linux-kernel - A question about page caching

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Message-ID: <8496f91a0803040857y78d70ca4o70704fa03e208823@mail.gmail.com>
Date:	Tue, 4 Mar 2008 19:57:06 +0300
From:	"Matvejchikov Ilya" <matvejchikov@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: A question about page caching

Hi folks,

I have a little question about such thing as a page caching while file
reading. As I know there is mechanism that optimise system performance
when file read occurs repeatedly. But I can't simulate it. The test
program that I wrote doesn't show acceptable results.
Here is it.

--- [main.c] ---
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <asm/msr.h>

#define RSIZE	(4096*1024*128)

int main()
{
    int fd;
    char * data;
    unsigned long long counter0;
    unsigned long long counter1;

    fd = open("junkfile", O_RDONLY);
    if (fd == -1) {
	printf("ERROR: open() = %d\n", fd);
	exit(1);
    }

    data = (char *)malloc(RSIZE);
    if (data == NULL) {
	printf("ERROR: malloc() fails\n");
	exit(1);
    }

    rdtscll(counter0);
    read(fd, (void *)data, RSIZE);
    rdtscll(counter1);

    printf("D = %lu\n", counter1 - counter0);

    fgetc(stdin);

    close(fd);

    return 0;
}
--- [main.c] ---

When I run first copy of program I have such result:
(1) D = 191860948
When I run second copy of program (i.e. when first copy waits on
fgetc()) I have the similar result:
(2) D = 192320613

Where I be wrong?

Thanks!
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