[libvirt-users] high memory guest issues - virsh start and QEMU_JOB_WAIT_TIME

[Daniel P. Berrange]

On Wed, Feb 15, 2017 at 10:27:46AM +0100, Michal Privoznik wrote:
> On 02/15/2017 03:43 AM, Blair Bethwaite wrote:
> > On 15 February 2017 at 00:57, Daniel P. Berrange <berrange at redhat.com> wrote:
> >> What is the actual error you're getting during startup.
> > 
> > # virsh -d0 start instance-0000037c
> > start: domain(optdata): instance-0000037c
> > start: found option <domain>: instance-0000037c
> > start: <domain> trying as domain NAME
> > error: Failed to start domain instance-0000037c
> > error: monitor socket did not show up: No such file or directory
> > 
> > Full libvirtd debug log at
> > https://gist.github.com/bmb/08fbb6b6136c758d027e90ff139d5701
> > 
> > On 15 February 2017 at 00:47, Michal Privoznik <mprivozn at redhat.com> wrote:
> >> I don't think I understand this. Who is running the other job? I mean,
> >> I'd expect qemu fail to create the socket and thus hitting 30s timeout
> >> in qemuMonitorOpenUnix().
> > 
> > Yes you're right, I just blindly started looking for 30s constants in
> > the code and that one seemed the most obvious but I had not tried to
> > trace it all the way back to the domain start job or checked the debug
> > logs yet, sorry. So looking a bit more carefully I see the real issue
> > is in src/qemu/qemu_monitor.c:
> > 
> > 321 static int
> > 322 qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
> > 323 {
> > 324     struct sockaddr_un addr;
> > 325     int monfd;
> > 326     int timeout = 30; /* In seconds */
> > 
> > Is this safe to increase? Is there any reason to keep it at 30s given
> > (from what I'm seeing on a fast 2-socket Haswell system) that hugepage
> > backed guests larger than ~160GB memory will not be able to start in
> > that time?
> > 
> 
> I recall some similar discussion took place in the past. But I just
> cannot find it now. I think the problem was that kernel is zeroing the
> pages on huge page allocation. Anyway, this timeout used to be 3 seconds
> and inly in fe89b687a0 it has been changed to 30 seconds.
> 
> We can increase the limit, but that would solve just this case until
> somebody tries to assign even more RAM to their domain. What if we would
> instead make this configurable? Have yet another variable living inside
> qemu.conf that by default has value of 30 and specifies how long should
> libvirt wait for qemu monitor to show up?
> 
> But frankly, on one hand I like this approach. But on the other I
> dislike it at the same time - we have just too much variables in
> qemu.conf because that's our answer to problems like these. We don't
> know so we offload the setting to the sys admin.

Honestly it is well overdue for us to come up with an improvement to
QEMU that lets us start QEMU & open the monitor in a race-free manner.
The obvious answer to this is to allow us to pass down a pre-opened
UNIX listener socket FD to QEMU. We can thus connect() immediately
with no race and then simply away the QMP greeting with no timeout,
safely getting EOF if QEMU fails to start.


Regards,
Daniel
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