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CONCURRENT PROGRAM-MING
Lecture 5
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Assignment 1
Having trouble? Resolve it as early as possible!
Assignment 2: Handling text-based protocol HTTP Proxy
Assignment 3: Concurrency Assignment 4: Distribute file system
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Last lecture
TCP Socket() Question: difference between send() and
write()? MSG_DONTWAIT, MSG_NOSIGNAL
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Concurrent Programming is Hard! The human mind tends to be sequential The notion of time is often misleading Thinking about all possible sequences of
events in a computer system is at least error prone and frequently impossible
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Concurrent Programming is Hard!
Classical problem classes of concurrent pro-grams: Races: outcome depends on arbitrary scheduling
decisions elsewhere in the system Example: who gets the last seat on the airplane?
Deadlock: improper resource allocation prevents forward progress Example: traffic gridlock
Livelock / Starvation / Fairness: external events and/or system scheduling decisions can prevent sub-task progress Example: people always jump in front of you in line
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Client / ServerSession
Iterative Echo ServerClient Serversocket socket
bind
listen
rio_readlineb
rio_writenrio_readlineb
rio_writen
Connectionrequest
rio_readlineb
close
closeEOF
Await connectionrequest fromnext client
open_listenfd
open_clientfd
acceptconnect
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Iterative Servers
Iterative servers process one request at a timeclient 1 server client 2
connect
accept connect
write read
call read
close
accept
write
read
close
Wait for Client 1
call read
write
ret read
writeret read
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Where Does Second Client Block?
Second client attempts to connect to iterative server
Call to connect returns Even though connection
not yet accepted Server side TCP manager
queues request Feature known as “TCP lis-
ten backlog” Call to rio_writen returns
Server side TCP manager buffers input data
Call to rio_readlineb blocks Server hasn’t written any-
thing for it to read yet.
Clientsocket
rio_readlineb
rio_writen
Connectionrequest
open_clientfd
connect
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Fundamental Flaw of Iterative Servers
Solution: use concurrent servers instead Concurrent servers use multiple concurrent flows to
serve multiple clients at the same time
User goesout to lunch
Client 1 blockswaiting for userto type in data
Client 2 blockswaiting to read from server
Server blockswaiting fordata fromClient 1
client 1 server client 2
connect
accept connect
write read
call readwrite
call readwriteret read
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Creating Concurrent Flows Allow server to handle multiple clients simultaneously
1. Processes Kernel automatically interleaves multiple logical flows Each flow has its own private address space
2. Threads Kernel automatically interleaves multiple logical flows Each flow shares the same address space
3. I/O multiplexing with select() Programmer manually interleaves multiple logical
flows All flows share the same address space Relies on lower-level system abstractions
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Concurrent Servers: Multiple Processes
Spawn separate process for each clientclient 1 server client 2
call connectcall accept
call read
ret connectret accept
call connect
call fgetsforkchild 1
User goesout to lunch
Client 1 blockswaiting for user to type in data
call acceptret connect
ret accept call fgets
writefork
call read
child 2
write
call read
end readclose
close
...
