From owner-freebsd-arch@FreeBSD.ORG  Tue Jun  5 15:44:39 2012
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: freebsd-arch@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52])
	by hub.freebsd.org (Postfix) with ESMTP id 166C710656E9;
	Tue,  5 Jun 2012 15:44:39 +0000 (UTC) (envelope-from des@des.no)
Received: from smtp.des.no (smtp.des.no [194.63.250.102])
	by mx1.freebsd.org (Postfix) with ESMTP id C54408FC16;
	Tue,  5 Jun 2012 15:44:38 +0000 (UTC)
Received: from ds4.des.no (smtp.des.no [194.63.250.102])
	by smtp.des.no (Postfix) with ESMTP id BF56D603A;
	Tue,  5 Jun 2012 15:44:37 +0000 (UTC)
Received: by ds4.des.no (Postfix, from userid 1001)
	id 93533961D; Tue,  5 Jun 2012 17:44:37 +0200 (CEST)
From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no>
To: John Baldwin <jhb@freebsd.org>
References: <CACfq090r1tWhuDkxdSZ24fwafbVKU0yduu1yV2+oYo+wwT4ipA@mail.gmail.com>
	<201206041053.51802.jhb@freebsd.org> <86y5o1vrer.fsf@ds4.des.no>
	<201206051008.29568.jhb@freebsd.org>
Date: Tue, 05 Jun 2012 17:44:37 +0200
In-Reply-To: <201206051008.29568.jhb@freebsd.org> (John Baldwin's message of
	"Tue, 5 Jun 2012 10:08:29 -0400")
Message-ID: <86haupvk4a.fsf@ds4.des.no>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (berkeley-unix)
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
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
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussion related to FreeBSD architecture <freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 05 Jun 2012 15:44:39 -0000

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