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SubjectRe: [PATCH 2 of 4] Introduce i386 fibril scheduling
On Fri, 2 Feb 2007, Linus Torvalds wrote:

> > You get some other funny things from co-routines which are very powerful,
> > very dangerous, or plain insane
>
> You forgot "very hard to think about".
>
> We DO NOT want coroutines in general. It's clever, but it's
> (a) impossible to do without language support that C doesn't have, or
> some really really horrid macro constructs that really only work for
> very specific and simple cases.
> (b) very non-intuitive unless you've worked with coroutines a lot (and
> almost nobody has)

Actually, coroutines are not too bad to program once you have a
total-coverage async scheduler to run them. The attached (very sketchy)
example uses libpcl ( http://www.xmailserver.org/libpcl.html ) and epoll
as scheduler (but here you can really use anything). You can implement
coroutines in many way, from C preprocessor macros up to anything, but in
the libpcl case they are simply switched stacks. Like fibrils are supposed
to be. The problem is that in order to make a real-life example of
coroutine-based application work, you need everything that can put you at
sleep (syscalls or any external library call you have no control on)
implemented in an async way. And what I ended up doing is exactly what Zab
did inside the kernel. In my case a dynamic pool of (userspace) threads
servicing any non-native potentially pre-emptive call, and signaling the
result to a pollable fd (pipe in my case) that is integrated in the epoll
(poll/select whatever) scheduler.
I personally find Zab idea a really good one, since it allows for generic
kernel async implementation, w/out the burden of dirtying kernel code
paths with AIO knowledge. Being it fibrils or real kthreads, it is IMO
definitely worth a very close look.




- Davide


struct eph_conn {
int sfd;
unsigned int events, revents;
coroutine_t co;
};


int eph_new_conn(int sfd, void *func) {
struct eph_conn *conn;
struct epoll_event ev;
conn = (struct eph_conn *) malloc(sizeof(struct eph_conn));
conn->sfd = sfd;
conn->co = co_create(func, conn, NULL, STACKSIZE);
ev.events = 0;
ev.data.ptr = conn;
epoll_ctl(kdpfd, EPOLL_CTL_ADD, sfd, &ev);
co_call(conn->co);
return 0;
}
void eph_exit_conn(struct eph_conn *conn) {
struct epoll_event ev;
epoll_ctl(kdpfd, EPOLL_CTL_DEL, conn->sfd, &ev);
co_exit();
}
int eph_connect(struct eph_conn *conn, const struct sockaddr *serv_addr, socklen_t addrlen) {
if (connect(conn->sfd, serv_addr, addrlen) == -1) {
if (errno != EWOULDBLOCK && errno != EINPROGRESS)
return -1;
co_resume();
if (conn->revents & (EPOLLERR | EPOLLHUP))
return -1;
}
return 0;
}
int eph_read(struct eph_conn *conn, void *buf, int nbyte) {
int n;
while ((n = read(conn->sfd, buf, nbyte)) < 0) {
if (errno == EINTR)
continue;
if (errno != EAGAIN && errno != EWOULDBLOCK)
return -1;
co_resume();
}
return n;
}
int eph_write(struct eph_conn *conn, void const *buf, int nbyte) {
int n;
while ((n = write(conn->sfd, buf, nbyte)) < 0) {
if (errno == EINTR)
continue;
if (errno != EAGAIN && errno != EWOULDBLOCK)
return -1;
co_resume();
}
return n;
}
int eph_accept(struct eph_conn *conn, struct sockaddr *addr, int *addrlen) {
int sfd;
while ((sfd = accept(conn->sfd, addr, (socklen_t *) addrlen)) < 0) {
if (errno == EINTR)
continue;
if (errno != EAGAIN && errno != EWOULDBLOCK)
return -1;
co_resume();
}
return sfd;
}
int eph_scheduler(int loop, long timeout) {
int i, nfds;
struct eph_conn *conn;
struct epoll_event *cevents;
do {
nfds = epoll_wait(kdpfd, events, maxfds, timeout);
for (i = 0, cevents = events; i < nfds; i++, cevents++) {
conn = cevents->data.ptr;
conn->revents = cevents->events;
if (conn->revents & conn->events)
co_call(conn->co);
}
} while (loop);
return 0;
}
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