Date: Tue, 05 Jun 2012 17:44:37 +0200 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: John Baldwin <jhb@freebsd.org> 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 Message-ID: <86haupvk4a.fsf@ds4.des.no> In-Reply-To: <201206051008.29568.jhb@freebsd.org> (John Baldwin's message of "Tue, 5 Jun 2012 10:08:29 -0400") References: <CACfq090r1tWhuDkxdSZ24fwafbVKU0yduu1yV2%2BoYo%2BwwT4ipA@mail.gmail.com> <201206041053.51802.jhb@freebsd.org> <86y5o1vrer.fsf@ds4.des.no> <201206051008.29568.jhb@freebsd.org>
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John Baldwin <jhb@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 --=20 Dag-Erling Sm=C3=B8rgrav - des@des.no
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