Hi all, In IRC last night Dan helpfully confirmed my analysis of an issue we are seeing attempting to launch high memory KVM guests backed by hugepages... In this case the guests have 240GB of memory allocated from two host NUMA nodes to two guest NUMA nodes. The trouble is that allocating the hugepage backed qemu process seems to take longer than the 30s QEMU_JOB_WAIT_TIME and so libvirt then most unhelpfully kills the barely spawned guest. Dan said there was currently no workaround available so I'm now looking at building a custom libvirt which sets QEMU_JOB_WAIT_TIME=60s.

I have two related questions: 1) will this change have any untoward side-effects?

2) if not, then is there any reason not to change it in master until a better solution comes along (or possibly better, alter qemuDomainObjBeginJobInternal to give a domain start job a little longer compared to other jobs)?

What is the actual error you're getting during startup.

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().

On 02/15/2017 03:43 AM, Blair Bethwaite wrote: > On 15 February 2017 at 00:57, Daniel P. Berrange <berrange(a)redhat.com&gt; 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(a)redhat.com&gt; 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.

On 15 February 2017 at 20:40, Daniel P. Berrange <berrange(a)redhat.com&gt; wrote: > 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(a)redhat.com&gt; 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(a)redhat.com&gt; 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. Wish I could volunteer to work on that but am afraid my day job has me now thinking about building a custom package to work around this for the moment, or even attempting to find the right hexedit against the existing shared object o_0... probably a line of thinking I should squash now. Would it be helpful to have this registered as a customer RFE with Red Hat?