Re: Default zone_reclaim_mode = 1 on NUMA kernel is bad forfile/email/web servers

["Bron Gondwana" <brong@fastmail.fm>]

On Tue, 28 Sep 2010 07:35 -0500, "Christoph Lameter" <cl@linux.com> wrote:
> > The problem we saw was purely with file caching. The application wasn't
> > actually allocating much memory itself, but it was reading lots of files
> > from disk (via mmap'ed memory mostly), and as most people would, we
> > expected that data would be cached in memory to reduce future reads from
> > disk. That was not happening.
> 
> Obviously and you have stated that numerous times. Problem that the use
> of
> a remote memory will reduced performance of reads so the OS (with
> zone_reclaim=1) defaults to the use of local memory and favors reclaim of
> local memory over the allocation from the remote node. This is fine if
> you have multiple applications running on both nodes because then each
> application will get memory local to it and therefore run faster. That
> does not work with a single app that only allocates from one node.

Is this what's happening, or is IO actually coming from disk in preference
to the remote node?  I can certainly see the logic behind preferring to
reclaim the local node if that's all that's happening - though the OS should
be allocating the different tasks more evenly across the nodes in that case.

> Control over memory allocations over the various nodes under NUMA
> for a process can occur via the numactl ctl or the libnuma C apis.
> 
> F.e.e
> 
> numactl --interleave ... command
> 
> will address that issue for a specific command that needs to go

Gosh what a pain.  While it won't kill us too much to add to our
startup, it does feel a lot like the tail is wagging the dog from here
still.  A task that doesn't ask for anything special should get sane
defaults, and the cost of data from the other node should be a lot
less than the cost of the same data from spinning rust.

Bron.
-- 
  Bron Gondwana
  brong@fastmail.fm

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