vmd: start vms defined in vm.conf in a staggered fashion

This addresses 'thundering herd' problem when a lot of
vms are configured in vm.conf.  A lot of vms booting in parallel can
overload the host and also mess up tsc calibration in openbsd guests as
it uses PIT which doesn't fire reliably if the host is overloaded.

We default to starting vms with parallelism of ncpuonline and a delay 30 seconds
between batches.  This is configurable in vm.conf.

ok mlarkin@ (also addressed comments from cheloha@)

1 files changed, 12 insertions, 2 deletions

diff --git a/usr.sbin/vmd/vm.conf.5 b/usr.sbin/vmd/vm.conf.5
index 2d2fe974dcf..8c864682329 100644
--- a/usr.sbin/vmd/vm.conf.5
+++ b/usr.sbin/vmd/vm.conf.5
@@ -1,4 +1,4 @@
-.\" $OpenBSD: vm.conf.5,v 1.44 2019/05/14 12:47:17 schwarze Exp $
+.\" $OpenBSD: vm.conf.5,v 1.45 2019/12/12 03:53:38 pd Exp $
 .\"
 .\" Copyright (c) 2015 Mike Larkin <mlarkin@openbsd.org>
 .\" Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
@@ -15,7 +15,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: May 14 2019 $
+.Dd $Mdocdate: December 12 2019 $
 .Dt VM.CONF 5
 .Os
 .Sh NAME
@@ -91,6 +91,16 @@ vm "vm1.example.com" {
 .Sh GLOBAL CONFIGURATION
 The following setting can be configured globally:
 .Bl -tag -width Ds
+.It Ic staggered start parallel Ar parallelism Ic delay Ar seconds
+Start all configured vms in staggered fashion with
+.Ar parallelism
+instances in parallel every
+.Ar delay
+seconds.  Defaults to
+.Ar parallelism
+equal to number of cpus and a
+.Ar delay
+of 30 seconds.
 .It Ic local prefix Ar address Ns Li / Ns Ar prefix
 Set the network prefix that is used to allocate subnets for
 local interfaces, see