Disable mutex spinning?

[Ivan Voras]

Hi

I'm not sure I'm thinking the right way here, so this could be just
nonsense.

Since I started using VMWare (and that was a long time ago) and other
virtual machine applications, I've noticed FreeBSD spends a lot of time
in system space (i.e. much time is allocated to "sys" in utilities like
top and time) during IO intensive operations. At first I thought this is
because the emulated IO devices are much slower than real devices, so IO
requests take a long time to complete, leading to sys time. It occurred
to me that this might be bogus thinking - yes, emulated IO devices take
a long time to perform IO but during that time the system doesn't do
anything, so that time should be effectively described as idle, waiting
for IO completion... or does it?

I'm thinking of experiment - removing all spinning from mutex-like code
and try running such kernel in a VMWare machine. For start, is there
anything else except turning off ADAPTIVE_MUTEXES, ADAPTIVE_GIANT and
ADAPTIVE_RWLOCKS I can do via the kernel config file?

If someone has more insight to share on this topic, I'm listening :)

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