Date: Tue, 5 Jun 2012 12:22:12 -0400 From: John Baldwin <jhb@freebsd.org> To: "Dag-Erling =?utf-8?q?Sm=C3=B8rgrav?=" <des@des.no> 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: <201206051222.12627.jhb@freebsd.org> In-Reply-To: <86haupvk4a.fsf@ds4.des.no> References: <CACfq090r1tWhuDkxdSZ24fwafbVKU0yduu1yV2%2BoYo%2BwwT4ipA@mail.gmail.com> <201206051008.29568.jhb@freebsd.org> <86haupvk4a.fsf@ds4.des.no>
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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
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