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

[Andreas Gustafsson]

A couple of comments...  First, the problem is not limited to Linux
guests.  In fact, I originally ran across it with a NetBSD guest, but
then reproduced it with a Linux guest for the bug report, because in my
experience, qemu bug reports involving non-Linux guests tend to be
ignored.

Second, the qemu man page says that "With sleep=on|off, the virtual time
will jump to the next timer deadline instantly whenever the virtual cpu
goes to sleep mode".  Although the text is confusing (see bug 1774412),
to me it suggests that when the mode in case is selected, qemu should
execute the guest as fast as possible and only provide it with an
illusion of the passage of time, and there should be no reason for qemu
itself to ever sleep.  Yet, regardless of whether I specify "sleep=on"
or "sleep=off", qemu sleeps.

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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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