On Mon, Feb 08, 2021 at 02:41:08PM +0100, Vlastimil Babka wrote:
When creating a new kmem cache, SLUB determines how large the slab pages will based on number of inputs, including the number of CPUs in the system. Larger slab pages mean that more objects can be allocated/free from per-cpu slabs before accessing shared structures, but also potentially more memory can be wasted due to low slab usage and fragmentation. The rough idea of using number of CPUs is that larger systems will be more likely to benefit from reduced contention, and also should have enough memory to spare.
<SNIP>
So this patch tries to determine the best available value without specific arch knowledge.
- num_present_cpus() if the number is larger than 1, as that means the arch is
likely setting it properly
- nr_cpu_ids otherwise
This should fix the reported regressions while also keeping the effect of 045ab8c9487b for PowerPC systems. It's possible there are configurations where num_present_cpus() is 1 during boot while nr_cpu_ids is at the same time bloated, so these (if they exist) would keep the large orders based on nr_cpu_ids as was before 045ab8c9487b.
Tested-by: Mel Gorman mgorman@techsingularity.net
Only x86-64 tested, three machines, all showing similar results as would be expected. One example;
hackbench-process-sockets 5.11.0-rc7 5.11.0-rc7 5.11.0-rc7 vanilla revert-v1r1 vbabka-fix-v1r1 Amean 1 0.3873 ( 0.00%) 0.4060 ( -4.82%) 0.3747 ( 3.27%) Amean 4 1.3767 ( 0.00%) 0.7700 * 44.07%* 0.7790 * 43.41%* Amean 7 2.4710 ( 0.00%) 1.2753 * 48.39%* 1.2680 * 48.68%* Amean 12 3.7103 ( 0.00%) 1.9570 * 47.26%* 1.9470 * 47.52%* Amean 21 5.9790 ( 0.00%) 2.9760 * 50.23%* 2.9830 * 50.11%* Amean 30 8.0467 ( 0.00%) 4.0590 * 49.56%* 4.0410 * 49.78%* Amean 48 12.8180 ( 0.00%) 6.5167 * 49.16%* 6.4070 * 50.02%* Amean 79 20.5150 ( 0.00%) 10.3580 * 49.51%* 10.3740 * 49.43%* Amean 110 25.5320 ( 0.00%) 14.0453 * 44.99%* 14.0577 * 44.94%* Amean 141 32.4170 ( 0.00%) 17.3267 * 46.55%* 17.4977 * 46.02%* Amean 172 40.0883 ( 0.00%) 21.0360 * 47.53%* 21.1480 * 47.25%* Amean 203 47.2923 ( 0.00%) 25.2367 * 46.64%* 25.4923 * 46.10%* Amean 234 55.2623 ( 0.00%) 29.0720 * 47.39%* 29.3273 * 46.93%* Amean 265 61.4513 ( 0.00%) 33.0260 * 46.26%* 33.0617 * 46.20%* Amean 296 73.2960 ( 0.00%) 36.6920 * 49.94%* 37.2520 * 49.18%*
Comparing just a revert and the patch
5.11.0-rc7 5.11.0-rc7 revert-v1r1 vbabka-fix-v1r1 Amean 1 0.4060 ( 0.00%) 0.3747 ( 7.72%) Amean 4 0.7700 ( 0.00%) 0.7790 ( -1.17%) Amean 7 1.2753 ( 0.00%) 1.2680 ( 0.58%) Amean 12 1.9570 ( 0.00%) 1.9470 ( 0.51%) Amean 21 2.9760 ( 0.00%) 2.9830 ( -0.24%) Amean 30 4.0590 ( 0.00%) 4.0410 ( 0.44%) Amean 48 6.5167 ( 0.00%) 6.4070 ( 1.68%) Amean 79 10.3580 ( 0.00%) 10.3740 ( -0.15%) Amean 110 14.0453 ( 0.00%) 14.0577 ( -0.09%) Amean 141 17.3267 ( 0.00%) 17.4977 * -0.99%* Amean 172 21.0360 ( 0.00%) 21.1480 ( -0.53%) Amean 203 25.2367 ( 0.00%) 25.4923 ( -1.01%) Amean 234 29.0720 ( 0.00%) 29.3273 ( -0.88%) Amean 265 33.0260 ( 0.00%) 33.0617 ( -0.11%) Amean 296 36.6920 ( 0.00%) 37.2520 ( -1.53%)
That's a negligible difference and all but one group (141) was within the noise. Even for 141, it's very marginal and with the degree of overload at that group count, it can be ignored.
Thanks!