[Libguestfs] [PATCH libnbd] copy: Set POSIX_FADV_SEQUENTIAL and POSIX_FADV_DONTNEED.
Nir Soffer
nsoffer at redhat.com
Fri Feb 19 19:08:08 UTC 2021
On Fri, Feb 19, 2021 at 7:57 PM Richard W.M. Jones <rjones at 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
>
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