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

From: Pavel Dovgalyuk
Subject: [Qemu-devel] [Bug 1774677] Re: -icount increases boot time by >10x
Date: Mon, 09 Jul 2018 07:26:05 -0000

I can reproduce it. But there is one interesting thing.
When I replace "-nographic" with "-serial stdio" everything is ok,
icount works normally.

But "-nographic" runs are very slow in icount mode.
It seems that GUI produces some events that force main loop to iterate.

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  -icount increases boot time by >10x

Status in QEMU:

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):

  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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