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Review: Iterative Echo Server
Accept a connection request Handle echo requests until client terminates
int main(int argc, char **argv) { int listenfd, connfd; int port = atoi(argv[1]); struct sockaddr_in clientaddr; int clientlen = sizeof(clientaddr);
listenfd = Open_listenfd(port); while (1) {
connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen);echo(connfd);Close(connfd);
} exit(0);}
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int main(int argc, char **argv) { int listenfd, connfd; int port = atoi(argv[1]); struct sockaddr_in clientaddr; int clientlen=sizeof(clientaddr);
Signal(SIGCHLD, sigchld_handler); listenfd = Open_listenfd(port); while (1) {
connfd = Accept(listenfd, (SA *) &clientaddr, &clientlen);if (Fork() == 0) { Close(listenfd); /* Child closes its listening socket */ echo(connfd); /* Child services client */ Close(connfd); /* Child closes connection with client */ exit(0); /* Child exits */}Close(connfd); /* Parent closes connected socket (important!) */
}}
Process-Based Concurrent Server
Fork separate process for each client
Does not allow any communication between different client handlers
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Process-Based Concurrent Server(cont)
Reap all zombie children
void sigchld_handler(int sig) { while (waitpid(-1, 0, WNOHANG) > 0)
; return;}
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Process Execution Model
Each client handled by independent process No shared state between them Both parent & child have copies of listenfd and connfd
Parent must close connfd Child must close listenfd
ServerProcess(Client 1)
ServerProcess(Client 2)
ServerProcess
(Listening)
Connection Requests
Client 1 data Client 2 data
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Concurrent Server: accept Illustratedlistenfd(3)
Client1. Server blocks in accept, waiting for connection request on listening descriptor listenfd
clientfd
Server
listenfd(3)
Client
clientfd
Server2. Client makes connection request by calling and blocking in connect
Connectionrequest
listenfd(3)
Client
clientfd
Server3. Server returns connfd from accept. Forks child to handle client. Client returns from connect. Connection is now established between clientfd and connfd
ServerChild
connfd(4)
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Implementation Must-dos With Process-Based Designs
Listening server process must reap zombie children to avoid fatal memory leak
Listening server process must close its copy of connfd Kernel keeps reference for each socket/open file After fork, refcnt(connfd) = 2 Connection will not be closed until refcnt(connfd) == 0
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View from Server’s TCP Manager
Client 1 ServerClient 2
cl1> ./echoclient greatwhite.ics.cs.cmu.edu 15213
srv> ./echoserverp 15213
srv> connected to (128.2.192.34), port 50437
cl2> ./echoclient greatwhite.ics.cs.cmu.edu 15213
srv> connected to (128.2.205.225), port 41656
Connec-tion
Host Port Host Port
Listening --- --- 128.2.220.10 15213
cl1 128.2.192.34 50437 128.2.220.10 15213
cl2 128.2.205.225 41656 128.2.220.10 15213
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Pros and Cons of Process-Based Designs
+ Handle multiple connections concurrently + Clean sharing model
file tables (yes) global variables (no)
+ Simple and straightforward – Additional overhead for process control – Nontrivial to share data between pro-
cesses Requires IPC (interprocess communication)
mechanisms FIFO’s (named pipes), System V shared memory
and semaphores
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#2) Event-Based Concurrent Servers Using I/O Multiplexing
Server maintains set of active connections Array of connfd’s
Repeat: Determine which connections have pending in-
puts If listenfd has input, then accept connection
Add new connfd to array Service all connfd’s with pending inputs
Library support Use library functions to construct scheduler
within single process
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Adding Concurency: Step One
Start with allowing address re-use
Then we set the socket to non-blocking
int sock, opts;sock = socket(…);// getting the current optionssetsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &opts, sizeof(opts));
// getting current optionsif (0 > (opts = fcntl(sock, F_GETFL)))
printf(“Error…\n”);// modifying and applyingopts = (opts | O_NONBLOCK);if (fcntl(sock, F_SETFL, opts))
printf(“Error…\n”);bind(…);
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Adding Concurrency: Step Two
Monitor sockets with select() int select(int maxfd, fd_set *readfds, fd_set
*writefds, fd_set *exceptfds, const struct timespec *timeout);
So what’s an fd_set? Bit vector with FD_SETSIZE bits
maxfd – Max file descriptor + 1 readfs – Bit vector of read descriptors to monitor writefds – Bit vector of write descriptors to monitor exceptfds – Read the manpage, set to NULL timeout – How long to wait with no activity before
returning, NULL for eternity
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How does the code change?
if (listen(sockfd, 5) < 0) { // listen for incoming connectionsprintf(“Error listening\n”);
init_pool(sockfd, &pool);while (1) {
pool.ready_set = &pool.read_set;pool.nready = select(pool.maxfd+1, &pool.ready_set,
&pool.write_set, NULL, NULL);if (FD_ISSET(sockfd, &pool.ready_set)) {
if (0 > (isock = accept(sockfd,…))) printf(“Error accepting\n”);
add_client(isock, &caddr, &pool);}check_clients(&pool);
}// close it up down here
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What was pool?
User defined structure used to store in-formation.
