[Qemu-devel] [Bug 1774677] Re: -icount increases boot time by >10x

[Peter Maydell]

The stalls in the test kernel seem to happen at and after the guest
prints

tsc: Refined TSC clocksource calibration: 1000.022 MHz
clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0xe6a2bd9634, 
max_idle_ns: 440795211s
hdc: QEMU DVD-ROM, ATAPI CD/DVD-ROM drive
clocksource: Switched to clocksource tsc

so I suspect a bad interaction between what icount is doing and in
particular our emulation of the x86 timestamp counter and what Linux
does with it.

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https://bugs.launchpad.net/bugs/1774677

Title:
  -icount increases boot time by >10x

Status in QEMU:
  Confirmed

Bug description:
  When I specify the -icount option, some guest operations such as
  booting a Linux kernel take more than 10 times longer than otherwise.
  For example, the following will boot Aboriginal Linux to the login
  prompt about 6 seconds on my system (using TCG, not KVM):

  wget 
http://landley.net/aboriginal/downloads/old/binaries/1.4.5/system-image-i686.tar.gz
  gunzip <system-image-i686.tar.gz | tar xfv -
  cd system-image-i686
  sh run-emulator.sh

  If I replace the last line with

  QEMU_EXTRA="-icount shift=auto" sh run-emulator.sh

  booting to the login prompt takes about 1 minute 20 seconds.

  I have tried different values for "shift" other than the "auto" used
  above, but have not been able to find one that gives reasonable
  performance.  Specifying "sleep=off" also did not help.

  During the slow boots, qemu appears to spend most of its time
  sleeping, not using the host CPU.

  I see this with multiple versions of qemu, including current git
  sources (c181ddaa176856b3cd2dfd12bbcf25fa9c884a97), and on multiple
  host OSes, including Debian 9 on x86_64.

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