Re: [PATCH resend^2] mm: increase RECLAIM_DISTANCE to 30

[KOSAKI Motohiro]

> On Mon, 11 Apr 2011 17:19:31 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> 
> > Recently, Robert Mueller reported zone_reclaim_mode doesn't work
> 
> It's time for some nagging.  
> 
> I'm trying to work out what the user-visible effect of this problem
> was, but it isn't described in the changelog and there is no link to
> any report and not even a Reported-by: or a Cc: and a search for Robert
> in linux-mm and linux-kernel turned up blank.

Here.
	http://lkml.org/lkml/2010/9/12/236


> 
> > properly on his new NUMA server (Dual Xeon E5520 + Intel S5520UR MB).
> > He is using Cyrus IMAPd and it's built on a very traditional
> > single-process model.
> > 
> >   * a master process which reads config files and manages the other
> >     process
> >   * multiple imapd processes, one per connection
> >   * multiple pop3d processes, one per connection
> >   * multiple lmtpd processes, one per connection
> >   * periodical "cleanup" processes.
> > 
> > Then, there are thousands of independent processes. The problem is,
> > recent Intel motherboard turn on zone_reclaim_mode by default and
> > traditional prefork model software don't work fine on it.
> > Unfortunatelly, Such model is still typical one even though 21th
> > century. We can't ignore them.
> > 
> > This patch raise zone_reclaim_mode threshold to 30. 30 don't have
> > specific meaning. but 20 mean one-hop QPI/Hypertransport and such
> > relatively cheap 2-4 socket machine are often used for tradiotional
> > server as above. The intention is, their machine don't use
> > zone_reclaim_mode.
> > 
> > Note: ia64 and Power have arch specific RECLAIM_DISTANCE definition.
> > then this patch doesn't change such high-end NUMA machine behavior.
> > 
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > Acked-by: Christoph Lameter <cl@xxxxxxxxx>
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > ---
> >  include/linux/topology.h |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/include/linux/topology.h b/include/linux/topology.h
> > index b91a40e..fc839bf 100644
> > --- a/include/linux/topology.h
> > +++ b/include/linux/topology.h
> > @@ -60,7 +60,7 @@ int arch_update_cpu_topology(void);
> >   * (in whatever arch specific measurement units returned by node_distance())
> >   * then switch on zone reclaim on boot.
> >   */
> > -#define RECLAIM_DISTANCE 20
> > +#define RECLAIM_DISTANCE 30
> 
> Any time we tweak a magic number to improve one platform, we risk
> causing deterioration on another.  Do we know that this risk is low
> with this patch?

In last thread, Robert Mueller who bug reporter explained he is only using
mere commodity whitebox hardware and very common workload.
Therefore, we agreed benefit is bigger than negative. IOW, mere whitebox
are used lots than special purpose one.



> Also, what are we doing setting
> 
> 	zone_relaim_mode = 1;
> 
> when we have nice enumerated constants for this?  It should be
> 
> 	zone_relaim_mode = RECLAIM_ZONE;
> 
> or, pedantically but clearer:
> 
> 	zone_relaim_mode = RECLAIM_ZONE & !RECLAIM_WRITE & !RECLAIM_SWAP;

Indeed.