Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

Linus Torvalds
2007-02-26T19:23:48Z

On Mon, 26 Feb 2007, Evgeniy Polyakov wrote:
>
> I want to say, that read() consists of tons of events, but programmer
> needs only one - data is ready in requested buffer. Programmer might
> not even know what is the object behind provided file descriptor.
> One only wans data in the buffer.

You're not following the discussion.

First off, I already *told* you that "read()" is the absolute simplest
case, and yes, we could make it an event if you also passed in the "which
range of the file do we care about" information (we could consider it
"f_pos", which the kernel already knows about, but that doesn't handle
pread()/pwrite(), so it's not very good for many cases).

But that's not THE ISSUE.

The issue is that it's a horrible interface from a users standpoint.
It's a lot better to program certain things as a thread. Why do you argue
against that, when that is just obviously true.

There's a reason that people write code that is functional, rather than
write code as a state machine.

We simply don't write code like

for (;;) {
switch (state) {
case Open:
fd = open();
if (fd < 0)
break;
state = Stat;
case Stat:
if (fstat(fd, &stat) < 0)
break;
state = Read;
case Read:
count = read(fd, buf + pos, size - pos));
if (count < 0)
break;
pos += count;
if (!count || pos == size)
state = Close;
continue;
case Close;
if (close(fd) < 0)
break;
state = Done;
return 0;
}
}
/* Returning 1 means wait in the event loop .. */
if (errno == EAGAIN || errno == EINTR)
return 1;
/* Else we had a real error */
state = Error;
return -1;

and instead we write it as

fd = open(..)
if (fd < 0)
return -1;
if (fstat(fd, &st)) < 0) {
close(fd);
return -1;
}
..

and if you cannot see the *reason* why people don't use event-based
programming for everything, I don't see the point of continuing this
discussion.

See? Stop blathering about how everything is an event. THAT'S NOT
RELEVANT. I've told you a hundred times - they may be "logically
equivalent", but that doesn't change ANYTHING. Event-based programming
simply isn't suitable for 99% of all stuff, and for the 1% where it *is*
suitable, it actually tends to be a very specific subset of the code that
you actually use events for (ie accept and read/write on pure streams).

Linus