Here are buildworld times on my test box with and without pre-zeroing,
single threaded (no -j option used):
/usr/bin/time -l make buildworld >& /tmp/bw.out
2306.94 real 1740.54 user 393.90 sys nocache=1, enable=1
2305.50 real 1738.80 user 397.38 sys nocache=1, enable=1
2306.85 real 1736.72 user 399.62 sys nocache=1, enable=1
2307.96 real 1737.63 user 398.24 sys nocache=1, enable=1
2306.22 real 1741.29 user 395.15 sys nocache=1, enable=1
2320.21 real 1739.50 user 396.49 sys nocache=1, enable=1
2311.48 real 1739.04 user 396.29 sys nocache=1, enable=1
2323.78 real 1735.54 user 399.61 sys nocache=1, enable=1
2331.38 real 1733.84 user 417.91 sys enable=0
2327.75 real 1733.74 user 415.24 sys enable=0
2322.29 real 1731.73 user 419.57 sys enable=0
2327.53 real 1729.05 user 416.27 sys enable=0
2335.71 real 1731.32 user 417.93 sys enable=0
As you can see it does seem to make a small difference. The system
time is about 20 seconds (4.8%) faster with pre-zeroing and the real
time is around ~18 seconds (0.7%) or so faster.
What this says, basically, is that the pre-zeroing helps out serialized
operations (such as shell scripts) which have a lot of zfod faults but
probably doesn't make much of a difference in the grand scheme of things.
I also tried some bursting tests but it turns out that once the system
is in a steady state the VM free page queue doesn't have enough pages
to really be able to handle the zfod fault rate. So I bumped
vm.v_free_target to an absurdly high number like 100000 (400MB) and
ran a little shell script which ran /bin/echo in a big loop and I
got:
#!/bin/csh
#
set i = 0
while ( $i < 10000 )
/bin/echo fubar > /dev/null
@ i = $i + 1
end
0.437u 1.148s 0:01.59 98.7% 342+164k 0+0io 0pf+0w idlezero enabled
0.460u 1.257s 0:01.71 100.0% 313+155k 0+0io 0pf+0w idlezero disabled
Or around a 7% execution time difference. The problem, though, is that
the system really needs a large number of free pages so the zfod zero
fault burst doesn't drain the pre-zerod page cache during the test.
With the default vm.v_free_target of 20000 on this box (40MB) the zfod
faults strip out the pre-zerod pages almost instantly and the pgzero
thread can't keep up. Run times wind up being 1.69 or only 1.2%
faster.
I then tried bumping up vm.idlezero_rate to infinity (well, 999999)
with a normal vm.v_free_target and that helped some. I got numbers
in the 1.61 to 1.65 range.
It may well be that the correct answer here is to remove the rate
limiting entirely.
-Matt
