SplittingPool: CPU allocations

[maleadt]

Looks like the new pool performs quite some CPU allocations (even though I did go through and made the code type stable):

[ Info: Epoch 2
 95.020749 seconds (223.41 M CPU allocations: 11.091 GiB, 7.04% gc time) (1.56 M GPU allocations: 2.084 TiB, 23.15% gc time of which 0.88% spent allocating)
 ────────────────────────────────────────────────────────────────────────────
                                     Time                   Allocations      
                             ──────────────────────   ───────────────────────
      Tot / % measured:           95.0s / 22.7%           11.1GiB / 24.7%    

 Section             ncalls     time   %tot     avg     alloc   %tot      avg
 ────────────────────────────────────────────────────────────────────────────
 pooled alloc         1.56M    21.6s   100%  13.8μs   2.74GiB  100%   1.83KiB
   1.1a repopulate        1   2.44ms  0.01%  2.44ms   2.08MiB  0.07%  2.08MiB
   1.1b compact           1   7.71ms  0.04%  7.71ms   1.63MiB  0.06%  1.63MiB
   1.2 scan           1.56M    1.46s  6.76%   933ns    408MiB  14.6%     273B
   1.3 alloc            256    347ms  1.61%  1.36ms   12.8KiB  0.00%    51.3B
   1.4a reclaim         747    120ms  0.56%   160μs   59.9KiB  0.00%    82.1B
   1.4b alloc           747    842ms  3.90%  1.13ms   23.4KiB  0.00%    32.0B
   1.5a reclaim         249    104ms  0.48%   418μs   58.4KiB  0.00%     240B
   1.5b alloc           249    271ms  1.25%  1.09ms   7.78KiB  0.00%    32.0B
   2.0 gc(false)        249    10.7s  49.6%  43.0ms    481MiB  17.2%  1.93MiB
     pooled free      1.56M    586ms  2.71%   376ns     0.00B  0.00%    0.00B
   2.1a repopulate      249    544ms  2.52%  2.19ms    568MiB  20.3%  2.28MiB
   2.1b compact         249    1.96s  9.06%  7.86ms    437MiB  15.6%  1.75MiB
   2.2 scan             249    693μs  0.00%  2.78μs    193KiB  0.01%     796B
 pooled free          5.70k   2.58ms  0.01%   452ns     0.00B  0.00%    0.00B
 ────────────────────────────────────────────────────────────────────────────
 ──────────────────────────────────────────────────────────────────
                           Time                   Allocations      
                   ──────────────────────   ───────────────────────
 Tot / % measured:      95.0s / 1.68%           11.1GiB / 0.21%    

 Section   ncalls     time   %tot     avg     alloc   %tot      avg
 ──────────────────────────────────────────────────────────────────
 alloc       391k    1.53s  95.6%  3.91μs   23.8MiB  100%     63.9B
 free        390k   70.5ms  4.40%   181ns      192B  0.00%    0.00B
 ──────────────────────────────────────────────────────────────────

vs

[ Info: Epoch 2
 89.118641 seconds (177.10 M CPU allocations: 8.993 GiB, 6.44% gc time) (1.56 M GPU allocations: 2.084 TiB, 13.51% gc time of which 1.48% spent allocating)
 ──────────────────────────────────────────────────────────────────────────
                                   Time                   Allocations      
                           ──────────────────────   ───────────────────────
     Tot / % measured:          89.1s / 13.2%           8.99GiB / 7.45%    

 Section           ncalls     time   %tot     avg     alloc   %tot      avg
 ──────────────────────────────────────────────────────────────────────────
 background task        1   20.3ms  0.17%  20.3ms    226KiB  0.03%   226KiB
   pooled free        626    272μs  0.00%   435ns   29.3KiB  0.00%    48.0B
   reclaim              1   1.22μs  0.00%  1.22μs     0.00B  0.00%    0.00B
   scan                 1    717ns  0.00%   717ns      336B  0.00%     336B
 pooled alloc       1.56M    11.8s   100%  7.53μs    686MiB  100%      460B
   1. try alloc       294    447ms  3.79%  1.52ms   13.8KiB  0.00%    48.0B
   2. gc(false)       294    9.62s  81.6%  32.7ms    555MiB  80.9%  1.89MiB
     pooled free    1.56M    527ms  4.47%   337ns   71.6MiB  10.4%    48.0B
 ──────────────────────────────────────────────────────────────────────────
 ──────────────────────────────────────────────────────────────────
                           Time                   Allocations      
                   ──────────────────────   ───────────────────────
 Tot / % measured:      89.1s / 0.66%           8.99GiB / 0.26%    

 Section   ncalls     time   %tot     avg     alloc   %tot      avg
 ──────────────────────────────────────────────────────────────────
 alloc       390k    509ms  86.0%  1.30μs   23.8MiB  100%     64.0B
 free        390k   82.8ms  14.0%   212ns     0.00B  0.00%    0.00B
 ──────────────────────────────────────────────────────────────────