PA3: Improving Performance
with I/O Multiplexing
Part 1-1: Nov. 7, Part 1-2: Nov. 10
Part 2-1: Nov. 17, Part 2-2: Nov.20
Dealing with Concurrency
• Creating new child or new thread
• Expensive
• Only handling 1 connection in a thread
• I/O multiplexing with event-driven programming
• Multiplex input/output between multiple files (or
sockets)
• Select()
• When event comes, select returns
• Buffered I/O (POSIX AIO)
• I/O asynchronously done in background
• Libevent
• When event comes, pre-designated function is called
Select()
• Select returns when
there’s new event
• After select returns,
it checks whether
the fd is set.
• If fd is set, process
the event (e.g.,
handle HTTP request)
// server example
ret = select(n, &fds, NULL, NULL, NULL);
if (ret < 0) {
perror(“select”);
} else if (ret) {
for (i = 0; i < n; i++) {
if (FD_ISSET(fd_set[i], &fds)) {
/* if listening fd, accept* /
/* otherwise, process request */
if (fd_set[i] == listenfd) {
accept(listenfd, …);
} else {
read(fd_set[i], …);
write(fd_set[i], …);
}
}
}
}
Part 1: POSIX AIO
• POSIX asynchronous I/O
• aio_read(), aio_write() immediately returns
• The actual read and write are processed in the
background
• The application can do another work without being
blocked by read() or write()
• The completion of I/O can be notified by a signal
Server Example using AIO
struct aiocb aiocbList[MAX_FD];
sa.sa_flags = SA_RESTART | SA_SIGINFO;
sa.sa_sigaction = aioSigHandler;
if (sigaction(IO_SIGNAL, &sa, NULL) == -1) exit(1);
while (1) {
fd = accept(listenfd, …);
if (fd < 0) continue;
/* build aio control block */
aiocbList[fd].aio_fildes = fd;
aiocbList[fd].aio_buf = malloc(BUF_SIZE);
aiocbList[fd].aio_nbytes = BUF_SIZE;
aiocbList[fd].aio_reqprio = 0;
aiocbList[fd].aio_offset = 0;
aiocbList[fd].aio_sigevent.sigev_notify = SIGEV_SIGNAL;
aiocbList[fd].aio_sigevent.sigev_signo = IO_SIGNAL;
aiocbList[fd].aio_sigevent.sigev_value.sival_ptr =
&aiocbList[fd];
s = aio_read(&aiocbList[fd]);
if (s == -1)
errExit("aio_read");
}
Server Example using AIO
/* Handler for I/O completion signal */
static void
aioSigHandler(int sig, siginfo_t *si, void *ucontext)
{
struct aiocb *acb = si->value.sival_ptr;
int fd = acb->aio_fildes;
/* process the request stored in acb->aio_buf
/* write a response using aio_write() */
/* if aio_write() completes, close the connection */
}
For more detail, refer to the man page of aio (in shell, $ man aio)
Part 2: Libevent
• Event notification library
• Execute a callback function when a specified event
occurs on a file descriptor
• Availability-based
•
•
•
•
If incoming connection available, it notifies
If incoming request available, it notifies
If sending buffer available, it notifies
Calls pre-registered callback function when there is event
• Use nonblocking for I/O operations
• Set socket fds to nonblocking by fcntl()
Server Example using libevent
#include <event.h> /* for libevent */
int main() {
struct event ev_accept;
…
/* create listening socket and set it nonblock */
…
event_init();
/* register the event */
event_set(&ev_accept, listenfd, EV_READ | EV_PERSIST,
OnAcceptEvent, NULL);
event_add(&ev_accept, NULL);
/* start event loop */
event_dispatch();
…
close(listenfd);
return 0;
}
Server Example using libevent
static void
OnAcceptEvent(int fd, short event, void *arg)
{
struct event ev_read;
int new_fd = accept(fd, NULL, NULL);
/* set new_fd nonblocking */
event_set(&ev_read, new_fd, EV_READ, OnReadEvent, NULL);
event_add(&ev_read, NULL);
}
static void
OnReadEvent(int fd, short event, void *arg)
{
struct event ev_write;
char buf[BUFSIZE];
read(fd, buf, BUFSIZE);
/* process the request in buffer */
/* write the response */
write(fd, response, len);
}
Buffer Management
• What if send buffer is full?
• write() will return -1
• errno will be set to EAGAIN or EWOULDBLOCK
• What you need to do?
• You need to send the response again later
• Remember the “state”
• Until where it was sent
• Register a write event
• Retry later when write event comes
• When send buffer becomes available, write event will come
Remembering the State
struct context
{
char w_buf[BUFSIZE];
/* write buffer */
int fd;
struct event ev_read;
struct event ev_write;
int off;
/* buffer offset */
int rem_len;
/* remaining length */
};
static void
OnReadEvent(int fd, short event, void *arg)
{
struct context *ctx = (struct context *)arg;
char buf[BUFSIZE];
rd = read(fd, buf, BUFSIZE);
/* process the request in buffer */
/* start writing the response */
/* if not sent fully, register an event */
wr = write(fd, ctx->w_buf, ctx->rem_len);
ctx->off += wr; ctx->rem_len -= wr;
event_del(ctx->ev_read, NULL);
event_set(ctx->ev_write, fd, EV_WRITE, OnWriteEvent, ctx);
event_add(ctx->ev_write, NULL);
}
Remembering the State
static void
OnWriteEvent(int fd, short event, void *arg)
{
struct context *ctx = (struct context *)arg;
wr = write(fd, ctx->w_buf + ctx->off, ctx->rem_len);
ctx->off += wr; ctx->rem_len -= wr;
/* if done writing */
if (ctx->rem_len == 0) {
event_del(ctx->ev_write, NULL);
close(fd);
free(ctx);
}
}
Evaluation
• 50 pt for each part
• 20 pt for basic functionality testing
• Ina.kaist.ac.kr, yahoo, naver
• 20 pt for performance and robustness testing
• Testing with ab (ApachBench)
• Many consecutive requests
• Many concurrent connections
• 10 pt for report
• Performance comparison
• Threaded vs. AIO vs. libevent