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Prestazioni accesso sequenziale a file
Programma per la creazione di un file contenente N interi casuali a 32 bit:
#include <stdio.h>
#include <stdlib.h>
/* type of file items */
typedef unsigned long ItemType;
int main
(int argc,
char** argv
){
FILE* f;
long long N, i;
/* check command line parameters */
if (argc <
3) exit
(printf("Usage: ./MakeRandFile fileName numItems\n"));
/* convert number of items from string to integer format */
N = atoll
(argv
[2]);
printf("file offset: %d bit\n",
(int)sizeof(off_t
)*
8);
printf("creating random file of %lld 32 bit integers...\n", N
);
/* open file for writing */
f = fopen
(argv
[1],
"w+");
if (f ==
NULL) exit
(printf("can't open file\n"));
/* make N sequential file accesses */
for (i=
0; i<N; ++i
) {
ItemType val = rand
();
fwrite
(&val,
sizeof(ItemType
),
1, f
);
}
fclose
(f
);
}
Analisi sperimentale
Obiettivo: stimare il tempo di scrittura sequenziale di un file di 2 miliardi di interi a 32 bit (7.45 GB)
Piattaforma:
- MacOS X 10.5.5
- 2.16 GHz Intel Core Duo
- 2GB SDRAM
- 2MB L2 cache
- HD Hitachi HTS542525K9SA00 232.89 GB serial ATA (speed 1.5 Gigabit)
- File system Journaled HFS+
- Compilatore gcc 4.0.1
Esperimento:
> gcc -o MakeRandFile MakeRandFile.c
> time ./MakeRandFile large.dat 2000000000
file offset: 64 bit
creating random file of 2000000000 32 bit integers...
real 4m10.685s
user 2m43.309s
sys 0m43.362s
>
Risultati:
Il tempo richiesto per generare sequenzialmente un file di 2 miliardi di interi a 32 bit (7.45 GB) è di 4 minuti e 10.685 secondi = 250.685 secondi, cioè 120 nanosecondi per intero, ovvero un throughput di circa 31.8 MB/sec.
L'uso della CPU durante l'esecuzione del programma si attesta intorno al 77%.
Si confrontino questi risultati con l'
accesso casuale a file.