Date: Thu, 07 Jun 2012 10:26:10 +0200 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: Bruce Evans <brde@optusnet.com.au> Cc: Gianni <gianni@FreeBSD.org>, John Baldwin <jhb@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: Fast vs slow syscalls (Re: Fwd: [RFC] Kernel shared variables) Message-ID: <86sje7sf31.fsf@ds4.des.no> In-Reply-To: <20120607064951.C1106@besplex.bde.org> (Bruce Evans's message of "Thu, 7 Jun 2012 07:14:06 %2B1000 (EST)") References: <CACfq090r1tWhuDkxdSZ24fwafbVKU0yduu1yV2%2BoYo%2BwwT4ipA@mail.gmail.com> <201206051008.29568.jhb@freebsd.org> <86haupvk4a.fsf@ds4.des.no> <201206051222.12627.jhb@freebsd.org> <20120605171446.GA28387@onelab2.iet.unipi.it> <20120606040931.F1050@besplex.bde.org> <864nqovoek.fsf@ds4.des.no> <20120607064951.C1106@besplex.bde.org>
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Bruce Evans <brde@optusnet.com.au> writes: > Now 2.44 nsec/call makes sense, but you really should add some volatiles > here to ensure that getpid() is not optimized away. As you can see from the disassembly I provided, it isn't. > SO it loops OK, but we can't see what getpid() does. It must not be > doing much. Umm, yes, that's the whole point of this conversation. Linux's getpid() is not a syscall, but a library function that returns a constant from a page shared by the kernel. > 5.4104 nsec/call for gettimeofday() is impossible if there is any > rdtsc() hardware call or much layering. It's gettimeofday(0, 0), actually, so it doesn't need to read the clock. If I pass a struct timeval as the first argument - so it *does* need to read the clock - it's a little bit slower but still faster than an actual system call. Here's another run that demonstrates this - a little bit slower than previous runs because I have other processes running: getpid(): 10,000,000 iterations in 30,377 us gettimeofday(0, 0): 10,000,000 iterations in 55,571 us gettimeofday(&tv, 0): 10,000,000 iterations in 302,634 us kill(pid, 0): 10,000,000 iterations in 1,291,793 us I can't test a static build since RHEL6 does not provide a static libc. DES --=20 Dag-Erling Sm=C3=B8rgrav - des@des.no
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