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From: John Baldwin <jhb@freebsd.org>
To: "Dag-Erling =?utf-8?q?Sm=C3=B8rgrav?=" <des@des.no>
Date: Tue, 5 Jun 2012 12:22:12 -0400
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Cc: Gianni <gianni@freebsd.org>, Alan Cox <alc@rice.edu>,
	Alexander Kabaev <kan@freebsd.org>, Attilio Rao <attilio@freebsd.org>,
	Konstantin Belousov <kib@freebsd.org>, freebsd-arch@freebsd.org,
	Konstantin Belousov <kostikbel@gmail.com>
Subject: Re: Fwd: [RFC] Kernel shared variables
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On Tuesday, June 05, 2012 11:44:37 am Dag-Erling Sm=C3=B8rgrav wrote:
> John Baldwin <jhb@freebsd.org> writes:
> > So you call getpid() on each access to a shared resource?
>=20
> 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:
>=20
> 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
>=20
> 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.
>=20
> (strace confirms that no system calls occur for either getpid() or
> gettimeofday(0, 0))
>=20
> Here is the same program running on FreeBSD 9.0-RELEASE in VirtualBox on
> an otherwise idle 3.4 GHz i7:
>=20
> 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=
=20
it's slow in something other than a microbenchmark.  Can you name the=20
application that you've seen use getpid()?

=2D-=20
John Baldwin