POSIX_FADV_DONTNEED causing excessive amounts of block I/O

[Lutz Vieweg]

I recently investigated the I/O performance of some software and
noticed that "iotop" and "pidstat -d" reported way more write activity
than the application could ever have written.

Further investigation revealed that the application was using
posix_fadvise(..., POSIX_FADV_DONTNEED) on regions of a file it
just wrote - which seems reasonable given that the data in question
is not expected to be read by this or any other application soon,
and the manual page of posix_fadvise states:

> The advice is not binding; it merely constitutes an expectation on behalf of the application.
...
>      POSIX_FADV_DONTNEED
>              The specified data will not be accessed in the near future.

The regions that the application used posix_fadvise() on where often smaller
than one page (4096 byte), but it seems as if the kernel (3.5.0) triggers an immediate
write out of a whole page for each call to posix_fadvise(), causing lots
of unneccessary I/O.

You can reproduce the effect by running the following tiny C program, while
you run "iotop"/"pidstat -d 1"/"iostat -dx 1" on the same system:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char ** argv) {

    int fd;
    off_t i;
    char c = 0;

    fd = open("dummytestfile", O_WRONLY | O_CREAT | O_TRUNC, 0777);

    for (i = 0; i < 10000; i++) {
         write(fd, &c, 1);
         posix_fadvise(fd, i, 1, POSIX_FADV_DONTNEED);

         usleep(1000);
    }

    close(fd);

    return 0;
}

This program writes no more than 10.000 bytes over a period of 10 seconds, but
the utilities report that it writes ~ 2 Megabytes per second!

I would have expected that posix_fadvise(..., POSIX_FADV_DONTNEED) just marks
a dirty page such that it is written out the next time it's convenient for the
I/O scheduler - but the multiplication of actual I/O is certainly not what the
application programmer could have expected, given the documentation of
posix_fadvise...

Regards,

Lutz Vieweg

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