Re: [PATCH 0/5] make slab gfp fair

Peter Zijlstra
2007-05-14T20:05:49Z

On Mon, 2007-05-14 at 12:44 -0700, Andrew Morton wrote:
> On Mon, 14 May 2007 11:12:24 -0500
> Matt Mackall <mpm@selenic.com> wrote:
>
> > If I understand this correctly:
> >
> > privileged thread unprivileged greedy process
> > kmem_cache_alloc(...)
> > adds new slab page from lowmem pool
> > do_io()
> > kmem_cache_alloc(...)
> > kmem_cache_alloc(...)
> > kmem_cache_alloc(...)
> > kmem_cache_alloc(...)
> > kmem_cache_alloc(...)
> > ...
> > eats it all
> > kmem_cache_alloc(...) -> ENOMEM
> > who ate my donuts?!
>
> Yes, that's my understanding also.
>
> I can see why it's a problem in theory, but I don't think Peter has yet
> revealed to us why it's a problem in practice. I got all excited when
> Christoph asked "I am not sure what the point of all of this is.", but
> Peter cunningly avoided answering that ;)
>
> What observed problem is being fixed here?

I'm moving towards swapping over networked storage. Admittedly a new
feature.

Like with pretty much all other swap solutions; there is the fundamental
vm deadlock: freeing memory requires memory. Current block devices get
around that by using mempools. This works well.

However with network traffic mempools are not easily usable; the network
stack uses kmalloc. By using reserve based allocation we can keep
operating in a similar matter.