Related to my other bug report today, calling posix_fadvise (which uses fadvise64) with the POSIX_FADV_NOREUSE flag does nothing. The pages are not dropped behind. I also tried call fadvise with POSIX_FADV_SEQUENTIAL first. This is expected as the POSIX_FADV_NOREUSE is a no-op in the recent kernels. Also, POSIX_FADV_SEQUENTIAL only does the readahead window. It doesn't hint the VM in any way to possibly drop-behind the pages. (See the previous bug report for more details of the test case) Relevant numbers: Copying (using fadvise_cp) a large file test: 1st run: 0m9.018s 2nd run: 0m3.444s Copying large file... 3rd run: 0m14.024s <<< page cache trashed 4th run: 0m3.449s Test programs and batch files are attached. - Eric
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int in;
int out;
int pagesize;
void *buf;
off_t pos;
if (argc != 3) {
printf("Usage: %s <src> <dest>\n", argv[0]);
return EXIT_FAILURE;
}
in = open(argv[1], O_RDONLY, 0);
out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0666);
posix_fadvise(in, 0, 0, POSIX_FADV_SEQUENTIAL);
posix_fadvise(out, 0, 0, POSIX_FADV_SEQUENTIAL);
pagesize = getpagesize();
buf = malloc(pagesize);
pos = 0;
for (;;) {
ssize_t count;
count = read(in, buf, pagesize);
if (!count || count == -1)
break;
write(out, buf, count);
/* right usage pattern? */
posix_fadvise(in, pos, count, POSIX_FADV_NOREUSE);
posix_fadvise(out, pos, count, POSIX_FADV_NOREUSE);
pos += count;
}
free(buf);
close(in);
close(out);
return EXIT_SUCCESS;
}
all: gcc fadvise_cp.c -o fadvise_cp gcc working_set_simul.c -o working_set_simul
Attachment:
use-once-test.sh
Description: application/shellscript
#include <fcntl.h>
#include <memory.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int fd;
off_t size;
char *mapping;
unsigned r;
unsigned i;
if (argc != 2) {
printf("Usage: %s <file>\n", argv[0]);
return EXIT_FAILURE;
}
fd = open(argv[1], O_RDONLY, 0);
size = lseek(fd, 0, SEEK_END);
mapping = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
/* access (read) the file a couple of times*/
for (r = 0; r < 4; r++) {
for (i = 0; i < size; i++) {
char t = mapping[i];
}
}
munmap(mapping, size);
close(fd);
return EXIT_SUCCESS;
}
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