Fwd: [RFC] Kernel shared variables

[Dag-Erling Smørgrav]

John Baldwin <jhb at freebsd.org> writes:
> So you call getpid() on each access to a shared resource?

I don't, but I've seen code that does, under the assumption that all the
world is Linux and getpid() is free.  Here's a sample from RHEL6 on a
3.1 GHz i5, using raise(0) as a baseline:

getpid(): 10,000,000 iterations in 24,400 ms
gettimeofday(0, 0): 10,000,000 iterations in 54,104 ms
raise(0): 10,000,000 iterations in 1,284,593 ms

The difference between the first two is due to the fact that while
getpid() just returns a constant, gettimeofday(0, 0) performs two
comparisons first.  Passing an actual struct timeval to gettimeofday()
slows it down by a factor of about 6.

(strace confirms that no system calls occur for either getpid() or
gettimeofday(0, 0))

Here is the same program running on FreeBSD 9.0-RELEASE in VirtualBox on
an otherwise idle 3.4 GHz i7:

getpid(): 10,000,000 iterations in 777,251 ms
gettimeofday(0, 0): 10,000,000 iterations in 799,808 ms
raise(0): 10,000,000 iterations in 2,142,275 ms

DES
-- 
Dag-Erling Smørgrav - des at des.no