During one of my long Agda runs (with GHC-7.4.2), I observed the following output, with run-time options +RTS -S -H11G -M11G -K256M : 7694558208 30623864 3833166176 0.11 0.11 234.75 234.79 0 0 (Gen: 0) 7678904688 29295168 3847737784 0.11 0.11 242.04 242.09 0 0 (Gen: 0) 7662481840 29195736 3861451856 0.11 0.11 249.31 249.35 0 0 (Gen: 0) 7647989280 26482704 3872463688 0.12 0.12 256.64 256.68 0 0 (Gen: 0) 4609865360 25764016 3886000448 0.09 0.09 261.04 261.09 0 0 (Gen: 0) 4581294920 19435032 3891512272 0.07 0.07 265.37 265.42 0 0 (Gen: 0) 4568757088 21095864 3902286000 0.08 0.08 269.70 269.74 0 0 (Gen: 0) 4546421608 21618856 3913923976 0.09 0.09 274.04 274.09 0 0 (Gen: 0) 4521518888 2894668056 3484748224 7.63 7.63 285.94 285.98 0 0 (Gen: 1) 8085358392 23776128 3499185336 0.11 0.11 293.49 293.53 0 0 (Gen: 0) 8064630856 32055112 3515876576 0.13 0.13 300.91 300.95 0 0 (Gen: 0) 8040500112 31477608 3528105088 0.12 0.12 308.37 308.41 0 0 (Gen: 0) 8031456296 29641328 3540632456 0.11 0.11 315.83 315.87 0 0 (Gen: 0) 8018447264 30187208 3554339600 0.12 0.12 323.26 323.31 0 0 (Gen: 0) To my untrained eye, this seems to be saying the following: In the first 4 lines, the heap runs (almost) full before (minor) collections. In lines 5 to 9 it apparently leaves 3G empty before collection, but ``those 3G'' then appear on line 9 in the ``amount of data copied during (major) collection'' column, and after that it runs up to fill all 11G again before the next few minor collections. What is really going on here? (Previously I had never seen such big numbers in the second column on major collections.) Wolfram P.S.: Same effect again, but more dramatic, later during the same Agda run: 448829488 4864536 5710435424 0.02 0.02 1422.80 1422.90 0 0 (Gen: 0) 445544064 3251712 5710248752 0.01 0.01 1423.23 1423.32 0 0 (Gen: 0) 450236784 4148864 5712696848 0.02 0.02 1423.68 1423.77 0 0 (Gen: 0) 445240152 3828120 5713606328 0.02 0.02 1424.10 1424.19 0 0 (Gen: 0) 443285616 5906448 5717731864 0.02 0.02 1424.52 1424.61 0 0 (Gen: 0) 430698248 4773500032 5363214440 9.30 9.30 1434.21 1434.30 0 0 (Gen: 1) 6148455592 13490304 5374609848 0.07 0.07 1439.83 1439.92 0 0 (Gen: 0) 6185350848 27419744 5389326896 0.11 0.11 1445.50 1445.59 0 0 (Gen: 0) 6168805736 23069072 5398725784 0.11 0.11 1451.22 1451.32 0 0 (Gen: 0) 6157744328 23451872 5408370152 0.09 0.09 1456.93 1457.03 0 0 (Gen: 0) 6151715272 25739584 5421044592 0.11 0.11 1462.62 1462.72 0 0 (Gen: 0) 6132589488 24541688 5428809632 0.10 0.10 1468.26 1468.37 0 0 (Gen: 0)