Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

[Glauber Costa <glommer@parallels.com>]

On 11/17/2012 01:57 AM, David Rientjes wrote:
> On Sat, 17 Nov 2012, Glauber Costa wrote:
> 
>>> I'm wondering if we should have more than three different levels.
>>>
>>
>> In the case I outlined below, for backwards compatibility. What I
>> actually mean is that memcg *currently* allows arbitrary notifications.
>> One way to merge those, while moving to a saner 3-point notification, is
>> to still allow the old writes and fit them in the closest bucket.
>>
> 
> Yeah, but I'm wondering why three is the right answer.
> 

This is unrelated to what I am talking about.
I am talking about pre-defined values with a specific event meaning (in
his patchset, 3) vs arbitrary numbers valued in bytes.

>>> Umm, why do users of cpusets not want to be able to trigger memory 
>>> pressure notifications?
>>>
>> Because cpusets only deal with memory placement, not memory usage.
> 
> The set of nodes that a thread is allowed to allocate from may face memory 
> pressure up to and including oom while the rest of the system may have a 
> ton of free memory.  Your solution is to compile and mount memcg if you 
> want notifications of memory pressure on those nodes.  Others in this 
> thread have already said they don't want to rely on memcg for any of this 
> and, as Anton showed, this can be tied directly into the VM without any 
> help from memcg as it sits today.  So why implement a simple and clean 
> mempressure cgroup that can be used alone or co-existing with either memcg 
> or cpusets?
> 
>> And it is not that moving a task to cpuset disallows you to do any of
>> this: you could, as long as the same set of tasks are mounted in a
>> corresponding memcg.
>>
> 
> Same thing with a separate mempressure cgroup.  The point is that there 
> will be users of this cgroup that do not want the overhead imposed by 
> memcg (which is why it's disabled in defconfig) and there's no direct 
> dependency that causes it to be a part of memcg.
> 
I think we should shoot the duck where it is going, not where it is. A
good interface is more important than overhead, since this overhead is
by no means fundamental - memcg is fixable, and we would all benefit
from it.

Now, whether or not memcg is the right interface is a different
discussion - let's have it!