Fwd: [RFC] Kernel shared variables

[John Baldwin]

On Tuesday, June 05, 2012 11:44:37 am Dag-Erling Smørgrav wrote:
> 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

Yes, we know getpid() is slow, I think the question is does it matter that 
it's slow in something other than a microbenchmark.  Can you name the 
application that you've seen use getpid()?

-- 
John Baldwin