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Re: [Qemu-discuss] qemu -> kvm eventually stops responding when VNC is p

From: Narcis Garcia
Subject: Re: [Qemu-discuss] qemu -> kvm eventually stops responding when VNC is prepped and waiting for a connection
Date: Sat, 25 Feb 2017 09:08:24 +0100

Oh, I believed this was caused by something I did wrong, but I see now
I'm not the only one to experience this problem.
I had this problem also with Ubuntu 14.04 (trusty)
QEMU emulator version 2.0.0

El 24/02/17 a les 20:55, Brad Barnett ha escrit:
> FYI, Debian Jessie.
> # qemu-system-x86_64 --version
> QEMU emulator version 2.1.2 (Debian 1:2.1+dfsg-12+deb8u6), Copyright (c)
> 2003-2008 Fabrice Bellard
> Through a variety of scripts and what not, I boot up multiple KVM
> instances on a variety of boxes.
> Over the 6 months, I've noticed that randomly KVM instances will stop
> responding.  They aren't using CPU.  Disk I/O seems non-existent.
> Yet, they do not respond to ping, or any network traffic.  Further, they
> seem to 'slowly' degrade.  It typically always starts with a variety of:
> Feb 24 13:54:36 xxx kernel: [111360.168074] INFO: task kworker/u16:0:6 
> blocked for more than 120 seconds.
> Feb 24 13:54:36 xxx kernel: [111360.168179]       Not tainted 3.16.0-4-amd64 
> #1
> Feb 24 13:54:36 xxx kernel: [111360.168244] "echo 0 > 
> /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> Feb 24 13:54:36 xxx kernel: [111360.168360] kworker/u16:0   D 
> ffff880236d504a8     0     6      2 0x00000000
> Feb 24 13:54:36 xxx kernel: [111360.168371] Workqueue: writeback 
> bdi_writeback_workfn (flush-254:0)
> The task could be a variety of things, but mostly seem to revolve around
> core kernel processes, or disk.
> For this reason, I've been poking at the wrong side of the tracks.  And,
> debugging disk i/o... even though these boxes are not heavily disk i/o
> laden.
> However, recently I discovered that the instant I connect to VNC?
> The box immediately and instantly recovers.
> I've now verified this three times over the last month (it can take weeks
> for a box to get into this state.)  In each case, the symptoms are identical.
> The box slows down, eventually starts to have issues as above, then
> eventually stops responding entirely.  Connect with VNC?  Instant recovery.
> It would seem that there is some sort of blocking going on, when VNC is
> up and running in my case... waiting for a connection.
> It should be noted that these boxes are prepped with a permanent VNC
> password, and ready to go.  I am not running a test run where qemu is
> prepped with the VNC command line, but no password/etc.
> Current command line (which causes this issue):
> qemu-system-x86_64 -enable-kvm -net nic,macaddr=xx:xx:xx:xx:xx:xx -net
> vde,sock=/var/run/vde2/mytap.ctl -m 2G -smp 2 -name xxxxxx -drive
> file=/xxx/xxx.raw,if=virtio,boot=on -D /xx/log/xxxxxxxx.logfile
> -pidfile /run/xxxxxx.pid -monitor stdio
> -display vnc=,password -vga qxl -qmp
> unix:/xxxx/xxx/xxx.sock,server,nowait
> Each qemu instance sits in its own screen instance.  Once qemu starts,
> the following script runs.  The 'stuff' screen command essentially sends
> commands to screens console/stdin.
> Yes, I know I can use the socket, but I want qemu/kvm + screen running
> anyhow in this way.  So, the scripts take advantage of that.. and address
> qemu/kvm's console directly.
> #!/bin/bash
> end=`echo -ne '\015'`;
> ( sleep 10
> screen -S screennameofinstance -X select '0'
> screen -S screennameofinstance -X stuff 'change vnc password'$end
> screen -S screennameofinstance -X stuff 'PASSWORD'$end
> screen -S screennameofinstance -X stuff 'expire_password vnc never'$end ) &
> I have just recently stopped using this script on all boxes.  The command
> line remains the same as above, but I am not sending the password
> setup/expire command now to each machine's console.
> Has anyone heard of this before?  I Googled, no luck..
> Thanks

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