Because Parallels Desktop 10 and Fusion 7 both run slow on Yosemite, there are KB articles which make the users execute

sudo nvram boot-args="debug=0x10"

For instance here: http://kb.parallels.com/122767

I have figured out that 0x10 will "make ddb the default debugger". But I have no idea what this means, yet it sounds scary.

What will this command do to Yosemite/the Mac? Can it have any other side effects?


sudo nvram boot-args="debug=0x144"

This is a combination of kernel debugging features that will show you extra information about the kernel's processes, which can be exceptionally useful if a system is experiencing kernel panics. Another option is to use debug=0x14e, which will display even more logging options. The primary use for this is that it enables old-style kernel panics that show scrolled text on the screen about why the system panicked, instead of displaying the gray backdrop and the message to merely restart your system. Alternative debug options are the following, though these will likely only be useful to kernel programmers:

  • 0x01 - Stop at boot time and wait for the debugger to attach
  • 0x02 - Send kernel debugging output to the console
  • 0x04 - Drop into debugger on a nonmaskable interrupt
  • 0x08 - Send kernel debugging information to a serial port
  • 0x10 - Make ddb the default debugger
  • 0x20 - Output diagnostics information to the system log
  • 0x40 - Allow the debugger to ARP and route
  • 0x80 - Support old versions of gdb on newer systems
  • 0x100 - Disable the graphical panic dialog screen

Source: Boot argument options in OS X

ddb is just a debugger. It's absolutely safe to enable those kinds of modes to collect information once you experienced system issues.

  • How would enabling a debugger and add more logging help to speed up a virtual machine? – Krumelur Nov 7 '14 at 22:21
  • No way of speeding doing that. You should enable to watch the logs after that, and look after errors, issues, whatever that let you fix your problem. – jherran Nov 7 '14 at 22:25
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    The thing is: it really works. Parallels has a tool that shows hundred thousands of interrupts which slow the VM down. Do the debug=0x10 and the issue is gone. – Krumelur Nov 8 '14 at 13:12
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    I can also confirm the change does make Parallels run normal. – Hackmodford Jan 23 '15 at 14:31
  • @Krumelur -- it's conceivable that by enabling the debugger, the interrupts would be shunted to the debugger's handler (and perhaps the default handler is to ignore them). – jhfrontz Jan 8 '16 at 16:14

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