Messages in this thread | | | Date | Fri, 15 May 2009 05:51:37 -0500 | From | Robin Holt <> | Subject | Re: [PATCH 4/4] zone_reclaim_mode is always 0 by default |
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> 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.
Thanks, Robin Holt
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