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Memory leak when using websocket over a low speed network
Description of problem
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When VNC is connected to QEMU via websocket over a low speed network (e.g. 300KB/S Wide Area Network), and there is a lot of frame buffer update, the VNC Client will get stuck, the cursor is almost impossible to move, which may result in accumulation of a large number of data in the QEMU process (memory consumption will keep increasing).
Environment
-------------------------
All of the following versions have been tested:
QEMU: 2.5.1 / 2.6.0 / 2.8.1.1 / 2.9.0 / 2.10.0
Host OS: Ubuntu 16.04 Server LTS / CentOS 7 x86_64_1611
Guest OS: Windows 7 64bit / Ubuntu 16.04 Desktop LTS
Client OS: Windows 7 64bit / Windows 10 64bit
Client Browser: IE 11.0.9600 / Chrome 60.0.3112 / Firefox 55.0.2
VNC Client: TigerVNC Viewer 1.8 / UltraVNC Viewer 1.2.1.5 / TightVNC Viewer 2.8.8
VNC Web Client: noVNC 0.5.1 / noVNC 0.61 / noVNC 0.62
VNC Server: TigerVNC 1.8 / x11vnc 0.9.13 / TightVNC 2.8.8
VNC Client: TigerVNC Viewer 1.8 / UltraVNC Viewer 1.2.1.5 / TightVNC Viewer 2.8.8
Steps to reproduce:
-------------------------
100% reproducible.
1. Launch a QEMU instance with websocket option:
qemu-system-x86_64 -enable-kvm -m 6G ./win_x64.qcow2 -vnc :1,websocket=5701
2. Open VNC Web Client (noVNC/vnc.html) in browser and connect to QEMU VM via websocket
3. Play a video (e.g. Watch YouTube) on VM (To produce a lot of frame buffer update)
4. Limit (e.g. Use NetLimiter) the client inbound bandwidth to 300KB/S (To simulate a low speed WAN)
5. Then client's output gets stuck(less than 1 fps), the cursor is almost impossible to move
6. Observe QEMU process on the host, more and more data are accumulated in the process, the consumption of memory continues to keep increasing
Current result:
-------------------------
[Top - Initial status]
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2725 root 20 0 7229144 5.910g 23024 S 16.3 18.9 0:12.84 qemu-system-x86_64
[Top - After an hour's playing w/o limit (6-8MB/S)]
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2725 root 20 0 7370284 6.046g 23132 S 28.0 19.3 35:58.15 qemu-system-x86_64
[Top - Limit the bandwidth and continue to playing for another an hour (300KB/S)]
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2725 root 20 0 11.029g 8.853g 23132 S 20.0 28.2 72:14.17 qemu-system-x86_64
Also test several other combinations in the same environment:
1. Client(VNC Viewer) - Server(QEMU)
2. Client(VNC Viewer) - Server(tigervnc/x11vnc/tightvnc)
3. Client(noVNC) - Server(tigervnc/x11vnc/tightvnc)
Likewise, the client's inbound bandwidth is limited to 300KB/S,
although a lot of frame are lost, all of they still works (at least the mouse is movable).
It's found that when connect to QEMU via websocket, it never drop any frames.
QEMU still sends a lot of data to its websocket even when the network is congested,
the process is continually consuming more memory, then it gets stack.
Expected results:
-------------------------
When the network is poor (non-LAN), QEMU would reduce the VNC data send to its websocket correspondingly, and the memory usage remains stable.
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