Date: Wed, 06 Jun 2012 10:24:19 +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: <864nqovoek.fsf@ds4.des.no> In-Reply-To: <20120606040931.F1050@besplex.bde.org> (Bruce Evans's message of "Wed, 6 Jun 2012 04:36:54 %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>
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Bruce Evans <brde@optusnet.com.au> writes:
> Dag-Erling Sm=C3=B8rgrav <des@des.no> writes:
> > 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
> That's one slow system or broken units.
Broken units, these are microseconds not milliseconds. Sorry.
> After adjusting by factors of 1000 here and there, this format is still
> hard to parse. I like the format of nsec/operation. 24400 10 million
> operations in 24400 moroseconds seems to scale to 2.44 nsec/call (if 1
> moro =3D 1 micro). But that is impossibly fast, unless getpid() is
> inlined to a load of the shared variable (it may also need the load to
> be moved outside the loop). I can't see any reasonable adjustment that
> gives 24.4 nsec/call.
#define ITERATIONS 10000000
struct timeval start, end;
int i;
gettimeofday(&start, NULL);
for (i =3D 0; i < ITERATIONS; ++i)
getpid();
gettimeofday(&end, NULL);
On Linux, gcc 4.4.6 compiles this to:
# gettimeofday(&start, NULL)
0x000000000040064b <+23>: lea -0x20(%rbp),%rax
0x000000000040064f <+27>: mov $0x0,%esi
0x0000000000400654 <+32>: mov %rax,%rdi
0x0000000000400657 <+35>: callq 0x400500 <gettimeofday@plt>
# i =3D 0
0x000000000040065c <+40>: movl $0x0,-0x4(%rbp)
0x0000000000400663 <+47>: jmp 0x40066e <main+58>
# getpid()
0x0000000000400665 <+49>: callq 0x400520 <getpid@plt>
# ++i
0x000000000040066a <+54>: addl $0x1,-0x4(%rbp)
# i < ITERATIONS
0x000000000040066e <+58>: cmpl $0x98967f,-0x4(%rbp)
0x0000000000400675 <+65>: jle 0x400665 <main+49>
# gettimeofday(&end, NULL)
0x0000000000400677 <+67>: lea -0x30(%rbp),%rax
0x000000000040067b <+71>: mov $0x0,%esi
0x0000000000400680 <+76>: mov %rax,%rdi
0x0000000000400683 <+79>: callq 0x400500 <gettimeofday@plt>
The code generated by gcc 4.2.1 on FreeBSD is almost identical:
# gettimeofday(&start, NULL)
0x00000000004006f7 <main+23>: lea -0x20(%rbp),%rdi
0x00000000004006fb <main+27>: mov $0x0,%esi
0x0000000000400700 <main+32>: callq 0x40057c <gettimeofday@plt>
# i =3D 0
0x0000000000400705 <main+37>: movl $0x0,-0x4(%rbp)
0x000000000040070c <main+44>: jmp 0x400717 <main+55>
# getpid()
0x000000000040070e <main+46>: callq 0x40059c <getpid@plt>
# ++i
0x0000000000400713 <main+51>: addl $0x1,-0x4(%rbp)
# i < ITERATIONS
0x0000000000400717 <main+55>: cmpl $0x98967f,-0x4(%rbp)
0x000000000040071e <main+62>: jle 0x40070e <main+46>
# gettimeofday(&end, NULL)
0x0000000000400720 <main+64>: lea -0x30(%rbp),%rdi
0x0000000000400724 <main+68>: mov $0x0,%esi
0x0000000000400729 <main+73>: callq 0x40057c <gettimeofday@plt>
I don't know why gcc 4.4.6 loads &start / &end into %rax before copying
it to %esi instead of loading it directly into %esi like 4.2.1 does. I
used the same command line (gcc -Wall -Wextra syscall.c) in both cases.
DES
--=20
Dag-Erling Sm=C3=B8rgrav - des@des.no
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