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

[Andreas Gustafsson]

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

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

Title:
  -icount increases boot time by >10x

Status in QEMU:
  New

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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