Re: How to get a sense of VM pressure

Peter Zijlstra
2008-07-28T07:36:10Z

On Fri, 2008-07-25 at 10:55 -0700, Jeremy Fitzhardinge wrote:
> 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?

Have a peek at this:

http://people.redhat.com/~riel/riel-OLS2006.pdf

The refault patches have been posted several times, but nobody really
tried to use them for your problem.