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

[David Rientjes]

On Fri, 16 Nov 2012, Anton Vorontsov wrote:

>  The main change is that I decided to go with discrete levels of the
>  pressure.
> 
>  When I started writing the man page, I had to describe the 'reclaimer
>  inefficiency index', and while doing this I realized that I'm describing
>  how the kernel is doing the memory management, which we try to avoid in
>  the vmevent. And applications don't really care about these details:
>  reclaimers, its inefficiency indexes, scanning window sizes, priority
>  levels, etc. -- it's all "not interesting", and purely kernel's stuff. So
>  I guess Mel Gorman was right, we need some sort of levels.
> 
>  What applications (well, activity managers) are really interested in is
>  this:
> 
>  1. Do we we sacrifice resources for new memory allocations (e.g. files
>     cache)?
>  2. Does the new memory allocations' cost becomes too high, and the system
>     hurts because of this?
>  3. Are we about to OOM soon?
> 
>  And here are the answers:
> 
>  1. VMEVENT_PRESSURE_LOW
>  2. VMEVENT_PRESSURE_MED
>  3. VMEVENT_PRESSURE_OOM
> 
>  There is no "high" pressure, since I really don't see any definition of
>  it, but it's possible to introduce new levels without breaking ABI.
> 
> Later I came up with the fourth level:
> 
>  Maybe it makes sense to implement something like PRESSURE_MILD/BALANCE
>  with an additional nr_pages threshold, which basically hits the kernel
>  about how many easily reclaimable pages userland has (that would be a
>  part of our definition for the mild/balance pressure level).
> 
> I.e. the fourth level can serve as a two-way communication w/ the kernel.
> But again, this would be just an extension, I don't want to introduce this
> now.
> 

That certainly makes sense, it would be too much of a usage and 
maintenance burden to assume that the implementation of the VM is to 
remain the same.

> > 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 
> 
> You meant 'why not'?
> 

Yes, sorry.

> > mempressure cgroup that can be used alone or co-existing with either memcg 
> > or cpusets?
> > 
> > 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.
> 
> There's also an API "inconvenince issue" with memcg's usage_in_bytes
> stuff: applications have a hard time resetting the threshold to 'emulate'
> the pressure notifications, and they also have to count bytes (like 'total
> - used = free') to set the threshold. While a separate 'pressure'
> notifications shows exactly what apps actually want to know: the pressure.
> 

Agreed.
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