ZFS scrub CPU bound?

[Daniel Kalchev]

I have built myself an server with SSD only drives. That thing goes 
"fast", like during scrub

# zpool status
   pool: system
  state: ONLINE
   scan: scrub in progress since Thu Aug  9 18:30:46 2012
         87.1G scanned out of 193G at 602M/s, 0h3m to go
         0 repaired, 45.06% done
config:

         NAME        STATE     READ WRITE CKSUM
         system      ONLINE       0     0     0
           raidz1-0  ONLINE       0     0     0
             da0p3   ONLINE       0     0     0
             da1p3   ONLINE       0     0     0
             da2p3   ONLINE       0     0     0
             da3p3   ONLINE       0     0     0

But, what I wonder is top -S

# top -S
last pid:  7912;  load averages:  9.89,  3.26,  
1.29                                                                                     
up 0+06:03:59  18:32:04
146 processes: 3 running, 142 sleeping, 1 waiting
CPU:  0.4% user,  0.0% nice, 51.4% system,  0.8% interrupt, 47.3% idle
Mem: 2171M Active, 1541M Inact, 5407M Wired, 25M Cache, 416K Buf, 22G Free
Swap: 8192M Total, 8192M Free

   PID USERNAME  THR PRI NICE   SIZE    RES STATE   C   TIME   WCPU COMMAND
    11 root       32 155 ki31     0K   512K CPU31  31 190.2H 1818.46% idle
     0 root      268  -8    0     0K  4288K -       0 141:25 1261.33% kernel
     4 root        4  -8    -     0K    80K CPU30  30   9:13 95.17% zfskern
    13 root        3  -8    -     0K    48K -       4   5:37 42.97% geom
    12 root       66 -84    -     0K  1056K WAIT    0   6:19 30.86% intr
[...]

It seems that zfskern will top to 100%. This is an 32 core system, and 
as you see scrub, at 600MB/sec is able to eat 16 cores (from 2x 2.2 GHz 
Opteron 6274). There is high load on geom as well... but geom does go 
over 100% CPU, so I suppose it scales.

What might be there in ZFS code that is single-CPU bound?

freebsd 9-stable.

Daniel