mm: page_alloc: default node-ordering on 64-bit NUMA, zone-ordering on 32-bit

Linux Kernel / Linux Kernel - Mel Gorman [suse.de] - 9 October 2014 21:25 UTC

Zones are allocated by the page allocator in either node or zone order. Node ordering is preferred in terms of locality and is applied automatically in one of three cases:

1. If a node has only low memory

2. If DMA/DMA32 is a high percentage of memory

3. If low memory on a single node is greater than 70% of the node size

Otherwise zone ordering is used to preserve low memory for devices that require it. Unfortunately a consequence of this is that applications running on a machine with balanced NUMA nodes will experience different performance characteristics depending on which node they happen to start from.

The point of zone ordering is to protect lower zones for devices that require DMA/DMA32 memory. When NUMA was first introduced, this was critical as 32-bit NUMA machines existed and exhausting low memory triggered OOMs easily as so many allocations required low memory. On 64-bit machines the primary concern is devices that are 32-bit only which is less severe than the low memory exhaustion problem on 32-bit NUMA. It seems there are really few devices that depends on it.

AGP -- I assume this is getting more rare but even then I think the allocations happen early in boot time where lowmem pressure is less of a problem

DRM -- If the device is 32-bit only then there may be low pressure. I didn't evaluate these in detail but it looks like some of these are mobile graphics card. Not many NUMA laptops out there. DRM folk should know better though.

Some TV cards -- Much demand for 32-bit capable TV cards on NUMA machines?

B43 wireless card -- again not really a NUMA thing.

I cannot find a good reason to incur a performance penalty on all 64-bit NUMA machines in case someone throws a brain damanged TV or graphics card in there. This patch defaults to node-ordering on 64-bit NUMA machines. I was tempted to make it default everywhere but I understand that some embedded arches may be using 32-bit NUMA where I cannot predict the consequences.

The performance impact depends on the workload and the characteristics of the machine and the machine I tested on had a large Normal zone on node 0 so the impact is within the noise for the majority of tests. The allocation stats show more allocation requests were from DMA32 and local node. Running SpecJBB with multiple JVMs and automatic NUMA balancing disabled the results were

specjbb 3.17.0-rc2 3.17.0-rc2
vanilla nodeorder-v1r1 Min 1 29534.00 ( 0.00%) 30020.00 ( 1.65%) Min 10 115717.00 ( 0.00%) 134038.00 ( 15.83%) Min 19 109718.00 ( 0.00%) 114186.00 ( 4.07%) Min 28 104459.00 ( 0.00%) 103639.00 ( -0.78%) Min 37 98245.00 ( 0.00%) 103756.00 ( 5.61%) Min 46 97198.00 ( 0.00%) 96197.00 ( -1.03%) Mean 1 30953.25 ( 0.00%) 31917.75 ( 3.12%) Mean 10 124432.50 ( 0.00%) 140904.00 ( 13.24%) Mean 19 116033.50 ( 0.00%) 119294.75 ( 2.81%) Mean 28 108365.25 ( 0.00%) 106879.50 ( -1.37%) Mean 37 102984.75 ( 0.00%) 106924.25 ( 3.83%) Mean 46 100783.25 ( 0.00%) 105368.50 ( 4.55%) Stddev 1 1260.38 ( 0.00%) 1109.66 ( 11.96%) Stddev 10 7434.03 ( 0.00%) 5171.91 ( 30.43%) Stddev 19 8453.84 ( 0.00%) 5309.59 ( 37.19%) Stddev 28 4184.55 ( 0.00%) 2906.63 ( 30.54%) Stddev 37 5409.49 ( 0.00%) 3192.12 ( 40.99%) Stddev 46 4521.95 ( 0.00%) 7392.52 (-63.48%) Max 1 32738.00 ( 0.00%) 32719.00 ( -0.06%) Max 10 136039.00 ( 0.00%) 148614.00 ( 9.24%) Max 19 130566.00 ( 0.00%) 127418.00 ( -2.41%) Max 28 115404.00 ( 0.00%) 111254.00 ( -3.60%) Max 37 112118.00 ( 0.00%) 111732.00 ( -0.34%) Max 46 108541.00 ( 0.00%) 116849.00 ( 7.65%) TPut 1 123813.00 ( 0.00%) 127671.00 ( 3.12%) TPut 10 497730.00 ( 0.00%) 563616.00 ( 13.24%) TPut 19 464134.00 ( 0.00%) 477179.00 ( 2.81%) TPut 28 433461.00 ( 0.00%) 427518.00 ( -1.37%) TPut 37 411939.00 ( 0.00%) 427697.00 ( 3.83%) TPut 46 403133.00 ( 0.00%) 421474.00 ( 4.55%)

3.17.0-rc2 3.17.0-rc2
vanillanodeorder-v1r1 DMA allocs 0 0 DMA32 allocs 57 1491992 Normal allocs 32543566 30026383 Movable allocs 0 0 Direct pages scanned 0 0 Kswapd pages scanned 0 0 Kswapd pages reclaimed 0 0 Direct pages reclaimed 0 0 Kswapd efficiency 100% 100% Kswapd velocity 0.000 0.000 Direct efficiency 100% 100% Direct velocity 0.000 0.000 Percentage direct scans 0% 0% Zone normal velocity 0.000 0.000 Zone dma32 velocity 0.000 0.000 Zone dma velocity 0.000 0.000 THP fault alloc 55164 52987 THP collapse alloc 139 147 THP splits 26 21 NUMA alloc hit 4169066 4250692 NUMA alloc miss 0 0

Note that there were more DMA32 allocations with the patch applied. In this particular case there was no difference in numa_hit and numa_miss. The expectation is that DMA32 was being used at the low watermark instead of falling into the slow path. kswapd was not woken but it's not worken for THP allocations.

On 32-bit, this patch defaults to zone-ordering as low memory depletion can be a serious problem on 32-bit large memory machines. If the default ordering was node then processes on node 0 will deplete the Normal zone due to normal activity. The problem is worse if CONFIG_HIGHPTE is not set. If combined with large amounts of dirty/writeback pages in Normal zone then there is also a high risk of OOM. The heuristics are removed as it's not clear they were ever important on 32-bit. They were only relevant for setting node-ordering on 64-bit.

3193913 mm: page_alloc: default node-ordering on 64-bit NUMA, zone-ordering on 32-bit
mm/page_alloc.c | 78 ++++++++++++++-----------------------------------------
1 file changed, 20 insertions(+), 58 deletions(-)

Upstream: git.kernel.org


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