optimized sbrk size. grade:

Results for mm malloc:
trace  valid  util     ops      secs  Kops
0       yes   99%    5694  0.001567  3634
1       yes   99%    5848  0.001503  3891
2       yes   99%    6648  0.001885  3528
3       yes  100%    5380  0.001419  3790
4       yes   66%   14400  0.002599  5540
5       yes   96%    4800  0.007916   606
6       yes   95%    4800  0.007996   600
7       yes   55%   12000  0.038866   309
8       yes   51%   24000  0.142658   168
9       yes   31%   14401  0.192439    75
10       yes   30%   14401  0.008111  1776
Total          75%  112372  0.406958   276

Perf index = 45 (util) + 11 (thru) = 56/100