A struct something like this:typedef struct s_pool {
int maxfd; // largest descriptor in setsfd_set read_set; // all active read descriptorsfd_set write_set; // all active write descriptorsfd_set ready_set; // descriptors ready for readingint nready; // return of select()int maxi; /* highwater index into client array */int clientfd[FD_SETSIZE]; /* set of active descriptors */rio_t clientrio[FD_SETSIZE]; /* set of active read buffers */
} pool;
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void init_pool(int sockfd, pool * p);{
int i;p->maxfd = -1;for (i=0;i<FD_SETSIZE;i++) p->clientfd[i]=-1;
p->maxfd = sockfd;FD_ZERO(&p->read_set);FD_SET(listenfd, &p->read_set);
}// close it up down here
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void add_client(int connfd, pool *p) { int i; p->nready--; for (i = 0; i < FD_SETSIZE; i++) /* Find an available slot */
if (p->clientfd[i] < 0) { /* Add connected descriptor to the pool */ p->clientfd[i] = connfd; Rio_readinitb(&p->clientrio[i], connfd);
/* Add the descriptor to descriptor set */ FD_SET(connfd, &p->read_set);
/* Update max descriptor and pool highwater mark */ if (connfd > p->maxfd)
p->maxfd = connfd; if (i > p->maxi)
p->maxi = i; break;}
if (i == FD_SETSIZE) /* Couldn't find an empty slot */app_error("add_client error: Too many clients");
}// close it up down here
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void check_clients(pool *p) { int i, connfd, n; char buf[MAXLINE]; rio_t rio;
for (i = 0; (i <= p->maxi) && (p->nready > 0); i++) {connfd = p->clientfd[i];rio = p->clientrio[i];
/* If the descriptor is ready, echo a text line from it */if ((connfd > 0) && (FD_ISSET(connfd, &p->ready_set))) { p->nready--; if ((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) {
byte_cnt += n; //line:conc:echoservers:beginechoprintf("Server received %d (%d total) bytes on fd %d\n", n, byte_cnt, connfd);Rio_writen(connfd, buf, n); //line:conc:echoservers:endecho
}
/* EOF detected, remove descriptor from pool */ else {
Close(connfd); //line:conc:echoservers:closeconnfdFD_CLR(connfd, &p->read_set); //line:conc:echoservers:beginremovep->clientfd[i] = -1; //line:conc:echoservers:endremove
}}
}}
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So what about bit vectors?
void FD_ZERO(fd_set *fdset); Clears all the bits
void FD_SET(int fd, fd_set *fdset); Sets the bit for fd
void FD_CLR(int fd, fd_set *fdset); Clears the bit for fd
int FD_ISSET(int fd, fd_set *fdset); Checks whether fd’s bit is set
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What about checking clients?
The code only tests for new incoming connec-tions But we have many more to test!
Store all your client file descriptors In pool is a good idea!
Several scenarios Clients are sending us data We may have pending data to write in a buffer
Keep the while(1) thin Delegate specifics to functions that access the ap-
propriate data Keep it orthogonal!
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Back to that lifecycle…
ServerClient(s)socket()
connect()
write()read()
close()
socket()
bind()
listen()
select()
write()
read()
close()
read()EOF
Connection Request
Client / Server Session(s)
FD_ISSET(sfd)
accept()
check_clients() main loop
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I/O Multiplexed Event Pro-cessing
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clientfd
7
4
-1
-1
12
5
-1
-1
-1
0
1
2
3
4
5
6
7
8
9
Active
Inactive
Active
Never Used
listenfd = 3
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clientfd
7
4
-1
-1
12
5
-1
-1
-1
listenfd = 3
Active Descriptors Pending Inputs
Read
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Time out?
int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
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Pros and Cons of I/O Multi-plexing
+ One logical control flow. + Can single-step with a debugger. + No process or thread control overhead.
Design of choice for high-performance Web servers and search engines.
– Significantly more complex to code than process- or thread-based designs.
– Hard to provide fine-grained concur-rency E.g., our example will hang up with partial
lines. – Cannot take advantage of multi-core
Single thread of control
