Is evacuate for StgMutArrPtrs and StgArrPtrs expensive to GC?

[Johan Tibell]

The code for 'allocate' in rts/sm/Storage.c doesn't seem that
expensive. An extra branch compared to inline allocation and
allocation is done in the next nursery block (risking fragmentation?).

-- Johan

On Mon, Sep 30, 2013 at 9:50 PM, Johan Tibell <johan.tibell at gmail.com> wrote:
> Hi,
>
> When I benchmark Data.HashMap.insert from unordered-containers
> (inserting the keys [0..10000]) the runtime is dominated by GC:
>
> $ cat Test.hs
> module Main where
>
> import           Control.DeepSeq
> import           Control.Exception
> import           Control.Monad
> import qualified Data.HashMap.Strict as HM
> import           Data.List (foldl')
>
> main = do
>     let ks = [0..10000] :: [Int]
>     evaluate (rnf ks)
>     forM_ ([0..1000] :: [Int]) $ \ x -> do
>         evaluate $ HM.null $ foldl' (\ m k -> HM.insert k x m) HM.empty ks
>
> $ perf record -g ./Test +RTS -s
>    6,187,678,112 bytes allocated in the heap
>    3,309,887,128 bytes copied during GC
>        1,299,200 bytes maximum residency (1002 sample(s))
>          118,816 bytes maximum slop
>                5 MB total memory in use (0 MB lost due to fragmentation)
>
>                                     Tot time (elapsed)  Avg pause  Max pause
>   Gen  0     11089 colls,     0 par    1.31s    1.30s     0.0001s    0.0005s
>   Gen  1      1002 colls,     0 par    0.49s    0.51s     0.0005s    0.0022s
>
>   INIT    time    0.00s  (  0.00s elapsed)
>   MUT     time    1.02s  (  1.03s elapsed)
>   GC      time    1.80s  (  1.80s elapsed)
>   EXIT    time    0.00s  (  0.00s elapsed)
>   Total   time    2.82s  (  2.84s elapsed)
>
>   %GC     time      63.7%  (63.5% elapsed)
>
>   Alloc rate    6,042,264,963 bytes per MUT second
>
>   Productivity  36.3% of total user, 36.1% of total elapsed
>
> $ perf report
> 41.46%  Test  Test               [.] evacuate
> 15.47%  Test  Test               [.] scavenge_block
> 11.04%  Test  Test               [.] s3cN_info
>  8.74%  Test  Test               [.] s3aZ_info
>  3.59%  Test  Test               [.] 0x7ff5
>  2.83%  Test  Test               [.] scavenge_mut_arr_ptrs
>  2.69%  Test  libc-2.15.so       [.] 0x147fd9
>  2.51%  Test  Test               [.] allocate
>  2.00%  Test  Test               [.] s3oo_info
>  0.91%  Test  Test               [.] todo_block_full
>  0.87%  Test  Test               [.] hs_popcnt64
>  0.80%  Test  Test               [.] s3en_info
>  0.62%  Test  Test               [.] s3el_info
>
> Is GC:ing StgMutArrPtrs and StgArrPtrs, which I create a lot of, more
> expensive than GC:ing normal heap objects (i.e. for standard data
> types)?
>
> -- Johan