Messages in this thread | | | Subject | madvise(2) MADV_SEQUENTIAL behavior | From | Eric Rannaud <> | Date | Tue, 15 Jul 2008 23:03:42 +0000 |
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mm/madvise.c and madvise(2) say:
* MADV_SEQUENTIAL - pages in the given range will probably be accessed * once, so they can be aggressively read ahead, and * can be freed soon after they are accessed.
But as the sample program at the end of this post shows, and as I understand the code in mm/filemap.c, MADV_SEQUENTIAL will only increase the amount of read ahead for the specified page range, but will not influence the rate at which the pages just read will be freed from memory.
Running the sample program on a large file, say 4GB on a machine with 3GB of RAM, the resident size of the program will grow enough to evict pretty much everything else. (on 2.6.25.9-40.fc8)
Right before the program below is done reading the 4GB file:
7f6c3e654000-7f6d3e654000 r--s 00000000 fd:02 98125 /tmp/bigfile Size: 4194304 kB Rss: 2472220 kB Pss: 2472220 kB Shared_Clean: 0 kB Shared_Dirty: 0 kB Private_Clean: 2472220 kB Private_Dirty: 0 kB Referenced: 718748 kB
I'm well aware that the kernel is free to ignore the advice given through madvise(2) (fadvise(2) seems to behave similarly, btw), so I'm certainly not claiming this is a bug. However, I was wondering what was the rationale behind it, and whether the manpages should be updated to be more accurate.
There is a very straightforward workaround: MADV_DONTNEED on the range just read, every so often, will be very effective at controlling the resident size of the mapping. (mm/madvise.c:madvise_dontneed() calls zap_page_range())
Thanks.
--- # dd if=/dev/zero of=/tmp/bigfile bs=1024 count=$((4*1024*1024)) # gcc test.c # Run: file=/tmp/bigfile; ./a.out $file & pid=$! ; while true; do cat /proc/$pid/smaps | grep -A 8 $file; sleep 1; done
# cat test.c
#include <sys/types.h> #include <sys/stat.h> #include <sys/mman.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h>
int main(int argc, char **argv) { if (argc != 2) return -EINVAL;
char *fn = argv[1]; int fd = open(fn, O_RDONLY); if (fd < 0) return -errno;
struct stat st; int ret = fstat(fd, &st); if (ret) return -errno;
unsigned char *map = mmap(0, st.st_size, PROT_READ, MAP_SHARED, fd, 0); if (map == MAP_FAILED) return -errno;
ret = madvise(map, st.st_size, MADV_SEQUENTIAL); if (ret) { fprintf(stderr, "madvise failed\n"); return -errno; }
const int pagesize = sysconf(_SC_PAGESIZE); unsigned char dummy = 0; off_t i;
for (i = 0; i < st.st_size; i += pagesize) { dummy += map[i]; }
munmap(map, st.st_size); close(fd);
return dummy; }
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