vkernel & GSoC, some questions

Matthew Dillon dillon at apollo.backplane.com
Thu Mar 20 16:45:07 UTC 2008

:Except that you still need "real" hardware concurrency to see some races and 
:that is important for testing.  I'd worry about the overhead of any 
:non-hardware assisted virtualization basically enforcing more serialization 
:and coherency than is present in real-world systems meaning that code will 
:work fine in the virtual environment and only break on real hardware.  For 
:example, when I worked on the rwlock implementation for FreeBSD 6/7, I wrote 
:a custom kernel module that banged on the locks a lot and used KTR traces to 
:verify that every single code path in the lock and unlock routines was 
:exercised on a 4-way machine.
:I do think that vitualized environments can certainly be useful for many 
:things (I used qemu for the recent GPT boot stuff and BTX changes and being 
:able to single step in gdb for that was extremely useful and time-saving), 
:but with concurrency races I think it is very important to make sure there is 
:a lot of testing on full-speed, concurrent hardware.
:John Baldwin

    Hardware and vkernel/qemu environments exercise different code paths
    and different timing mechanics.  Certain bugs show up on vkernel's
    more readily then on real hardware, while other bugs show up more 
    readily on real hardware then on vkernel's.  I don't think one is 
    particularly better then another with regards to testing, they just
    cover different continuums.

					Matthew Dillon 
					<dillon at backplane.com>

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