[android] Mir servers use non-zero CPU (in HWC vsync thread) even when completely idle

[Daniel van Vugt]

Mir servers use non-zero CPU when completely idle

For example, on krillin with no clients and no touches, the minimal server sits between 0.3 and 0.7% CPU:

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10476 phablet 20 0 6212 1248 820 R 1.3 0.1 0:00.68 top
 1721 root 20 0 37356 3876 3024 S 0.7 0.4 0:05.75 upowerd
10444 phablet 20 0 163816 8552 6628 S 0.7 0.9 0:00.30 mir_demo_server_minimal

[Alexandros Frantzis (afrantzis)]

The culprit is a thread called "VSyncThreadHWC". Not sure if this is something we can control from the Android backend or it's completely internal to the krillin driver. Kevin, Alberto?

[Alexandros Frantzis (afrantzis)]

Note that non-zero CPU usage can also be seen on the Nexus4, in this case the culprit being a thread called "hwcVsyncThread".

[Daniel van Vugt (vanvugt)]

Sounds like an unavoidable penalty of using the Android graphics platform. Not an issue for Mesa or desktop in general.

[Daniel van Vugt (vanvugt)]

This still needs investigating. I'm not sure it's justifiable that VSyncThreadHWC should keep waking up when we're not feeding it anything.

[Daniel van Vugt (vanvugt)]

Actually I think the HWC vsync callback is optional. We just happen to have it always turned on for Mir.

[Daniel van Vugt (vanvugt)]

... which is useful because I now need it in:
https://code.launchpad.net/~vanvugt/mir/physical-frame/+merge/306199

[Daniel van Vugt (vanvugt)]

The Android platform was deleted from lp:mir at revision 4155.