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

[Andreas Gustafsson]

I tested Pavel's patch, applying it to master
(c447afd5783b9237fa51b7a85777007d8d568bfc), but I'm afraid it only made
things worse - qemu has now been booting the test kernel for 30 minutes
but the boot has still not completed.  The last console messages printed
were:

piix 0000:00:01.1: not 100% native mode: will probe irqs later
    ide0: BM-DMA at 0xc100-0xc107
    ide1: BM-DMA at 0xc108-0xc10f

Running strace -p on the qemu process shows it calling ppoll once per
second:

ppoll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, 
{fd=10, events=POLLIN}, {fd=11, events=POLLIN}], 5, {tv_sec=1, tv_nsec=0}, 
NULL, 8) = 0 (Timeout)
ppoll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, 
{fd=10, events=POLLIN}, {fd=11, events=POLLIN}], 5, {tv_sec=1, tv_nsec=0}, 
NULL, 8) = 0 (Timeout)
ppoll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, 
{fd=10, events=POLLIN}, {fd=11, events=POLLIN}], 5, {tv_sec=1, tv_nsec=0}, 
NULL, 8) = 0 (Timeout)
ppoll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, 
{fd=10, events=POLLIN}, {fd=11, events=POLLIN}], 5, {tv_sec=1, tv_nsec=0}, 
NULL, 8) = 0 (Timeout)
ppoll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, 
{fd=10, events=POLLIN}, {fd=11, events=POLLIN}], 5, {tv_sec=1, tv_nsec=0}, 
NULL, 8) = 0 (Timeout)
ppoll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, 
{fd=10, events=POLLIN}, {fd=11, events=POLLIN}], 5, {tv_sec=1, tv_nsec=0}, 
NULL, 8) = 0 (Timeout)

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1774677/+subscriptions