[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [Libguestfs] [PATCH libnbd] copy: Set POSIX_FADV_SEQUENTIAL and POSIX_FADV_DONTNEED.



On Fri, Feb 19, 2021 at 7:57 PM Richard W.M. Jones <rjones redhat com> wrote:
>
> Set POSIX_FADV_SEQUENTIAL flag on the file when copying to or from a
> local file.  This tells the operating system that we intend to read or
> write sequentially and to optimize for that case.
>
> Avoid polluting the page cache when copying to or from a local file or
> block device by calling POSIX_FADV_DONTNEED.  This is similar to what
> we do in nbdkit-file-plugin:
> https://gitlab.com/nbdkit/nbdkit/-/commit/aa5a2183a6d16afd919174b5ba8222b2bccf4039
>
> Before this change:
>
> $ cachedel /var/tmp/random
> $ cachestats /var/tmp/random
> pages in cache: 0/8388608 (0.0%)  [filesize=33554432.0K, pagesize=4K]
> $ free -m; time ./run nbdcopy /var/tmp/random null: ; free -m ; cachestats /var/tmp/random
>               total        used        free      shared  buff/cache   available
> Mem:          32080        1069       14432           3       16578       30550
> Swap:         16135          86       16049
>
> real    0m21.484s
> user    0m2.275s
> sys     0m25.837s
>               total        used        free      shared  buff/cache   available
> Mem:          32080        1041         297           3       30741       30581
> Swap:         16135          86       16049
> pages in cache: 4971577/8388608 (59.3%)  [filesize=33554432.0K, pagesize=4K]
>
> After this change:
>
> $ cachedel /var/tmp/random
> $ free -m; time ./run nbdcopy /var/tmp/random null: ; free -m ; cachestats /var/tmp/random
>               total        used        free      shared  buff/cache   available
> Mem:          32080        1071       14431           3       16578       30548
> Swap:         16135          86       16049
>
> real    0m22.423s
> user    0m2.180s
> sys     0m31.059s
>               total        used        free      shared  buff/cache   available
> Mem:          32080        1053       14238           3       16789       30566
> Swap:         16135          86       16049
> pages in cache: 53888/8388608 (0.6%)  [filesize=33554432.0K, pagesize=4K]
> ---
>  configure.ac    |  3 +++
>  copy/file-ops.c | 17 +++++++++++++++--
>  copy/main.c     | 11 +++++++++++
>  3 files changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/configure.ac b/configure.ac
> index 58a7724..b36ae09 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -79,6 +79,9 @@ AC_CHECK_HEADERS([\
>
>  AC_CHECK_HEADERS([linux/vm_sockets.h], [], [], [#include <sys/socket.h>])
>
> +dnl posix_fadvise helps to optimise linear reads and writes (optional).
> +AC_CHECK_FUNCS([posix_fadvise])
> +
>  dnl Check for strerrordesc_np (optional, glibc only).
>  dnl Prefer this over sys_errlist.
>  dnl https://lists.fedoraproject.org/archives/list/glibc lists fedoraproject org/thread/WJHGG2OO7ABNAYICGA5WQZ2Q34Q2FEHU/
> diff --git a/copy/file-ops.c b/copy/file-ops.c
> index 2a239d0..1310f08 100644
> --- a/copy/file-ops.c
> +++ b/copy/file-ops.c
> @@ -60,11 +60,12 @@ static size_t
>  file_synch_read (struct rw *rw,
>                   void *data, size_t len, uint64_t offset)
>  {
> +  int fd = rw->u.local.fd;
>    size_t n = 0;
>    ssize_t r;
>
>    while (len > 0) {
> -    r = pread (rw->u.local.fd, data, len, offset);
> +    r = pread (fd, data, len, offset);
>      if (r == -1) {
>        perror (rw->name);
>        exit (EXIT_FAILURE);
> @@ -72,6 +73,11 @@ file_synch_read (struct rw *rw,
>      if (r == 0)
>        return n;
>
> +#if defined (HAVE_POSIX_FADVISE) && defined (POSIX_FADV_DONTNEED)
> +    /* On Linux this will evict the pages we just read from the page cache. */
> +    posix_fadvise (fd, offset, r, POSIX_FADV_DONTNEED);

I don't think this is a good idea, since this affects the current page
cache, for
the entire host.

So if the host is having an image in cache for good reason, running nbdcopy
will drop the cache since nbdcopy does not need it, but maybe the host will
need that cache after running nbdcopy.

The right way to avoid polluting the page cache is to bypass the cache using
O_DIRECT, so nbdcopy is not using or affecting the page cache.

This can be useful if the user can enable this with a flag.

> +#endif
> +
>      data += r;
>      offset += r;
>      len -= r;
> @@ -85,14 +91,21 @@ static void
>  file_synch_write (struct rw *rw,
>                    const void *data, size_t len, uint64_t offset)
>  {
> +  int fd = rw->u.local.fd;
>    ssize_t r;
>
>    while (len > 0) {
> -    r = pwrite (rw->u.local.fd, data, len, offset);
> +    r = pwrite (fd, data, len, offset);
>      if (r == -1) {
>        perror (rw->name);
>        exit (EXIT_FAILURE);
>      }
> +
> +#if defined (HAVE_POSIX_FADVISE) && defined (POSIX_FADV_DONTNEED)
> +    /* On Linux this will evict the pages we just wrote from the page cache. */
> +    posix_fadvise (fd, offset, r, POSIX_FADV_DONTNEED);

This is fine, since we are creating a new image, and nobody else needs it yet.
But when I tested similar code in the past it had no effect until you
call fsync().

> +#endif
> +
>      data += r;
>      offset += r;
>      len -= r;
> diff --git a/copy/main.c b/copy/main.c
> index 68a6030..502ad28 100644
> --- a/copy/main.c
> +++ b/copy/main.c
> @@ -460,6 +460,15 @@ open_null (struct rw *rw)
>    rw->size = INT64_MAX;
>  }
>
> +/* Set the POSIX_FADV_SEQUENTIAL flag on a file descriptor, but don't fail. */
> +static inline void
> +fadvise_sequential (int fd)
> +{
> +#if defined (HAVE_POSIX_FADVISE) && defined (POSIX_FADV_SEQUENTIAL)
> +  posix_fadvise (fd, 0, 0, POSIX_FADV_SEQUENTIAL);
> +#endif
> +}
> +
>  /* Open a local (non-NBD) file, ie. a file, device, or "-" for stdio.
>   * Returns the open file descriptor which the caller must close.
>   *
> @@ -524,12 +533,14 @@ open_local (const char *prog,
>      if (ioctl (fd, BLKSSZGET, &rw->u.local.sector_size))
>        fprintf (stderr, "warning: cannot get sector size: %s: %m", rw->name);
>  #endif
> +    fadvise_sequential (fd);
>    }
>    else if (S_ISREG (rw->u.local.stat.st_mode)) {
>      /* Regular file. */
>      rw->ops = &file_ops;
>      rw->size = rw->u.local.stat.st_size;
>      rw->u.local.seek_hole_supported = seek_hole_supported (fd);
> +    fadvise_sequential (fd);
>    }
>    else {
>      /* Probably stdin/stdout, a pipe or a socket.  Set size == -1
> --
> 2.29.0.rc2
>


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]