mod_log_config.c (original) (raw)

On Sun, Dec 05, 2004 at 01:58:44PM -0700, Paul Querna wrote:

Joe Orton wrote:

On Sun, Dec 05, 2004 at 07:05:23AM -0000, Paul Querna wrote:

Author: pquerna Date: Sat Dec 4 23:05:23 2004 New Revision: 109866

URL: http://svn.apache.org/viewcvs?view=rev&rev=109866 Log: mod_log_config.c: Use iovecs to write the log line to eliminate a memcpy

IIRC, writev'ing several small blocks to a file is actually generally more expensive than doing a memcpy in userspace and calling write. Did you benchmark this to be faster/better/...?

also that introduced a warning:

mod_log_config.c: In function `ap_default_log_writer': mod_log_config.c:1353: warning: assignment discards qualifiers from pointer target type

I did a local mini-benchmark of write w/ memcpy vs writev... and they came out to almost exactly the same on average with small sets of data.

I remember I checked this before too... try the attached compiled with or without -DUSE_WRITEV, copy-and-write comes out about ~10-20% faster than writev on Linux 2.6/ext3 here for a real log vector, or did I screw up the benchmark?

rm -f writev.out; sync; sleep 5 ./writev-copy copy+write: 7s330676. rm -f writev.out; sync; sleep 5 ./writev-copy copy+write: 7s327580. rm -f writev.out; sync; sleep 5 ./writev-copy copy+write: 7s360685. rm -f writev.out; sync; sleep 5 ./writev-writev writev: 8s893524. rm -f writev.out; sync; sleep 5 ./writev-writev writev: 8s808458. rm -f writev.out; sync; sleep 5 ./writev-writev writev: 9s052335.

joe

#include <sys/time.h> #include <sys/uio.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <stdio.h> #include <string.h> #include <stdlib.h>

char *strs[] = { "127.0.0.1", " ", "-", " ", "-", " ", "[22/Jul/2003:12:09:58 +0100]", " "", "GET / HTTP/1.0", "" ", "200", " ", "1456", "\n" };

#define NVECS (sizeof(strs) / sizeof(char *)) #define ITERS (1000000)

#ifdef USE_WRITEV static void do_writev(int fd, struct iovec *vec, size_t nvec, size_t total) { ssize_t ret = writev(fd, vec, nvec);

if (ret != total) {
    printf("writev got %d not %u\n", ret, total);
}

} #else static void do_writev(int fd, struct iovec *vec, size_t nvec, size_t total) { char *p, *buf; ssize_t ret;

p = buf = malloc(total);
while (nvec) {
    memcpy(p, vec[0].iov_base, vec[0].iov_len);
    p += vec[0].iov_len;
    nvec--;
    vec++;
}

ret = write(fd, buf, total);
free(buf);
if (ret != total) {
    printf("write got %d not %u\n", ret, total);
}

} #endif

#define BIG 100

#define TESTFN "./writev.out"

int main(int argc, char **argv) { int fd; size_t n, total = 0; struct iovec vecs[NVECS]; int count = 0; struct timeval start, end, diff;

unlink(TESTFN);

fd = open(TESTFN, O_CREAT | O_WRONLY | O_TRUNC, 0644);

if (fd < 0) {
    perror("open");
    return 1;
}

#if 0 for (n = 1; n < 5; n++) { strs[n] = malloc(BIG * n + 1); memset(strs[n], 'a' + n, BIG * n); strs[n][BIG * n] = 0; } #endif

for (n = 0; n < NVECS; n++) {
    vecs[n].iov_base = strs[n];
    vecs[n].iov_len = strlen(strs[n]);
    total += vecs[n].iov_len;
}

gettimeofday(&start, NULL);

while (count++ < ITERS)
    do_writev(fd, vecs, NVECS, total);

gettimeofday(&end, NULL);

timersub(&end, &start, &diff);

#ifdef USE_WRITEV printf("writev"); #else printf("copy+write"); #endif

printf(": %lds%06ld.\n", diff.tv_sec, diff.tv_usec);

close(fd);
return 0;

}