"Jim C. Nasby" <[EMAIL PROTECTED]> writes:
> Before we start debating merits of proposals based on random reads, can
> someone confirm that the sampling code actually does read randomly? I
> looked at it yesterday; there is a comment that states that blocks to be
> scanned are passed to the analyze function in physical order, and AFAICT
> the function that chooses blocks does so based strictly on applying a
> probability function to block numbers as it increments a counter. It
> seems that any reading is actually sequential and not random, which
> makes all the random_page_cost hand-waving null and void.
Hm. I'm curious just how much that behaves like a sequential scan actually. I
think I'll do some experiments.
Reading 1% (1267 read, 126733 skipped): 7748264us
Reading 2% (2609 read, 125391 skipped): 12672025us
Reading 5% (6502 read, 121498 skipped): 19005678us
Reading 5% (6246 read, 121754 skipped): 18509770us
Reading 10% (12975 read, 115025 skipped): 19305446us
Reading 20% (25716 read, 102284 skipped): 18147151us
Reading 50% (63656 read, 64344 skipped): 18089229us
Reading 100% (128000 read, 0 skipped): 18173003us
These numbers don't make much sense to me. It seems like 5% is about as slow
as reading the whole file which is even worse than I expected. I thought I was
being a bit pessimistic to think reading 5% would be as slow as reading 20% of
the table.
Anyone see anything wrong my my methodology?
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <time.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#define BLOCKSIZE 8192
int main(int argc, char *argv[], char *arge[])
{
char *fn;
int fd;
int perc;
struct stat statbuf;
struct timeval tv1,tv2;
off_t size, offset;
char *buf[BLOCKSIZE];
int b_read=0, b_skipped=0;
fn = argv[1];
perc = atoi(argv[2]);
fd = open(fn, O_RDONLY);
fstat(fd, &statbuf);
size = statbuf.st_size;
size = size/BLOCKSIZE*BLOCKSIZE;
gettimeofday(&tv1, NULL);
srandom(getpid()^tv1.tv_sec^tv1.tv_usec);
for(offset=0;offset<size;offset+=BLOCKSIZE) {
if (random()%100 < perc) {
lseek(fd, offset, SEEK_SET);
read(fd, buf, BLOCKSIZE);
b_read++;
} else {
b_skipped++;
}
}
gettimeofday(&tv2, NULL);
fprintf(stderr,
"Reading %d%% (%d read, %d skipped): %ldus\n",
(int)perc, b_read, b_skipped,
(tv2.tv_sec-tv1.tv_sec)*1000000 + (tv2.tv_usec-tv1.tv_usec)
);
exit(0);
}
--
greg
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