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    Subject[PATCH 00/29] RFC: iov_iter: Switch to using an ops table
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    Hi Pavel, Willy, Jens, Al,

    I had a go switching the iov_iter stuff away from using a type bitmask to
    using an ops table to get rid of the if-if-if-if chains that are all over
    the place. After I pushed it, someone pointed me at Pavel's two patches.

    I have another iterator class that I want to add - which would lengthen the
    if-if-if-if chains. A lot of the time, there's a conditional clause at the
    beginning of a function that just jumps off to a type-specific handler or
    to reject the operation for that type. An ops table can just point to that
    instead.

    As far as I can tell, there's no difference in performance in most cases,
    though doing AFS-based kernel compiles appears to take less time (down from
    3m20 to 2m50), which might make sense as that uses iterators a lot - but
    there are too many variables in that for that to be a good benchmark (I'm
    dealing with a remote server, for a start).

    Can someone recommend a good way to benchmark this properly? The problem
    is that the difference this makes relative to the amount of time taken to
    actually do I/O is tiny.

    I've tried TCP transfers using the following sink program:

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <fcntl.h>
    #include <unistd.h>
    #include <netinet/in.h>
    #define OSERROR(X, Y) do { if ((long)(X) == -1) { perror(Y); exit(1); } } while(0)
    static unsigned char buffer[512 * 1024] __attribute__((aligned(4096)));
    int main(int argc, char *argv[])
    {
    struct sockaddr_in sin = { .sin_family = AF_INET, .sin_port = htons(5555) };
    int sfd, afd;
    sfd = socket(AF_INET, SOCK_STREAM, 0);
    OSERROR(sfd, "socket");
    OSERROR(bind(sfd, (struct sockaddr *)&sin, sizeof(sin)), "bind");
    OSERROR(listen(sfd, 1), "listen");
    for (;;) {
    afd = accept(sfd, NULL, NULL);
    if (afd != -1) {
    while (read(afd, buffer, sizeof(buffer)) > 0) {}
    close(afd);
    }
    }
    }

    and send program:

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <fcntl.h>
    #include <unistd.h>
    #include <netdb.h>
    #include <netinet/in.h>
    #include <sys/stat.h>
    #include <sys/sendfile.h>
    #define OSERROR(X, Y) do { if ((long)(X) == -1) { perror(Y); exit(1); } } while(0)
    static unsigned char buffer[512*1024] __attribute__((aligned(4096)));
    int main(int argc, char *argv[])
    {
    struct sockaddr_in sin = { .sin_family = AF_INET, .sin_port = htons(5555) };
    struct hostent *h;
    ssize_t size, r, o;
    int cfd;
    if (argc != 3) {
    fprintf(stderr, "tcp-gen <server> <size>\n");
    exit(2);
    }
    size = strtoul(argv[2], NULL, 0);
    if (size <= 0) {
    fprintf(stderr, "Bad size\n");
    exit(2);
    }
    h = gethostbyname(argv[1]);
    if (!h) {
    fprintf(stderr, "%s: %s\n", argv[1], hstrerror(h_errno));
    exit(3);
    }
    if (!h->h_addr_list[0]) {
    fprintf(stderr, "%s: No addresses\n", argv[1]);
    exit(3);
    }
    memcpy(&sin.sin_addr, h->h_addr_list[0], h->h_length);
    cfd = socket(AF_INET, SOCK_STREAM, 0);
    OSERROR(cfd, "socket");
    OSERROR(connect(cfd, (struct sockaddr *)&sin, sizeof(sin)), "connect");
    do {
    r = size > sizeof(buffer) ? sizeof(buffer) : size;
    size -= r;
    o = 0;
    do {
    ssize_t w = write(cfd, buffer + o, r - o);
    OSERROR(w, "write");
    o += w;
    } while (o < r);
    } while (size > 0);
    OSERROR(close(cfd), "close/c");
    return 0;
    }

    since the socket interface uses iterators. It seems to show no difference.
    One side note, though: I've been doing 10GiB same-machine transfers, and it
    takes either ~2.5s or ~0.87s and rarely in between, with or without these
    patches, alternating apparently randomly between the two times.

    The patches can be found here:

    https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=iov-ops

    David
    ---
    David Howells (29):
    iov_iter: Switch to using a table of operations
    iov_iter: Split copy_page_to_iter()
    iov_iter: Split iov_iter_fault_in_readable
    iov_iter: Split the iterate_and_advance() macro
    iov_iter: Split copy_to_iter()
    iov_iter: Split copy_mc_to_iter()
    iov_iter: Split copy_from_iter()
    iov_iter: Split the iterate_all_kinds() macro
    iov_iter: Split copy_from_iter_full()
    iov_iter: Split copy_from_iter_nocache()
    iov_iter: Split copy_from_iter_flushcache()
    iov_iter: Split copy_from_iter_full_nocache()
    iov_iter: Split copy_page_from_iter()
    iov_iter: Split iov_iter_zero()
    iov_iter: Split copy_from_user_atomic()
    iov_iter: Split iov_iter_advance()
    iov_iter: Split iov_iter_revert()
    iov_iter: Split iov_iter_single_seg_count()
    iov_iter: Split iov_iter_alignment()
    iov_iter: Split iov_iter_gap_alignment()
    iov_iter: Split iov_iter_get_pages()
    iov_iter: Split iov_iter_get_pages_alloc()
    iov_iter: Split csum_and_copy_from_iter()
    iov_iter: Split csum_and_copy_from_iter_full()
    iov_iter: Split csum_and_copy_to_iter()
    iov_iter: Split iov_iter_npages()
    iov_iter: Split dup_iter()
    iov_iter: Split iov_iter_for_each_range()
    iov_iter: Remove iterate_all_kinds() and iterate_and_advance()


    lib/iov_iter.c | 1440 +++++++++++++++++++++++++++++++-----------------
    1 file changed, 934 insertions(+), 506 deletions(-)


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