BUFSIZ = 1024, still ?

[John Baldwin]

On Saturday, August 18, 2012 5:05:11 pm Poul-Henning Kamp wrote:
> In message <5030033B.4060705 at feral.com>, Matthew Jacob writes:
> >On 8/18/2012 1:32 PM, Poul-Henning Kamp wrote:
> >> Shouldn't we at least increase it to pagesize ?
> >>
> >What data suggests to you it would be better at pagesize?
> 
> The number of system calls to fwrite() a big file ?

Have you looked at an actual ktrace? :)  stdio doesn't use BUFSIZ for
regular files:

head/lib/libc/stdio/makebuf.c:
/*
 * Internal routine to determine `proper' buffering for a file.
 */
int
__swhatbuf(fp, bufsize, couldbetty)
        FILE *fp;
        size_t *bufsize;
        int *couldbetty;
{
        struct stat st;

        if (fp->_file < 0 || _fstat(fp->_file, &st) < 0) {
               ...
                *bufsize = BUFSIZ;
                return (__SNPT);
        }

        ...
        if (st.st_blksize <= 0) {
                *bufsize = BUFSIZ;
                return (__SNPT);
        }

        *bufsize = st.st_blksize;
        ...
}

For a regular file stdio will use the filesystem's block size, not BUFSIZ.

Test program:

#include <stdio.h>
#include <err.h>

int
main(int ac, char **av)
{
        char junk;
        FILE *fp;
        int i;

        fp = fopen("/tmp/junk", "w");
        if (fp == NULL)
                err(1, "fopen");
        for (i = 0; i < 1024 * 1024; i++)
                if (fwrite(&junk, sizeof(junk), 1, fp) != 1)
                        errx(1, "fwrite failed");
        return (0);
}

ktrace excerpt:

 42599 a.out    CALL  write(0x3,0x800a04000,0x4000)
 42599 a.out    RET   write 16384/0x4000
 42599 a.out    CALL  write(0x3,0x800a04000,0x4000)
 42599 a.out    RET   write 16384/0x4000
 42599 a.out    CALL  write(0x3,0x800a04000,0x4000)
 42599 a.out    RET   write 16384/0x4000
 42599 a.out    CALL  write(0x3,0x800a04000,0x4000)
 42599 a.out    RET   write 16384/0x4000
 42599 a.out    CALL  write(0x3,0x800a04000,0x4000)
 42599 a.out    RET   write 16384/0x4000
 42599 a.out    CALL  write(0x3,0x800a04000,0x4000)
 42599 a.out    RET   write 16384/0x4000

This hint also works for pipes (they set st_blksize to PAGE_SIZE).  Given 
that, I think BUFSIZ should just be left alone.

-- 
John Baldwin