+RTS -S heap reporting oddity

[Simon Marlow]

On 17/08/2012 17:08, Wolfram Kahl wrote:
> 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.)

It looks like on line 5, the GC thought it was going to do a major 
collection the next time, so it left 3G free to copy the contents of the 
old generation.  But then it didn't do a major GC until line 9.  I've 
just checked the code, and I think this might be due to a slight 
inaccuracy in the way that we estimate whether the next GC will be a 
major one, and at these huge sizes the discrepancy becomes significant. 
  Thanks for pointing it out, I'll fix it to use the same calculation in 
both places.

Cheers,
	Simon



>
> 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)
>
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