Dirty deleted files cause pointless I/O storms (unless truncated first)

Dirty deleted files cause pointless I/O storms (unless truncated first)

[Andy Lutomirski]

The code below runs quickly for a few iterations, and then it slows
down and the whole system becomes laggy for far too long.

Removing the sync_file_range call results in no I/O being performed at
all (which means that the kernel isn't totally screwing this up), and
changing "4096" to SIZE causes lots of I/O but without
the going-out-to-lunch bit (unsurprisingly).

Surprisingly, uncommenting the ftruncate call seems to fix the
problem.  This suggests that all the necessary infrastructure to avoid
wasting time writing to deleted files is there but that it's not
getting used.


#define _GNU_SOURCE
#include <sys/mman.h>
#include <err.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

#define SIZE (16 * 1048576)

static void hammer(const char *name)
{
  int fd = open(name, O_RDWR | O_CREAT | O_EXCL, 0600);
  if (fd == -1)
    err(1, "open");

  fallocate(fd, 0, 0, SIZE);

  void *addr = mmap(NULL, SIZE, PROT_WRITE, MAP_SHARED, fd, 0);
  if (addr == MAP_FAILED)
    err(1, "mmap");

  memset(addr, 0, SIZE);

  if (munmap(addr, SIZE) != 0)
    err(1, "munmap");

  if (sync_file_range(fd, 0, 4096,
              SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |
              SYNC_FILE_RANGE_WAIT_AFTER) != 0)
    err(1, "sync_file_range");

  if (unlink(name) != 0)
    err(1, "unlink");

  //  if (ftruncate(fd, 0) != 0)
  //    err(1, "ftruncate");

  close(fd);
}

int main(int argc, char **argv)
{
  if (argc != 2) {
    printf("Usage: hammer_and_delete FILENAME\n");
    return 1;
  }

  while (true) {
    hammer(argv[1]);
    write(1, ".", 1);
  }
}

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: Dirty deleted files cause pointless I/O storms (unless truncated first)

[Dave Chinner]

On Mon, Jan 20, 2014 at 04:59:23PM -0800, Andy Lutomirski wrote:
> The code below runs quickly for a few iterations, and then it slows
> down and the whole system becomes laggy for far too long.
> 
> Removing the sync_file_range call results in no I/O being performed at
> all (which means that the kernel isn't totally screwing this up), and
> changing "4096" to SIZE causes lots of I/O but without
> the going-out-to-lunch bit (unsurprisingly).

More details please. hardware, storage, kernel version, etc.

I can't reproduce any slowdown with the code as posted on a VM
running 3.31-rc5 with 16GB RAM and an SSD w/ ext4 or XFS. The
workload is only generating about 80 IOPS on ext4 so even a slow
spindle should be able handle this without problems...

> Surprisingly, uncommenting the ftruncate call seems to fix the
> problem.  This suggests that all the necessary infrastructure to avoid
> wasting time writing to deleted files is there but that it's not
> getting used.

Not surprising at all - if it's stuck in a writeback loop somewhere,
truncating the file will terminate writeback because it end up being
past EOF and so stops immediately...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>