Re: [9fans] Calling vac from C

[erik quanstrom <quanstro@quanstro.net>]

[-- Attachment #1: Type: text/plain, Size: 952 bytes --]

> I believe that
> 1) Its too much trouble parsing the output everytime.

i don't buy that.  that takes very little code.  since you
have evidently already written the code, the cost
is zero.

(if you're worried about runtime, i measure parsing
time at 338ns on a core i7 920.  cf. attached digestspd.c)

> 2) Calling some function from an included library will be faster.

maybe.  are you sure that it matters?  i measure
base fork/exec latency on a 1.8ghz xeon5000 at 330µs.
(files served from the fileserver, not a ram disk.)
the attached fork.c and nop.c were used to do the
measurement.  i measure vac throughput at ~3mb/s
for small files from a brand new venti running from a
ramdisk.  the venti was tiny with 5mb isect and 100mb
arenas, and empty.  at that rate, 330µs will cost you
1038 bytes, or 0.3%.

remember that dynamic linking isn't free.  that cost
assumes that dynamic linking is free, and it is not.

- erik

[-- Attachment #2: digestspd.c --]
[-- Type: text/plain, Size: 815 bytes --]

#include <u.h>
#include <libc.h>
#include <libsec.h>

static int
nibble(int c)
{
	if(c >= '0' && c <= '9')
		return c - '0';
	if(c < 0x20)
		c += 0x20;
	if(c >= 'a' && c <= 'f')
		return c - 'a'+10;
	return 0xff;
}

static void
bindigest(char *s, uchar *t)
{
	int i;

	if(strlen(s) != 2*SHA1dlen)
		sysfatal("bad digest %s", s);
	for(i = 0; i < SHA1dlen; i++)
		t[i] = nibble(s[2*i])<<4 | nibble(s[2*i + 1]);
}

static char *vs = "vac:da6b4b5549383cffc1b5691d824fc4bd381f0f6b";

void
main(void)
{
	int i, n;
	uchar score[SHA1dlen];
	uvlong t0, t1;

	n = 1000*1000;
	t0 = nsec();
	for(i = 0; i < n; i++){
		if(strncmp(vs, "vac:", 4) == 0)
			bindigest(vs + 4, score);
		else
			sysfatal("bad digest");
	}
	t1 = nsec();
	print("%g\n", 1.*(t1 - t0)/(1.*n));
	exits("");
}

[-- Attachment #3: fork.c --]
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#include <u.h>
#include <libc.h>

char *argv[] = {"nop", 0};

void
main(void)
{
	int i, n;
	uvlong t0, t1;

	n = 10000;
	t0 = nsec();
	for(i = 0; i < n; i++)
		switch(fork()){
		case 0:
			exec(*argv, argv);
			_exits("exec");
		case -1:
			sysfatal("fork");
		default:
			free(wait());
		}
	t1 = nsec();
	print("%g\n", 1.*(t1 - t0)/(1.*n));
	exits("");
}

[-- Attachment #4: nop.c --]
[-- Type: text/plain, Size: 70 bytes --]

#include <u.h>
#include <libc.h>
void
main(void)
{
	exits("");
}