Is evacuate for StgMutArrPtrs and StgArrPtrs expensive to GC?

[Johan Tibell]

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