Re: [PATCH 4/4] zone_reclaim_mode is always 0 by default

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hi

> > Current linux policy is, zone_reclaim_mode is enabled by default if the machine
> > has large remote node distance. it's because we could assume that large distance 
> > mean large server until recently.
> > 
> > Unfortunately, recent modern x86 CPU (e.g. Core i7, Opeteron) have P2P transport
> > memory controller. IOW it's seen as NUMA from software view.
> > 
> > Some Core i7 machine has large remote node distance, but zone_reclaim don't
> > fit desktop and small file server. it cause performance degression.
> > 
> > Thus, zone_reclaim == 0 is better by default if the machine is small.
> 
> What if I had a node 0 with 32GB or 128GB of memory.  In that case,
> we would have 3GB for DMA32, 125GB for Normal and then a node 1 with
> 128GB.  I would suggest that zone reclaim would perform normally and
> be beneficial.
> 
> You are unfairly classifying this as a size of machine problem when it is
> really a problem with the underlying zone reclaim code being triggered
> due to imbalanced node/zones, part of which is due to a single node
> having multiple zones and those multiple zones setting up the conditions
> for extremely agressive reclaim.  In other words, you are putting a
> bandage in place to hide a problem on your particular hardware.
> 
> Can RECLAIM_DISTANCE be adjusted so your Ci7 boxes are no longer caught?
> Aren't 4 node Ci7 boxes soon to be readily available?  How are your apps
> different from my apps in that you are not impacted by node locality?
> Are you being too insensitive to node locality?  Conversely am I being
> too sensitive?
> 
> All that said, I would not stop this from going in.  I just think the
> selection criteria is rather random.  I think we know the condition we
> are trying to avoid which is a small Normal zone on one node and a larger
> Normal zone on another causing zone reclaim to be overly agressive.
> I don't know how to quantify "small" versus "large".  I would suggest
> that a node 0 with 16 or more GB should have zone reclaim on by default
> as well.  Can that be expressed in the selection criteria.

I post my opinion as another mail. please see it.







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