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

From: Andreas Gustafsson
Subject: [Qemu-devel] [Bug 1774677] Re: -icount increases boot time by >10x
Date: Sun, 08 Jul 2018 12:51:58 -0000

I bisected this using the following test case (after the setup shown in
the original bug report, and replacing "e1000" by "ne2k_pci" in the run-
emulator.sh script to avoid a panic in the e1000 driver with some qemu

QEMU_EXTRA="-icount shift=3,sleep=off" sh run-emulator.sh

The bisection identified the following commit:

281b2201e4e18d5b9a26e1e8d81b62b5581a13be is the first bad commit
commit 281b2201e4e18d5b9a26e1e8d81b62b5581a13be
Author: Pavel Dovgalyuk <address@hidden>
Date:   Thu Mar 10 14:56:03 2016 +0300

    icount: remove obsolete warp call

    qemu_clock_warp call in qemu_tcg_wait_io_event function is not needed
    anymore, because it is called in every iteration of main_loop_wait.

    Reviewed-by: Paolo Bonzini <address@hidden>

    Signed-off-by: Pavel Dovgalyuk <address@hidden>
    Message-Id: <address@hidden>
    Signed-off-by: Paolo Bonzini <address@hidden>

With revision 281b2201e4e18d5b9a26e1e8d81b62b5581a13be, the test case
takes 141 seconds to boot.  With the previous revision,
33577b47c64435fcc2a1bc01c7e82534256f1fc3, it takes 1.7 seconds.

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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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