Re: [PATCH 02/10] mm: system wide ALLOC_NO_WATERMARK

Andrew Morton
2007-08-06T20:28:49Z

On Mon, 6 Aug 2007 13:19:26 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:

> On Mon, 6 Aug 2007, Matt Mackall wrote:
>
> > > > Because a block device may have deadlocked here, leaving the system
> > > > unable to clean dirty memory, or unable to load executables over the
> > > > network for example.
> > >
> > > So this is a locking problem that has not been taken care of?
> >
> > No.
> >
> > It's very simple:
> >
> > 1) memory becomes full
>
> We do have limits to avoid memory getting too full.
>
> > 2) we try to free memory by paging or swapping
> > 3) I/O requires a memory allocation which fails because memory is full
> > 4) box dies because it's unable to dig itself out of OOM
> >
> > Most I/O paths can deal with this by having a mempool for their I/O
> > needs. For network I/O, this turns out to be prohibitively hard due to
> > the complexity of the stack.
>
> The common solution is to have a reserve (min_free_kbytes). The problem
> with the network stack seems to be that the amount of reserve needed
> cannot be predicted accurately.
>
> The solution may be as simple as configuring the reserves right and
> avoid the unbounded memory allocations. That is possible if one
> would make sure that the network layer triggers reclaim once in a
> while.

Such a simple fix would be attractive. Some of the net drivers now have
remarkably large rx and tx queues. One wonders if this is playing a part
in the problem and whether reducing the queue sizes would help.

I guess we'd need to reduce the queue size on all NICs in the machine
though, which might be somewhat of a performance problem.

I don't think we've seen a lot of justification for those large queues.
I'm suspecting it's a few percent in carefully-chosen workloads (of the
microbenchmark kind?)