Hello Rik,
>
> Thanks for looking into this!
>
> On Tue, May 01, 2012 at 05:04:21PM -0400, Rik van Riel wrote:
>> On 05/01/2012 09:18 AM, Anton Vorontsov wrote:
>>> This patch implements a new event type, it will trigger whenever a
>>> value becomes greater than user-specified threshold, it complements
>>> the 'less-then' trigger type.
>>>
>>> Also, let's implement the one-shot mode for the events, when set,
>>> userspace will only receive one notification per crossing the
>>> boundaries.
>>>
>>> Now when both LT and GT are set on the same level, the event type
>>> works as a cross event type: it triggers whenever a value crosses
>>> the threshold from a lesser values side to a greater values side,
>>> and vice versa.
>>>
>>> We use the event types in an userspace low-memory killer: we get a
>>> notification when memory becomes low, so we start freeing memory by
>>> killing unneeded processes, and we get notification when memory hits
>>> the threshold from another side, so we know that we freed enough of
>>> memory.
>>
>> How are these vmevents supposed to work with cgroups?
>
> Currently these are independent subsystems, if you have memcg enabled,
> you can do almost anything* with the memory, as memg has all the needed
> hooks in the mm/ subsystem (it is more like "memory management tracer"
> nowadays :-).
>
> But cgroups have its cost, both performance penalty and memory wastage.
> For example, in the best case, memcg constantly consumes 0.5% of RAM to
> track memory usage, this is 5 MB on a 1 GB "embedded" machine.  To some
> people it feels just wrong to waste that memory for mere notifications.
>
> Of course, this alone can be considered as a lame argument for making
> another subsystem (instead of "fixing" the current one). But see below,
> vmevent is just a convenient ABI.
>
>> What do we do when a cgroup nears its limit, and there
>> is no more swap space available?
>>
>> What do we do when a cgroup nears its limit, and there
>> is swap space available?
>
> As of now, this is all orthogonal to vmevent. Vmevent doesn't know
> about cgroups. If kernel has the memcg enabled, one should probably*
> go with it (or better, with its ABI). At least for now.
>
>> It would be nice to be able to share the same code for
>> embedded, desktop and server workloads...
>
> It would be great indeed, but so far I don't see much that
> vmevent could share. Plus, sharing the code at this point is not
> that interesting; it's mere 500 lines of code (comparing to
> more than 10K lines for cgroups, and it's not including memcg_
> hooks and logic that is spread all over mm/).
>
> Today vmevent code is mostly an ABI implementation, there is
> very little memory management logic (in contrast to the memcg).

But, if it doesn't work desktop/server area, it shouldn't be merged.
We have to consider the best design before kernel inclusion. They cann't
be separeted to discuss.



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