Re: [PATCH] vm: enhance __alloc_pages to prioritize pagecache eviction when pressed for memory

[Andrew Morton <akpm@osdl.org>]

Neil Horman <nhorman@tuxdriver.com> wrote:
>
> On Fri, Dec 09, 2005 at 04:29:01PM -0800, Andrew Morton wrote:
> > Neil Horman <nhorman@tuxdriver.com> wrote:
> > >
> > > Hey all-
> > >      I was recently shown this issue, wherein, if the kernel was kept full of
> > > pagecache via applications that were constantly writing large amounts of data to
> > > disk, the box could find itself in a position where the vm, in __alloc_pages
> > > would invoke the oom killer repetatively within try_to_free_pages, until such
> > > time as the box had no candidate processes left to kill, at which point it would
> > > panic.
> > 
> > That's pretty bad.  Are you able to provide a description which would permit
> > others to reproduce this?
> 
> As promised, heres the reproducer that was given to me, and used to reproduce
> this problem:
> 
> 1) setup an nfs serve with a thread count of 2.  Of course, 1 thread might make
> the problem more easy to reproduce.  I haven't tried it yet.
> 
> 2) Setup 4 nodes to hammer the nfs mounted directory.  The 4 nodes should hammer
> out 4 gigs.  2 gigs didn't seem to be enough.
> 
> I used a locally developed tool called ior to reproduce this problem.  The tool
> can be found here:
> 
> http://www.llnl.gov/asci/platforms/purple/rfp/benchmarks/limited/ior/
> 
> I suppose anything that can write to NFS fast should be fine.  But that's what I
> did.
> 
> 
> If you do this, any node writing to the server that has more than 4GB of RAM
> should start oom killing to the point where it runs out of candidate processes
> and panics

We merged an NFS fix last week which will help throttling under heavy
writeout conditions..
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