How to get a sense of VM pressure

[Jeremy Fitzhardinge]

I'm thinking about ways to improve the Xen balloon driver.  This is the 
driver which allows the guest domain to expand or contract by either 
asking for more memory from the hypervisor, or giving unneeded memory 
back.  From the kernel's perspective, it simply looks like a driver 
which allocates and frees pages; when it allocates memory it gives the 
underlying physical page back to the hypervisor.  And conversely, when 
it gets a page from the hypervisor, it glues it under a given pfn and 
releases that page back to the kernel for reuse.

At the moment it's very dumb, and is pure mechanism.  It's told how much 
memory to target, and it either allocates or frees memory until the 
target is reached.  Unfortunately, that means if it's asked to shrink to 
an unreasonably small size, it will do so without question, killing the 
domain in a thrash-storm in the process.

There are several problems:

  1. it doesn't know what a reasonable lower limit is, and
  2. it doesn't moderate the rate of shrinkage to give the rest of the
     VM time to adjust to having less memory (by paging out, dropping
     inactive, etc)

And possibly the third point is that the only mechanism it has for 
applying memory pressure to the system is by allocating memory.  It 
allocates with (GFP_HIGHUSER | __GFP_NOWARN | __GFP_NORETRY | 
__GFP_NOMEMALLOC), trying not to steal memory away from things that 
really need it.  But in practice, it can still easy drive the machine 
into a massive unrecoverable swap storm.

So I guess what I need is some measurement of "memory use" which is 
perhaps akin to a system-wide RSS; a measure of the number of pages 
being actively used, that if non-resident would cause a large amount of 
paging.  If you shrink the domain down to that number of pages + some 
padding (x%?), then the system will run happily in a stable state.  If 
that number increases, then the system will need new memory soon, to 
stop it from thrashing.  And if that number goes way below the domain's 
actual memory allocation, then it has "too much" memory.

Is this what "Active" accounts for?  Is Active just active 
usermode/pagecache pages, or does it also include kernel allocations?  
Presumably Inactive Clean memory can be freed very easily with little 
impact on the system, Inactive Dirty memory isn't needed but needs IO to 
free; is there some way to measure how big each class of memory is?

If you wanted to apply gentle memory pressure on the system to attempt 
to accelerate freeing memory, how would you go about doing that?  Would 
simply allocating memory at a controlled rate achieve it?

I guess it also gets more complex when you bring nodes and zones into 
the picture.  Does it mean that this computation would need to be done 
per node+zone rather than system-wide?

Or is there some better way to implement all this?

Thanks,
   J
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