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Prestazioni accesso casuale a file
Programma di test per l'accesso casuale a un file:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
/* type of file items */
typedef unsigned long ItemType;
int main
(int argc,
char** argv
){
FILE* f;
long long i, N, R;
/* check command line parameters */
if (argc <
3) exit
(printf("Usage: ./RandFileScan fileName numReads\n"));
/* convert number of accesses from string to integer format */
R = atoll
(argv
[2]);
/* open file for reading */
f = fopen
(argv
[1],
"r");
if (f ==
NULL) exit
(printf("can't open file %s\n", argv
[1]));
/* compute number N of elements in the file */
fseeko
(f, 0LL, SEEK_END
);
N = ftello
(f
)/
sizeof(ItemType
);
printf("file offset: %d bit\n",
(int)sizeof(off_t
)*
8);
printf("make %lld random accesses to file of %lld 32 bit integers...\n", R, N
);
/* init pseudo-random generator seed */
srand
(time
(NULL));
/* make R random file accesses */
for (i=
0; i<R; ++i
) {
ItemType val;
long long j =
(long long)(N*
((double)rand
()/RAND_MAX
));
fseeko
(f, j*
sizeof(ItemType
), SEEK_SET
);
fread
(&val,
sizeof(ItemType
),
1, f
);
}
fclose
(f
);
}
Analisi sperimentale
Obiettivo: stimare il tempo di lettura casuale di 10000 interi a 32 bit in 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
- File system Journaled HFS+
- Compilatore gcc 4.0.1
Esperimento:
> gcc -o RandFileScan RandFileScan.c
> time ./RandFileScan large.dat 10000
file offset: 64 bit
make 10000 random accesses to file of 2000000000 32 bit integers...
real 1m58.419s
user 0m0.303s
sys 0m1.671s
>
Risultati:
Il tempo richiesto per leggere in modo casuale 10000 interi a 32 bit in un file di 2 miliardi di interi a 32 bit (7.45 GB) è di 1 minuti e 58.419 secondi = 118.419 secondi, cioè 11.8 millisecondi per intero, ovvero un throughput di circa 337.8 byte/sec =
0.0003 MB/sec.
L'uso della CPU durante l'esecuzione del programma si attesta intorno all'1.6%, contro il 77% dell'
accesso sequenziale.
Il throughput per l'
accesso sequenziale sulla stessa macchina è di circa
31.8 MB/sec, circa 100000 volte più alto!