Boot on various Amazon M6g (Graviton2 based) instances.
On a 64-core instance I observed a 10s reduction in buildworld times with
a previous version of this patch (only one build with and without the change).
I timed 10 make buildkernel -j64 runs with an empty obj. I ignored the first
tun as it was used to warm the cache. The user and real times showed no
significant change. The sys time shows the following improvement from an
earlier version of this patch. isync.sys is a full icache sync, no_isync
is with just a dsb & isb.
```
x isync.sys
+ no_isync.sys
+------------------------------------------------------------------------------+
| + x |
|+ + + + + x x x * ++x x x x|
| |___________M_______A___________|_______|_____A____________| |
+------------------------------------------------------------------------------+
N Min Max Median Avg Stddev
x 9 151.36 167.08 155.41 157.94778 5.6033289
+ 9 135.34 158.05 144.11 147.28333 8.2816167
Difference at 95.0% confidence
-10.6644 +/- 7.06605
-6.75188% +/- 4.38105%
(Student's t, pooled s = 7.07045)
```
Running `make buildworld -j4` on a 4 core M6g instance with a clean obj 10 times (after running once and ignoring the result to prime caches) I get the following results. isync is with the sysctls set to 0, no_isync is with them set to 1.
```
x isync.user
+ no_isync.user
+------------------------------------------------------------------------------+
|x * x xx * ++ + + + x ++ x + x|
| |_______________M___|_____A__________MA_____________|____| |
+------------------------------------------------------------------------------+
N Min Max Median Avg Stddev
x 10 1660.01 1662.96 1660.655 1661.061 0.99780259
+ 10 1660.07 1662.52 1661.455 1661.516 0.71498562
No difference proven at 95.0% confidence
x isync.sys
+ no_isync.sys
+------------------------------------------------------------------------------+
|+ + + + + + ++ + + x x x x x x xx|
| |________AM________| |_________A________| |
+------------------------------------------------------------------------------+
N Min Max Median Avg Stddev
x 10 95.5 97.68 96.95 96.947 0.67774217
+ 10 92.13 94.15 93.39 93.305 0.67871202
Difference at 95.0% confidence
-3.642 +/- 0.637259
-3.75669% +/- 0.645117%
(Student's t, pooled s = 0.678227)
x isync.real
+ no_isync.real
+------------------------------------------------------------------------------+
|+ + + + +x +x + x x + +x + xx x xx|
| |_______________AM_____|________|___________A_M_________________| |
+------------------------------------------------------------------------------+
N Min Max Median Avg Stddev
x 10 445.98 447.92 447.1 447.003 0.79998681
+ 10 444.91 446.69 445.985 445.92 0.63177
Difference at 95.0% confidence
-1.083 +/- 0.677263
-0.24228% +/- 0.151286%
(Student's t, pooled s = 0.720802)
```