Pipes, cat buffer size

[Ivan Voras]

2008/10/19 Dan Nelson <dnelson at allantgroup.com>:
> In the last episode (Oct 19), Ivan Voras said:

>> Of course. But that's not the point :) From what I see (didn't look at
>> the code), Linux for example does some kind of internal buffering that
>> decouples how the reader and the writer interact. I think that with
>> FreeBSD's current behaviour the writer could write 1-byte buffers and
>> the reader will be forced to read each byte individually. I don't know
>> if there's some ulterior reason for this.
>
> No; take a look at /sys/kern/sys_pipe.c .  Depending on how much data
> is in the pipe, it switches between async in-kernel buffering (<8192
> bytes), and direct page wiring between sender and receiver (basically
> zero-copy).

Ok, maybe it's just not behaving as I thought it should. See this test program:

----
#include <sys/fcntl.h>
#include <stdlib.h>
#include <stdio.h>

#define BSIZE (1024*1024)

void main() {
        int r;
        char buf[BSIZE];

        while (1) {
                r = read(0, buf, BSIZE);
                fprintf(stderr, "read %d bytes\n", r);
                if (r <= 0)
                        break;
        }
}
----

and this command line:

> dd bs=1 if=/dev/zero| ./reader

The output of this on RELENG_7 is:

read 8764 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
read 1 bytes
...

The first value puzzles me - so it actually is doing some kind of
buffering. Linux isn't actually much better, but the intention is
there:

$ dd if=/dev/zero bs=1 | ./bla
read 1 bytes
read 38 bytes
read 8 bytes
read 2 bytes
read 2 bytes
read 2 bytes
read 2 bytes
read 4 bytes
read 3 bytes
read 2 bytes
read 2 bytes
read 2 bytes
read 2 bytes
read 2 bytes
read 2 bytes
read 3 bytes
read 3 bytes
read 112 bytes
read 2 bytes
read 2 bytes
...

Maybe FreeBSD switches between the writer and the reader too soon so
the buffer doesn't get filled?

Using cat (which started all this), FreeBSD consistently processes
4096 byte buffers, while Linux's sizes are all over the place - from 4
kB to 1 MB, randomly fluctuating. My goal would be (if it's possible -
it might not be) to maximize coalescing in an environment where the
reader does something with the data (e.g. compression) so there should
be a reasonable amount of backlogged input data.

But if it works in general, it may simply be that it isn't really
applicable to my purpose (and I should modify the reader to read
multiple blocks).

Though it won't help me, I still think that modifying cat is worth it :)