1. Web Services 2. Concurrency and threads. Web History 1989: Tim Berners-Lee (CERN) writes...
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1. Web Services
2. Concurrency and threads
Web History
1989: Tim Berners-Lee (CERN) writes internal proposal to develop a
distributed hypertext system. Connects “a web of notes with links.” Intended to help CERN physicists in large projects share and
manage information 1990:
Tim BL writes a graphical browser for Next machines.
Web History (cont) 1992
NCSA server released 26 WWW servers worldwide
1993 Marc Andreessen releases first version of NCSA Mosaic browser Mosaic version released for (Windows, Mac, Unix). Web (port 80) traffic at 1% of NSFNET backbone traffic. Over 200 WWW servers worldwide.
1994 Andreessen and colleagues leave NCSA to form “Mosaic
Communications Corp” (predecessor to Netscape).
Internet Hosts
Web Servers
Webserver
HTTP request
HTTP response(content)
Clients and servers communicate using the HyperText Transfer Protocol (HTTP) Client and server establish TCP
connection Client requests content Server responds with
requested content Client and server close
connection (eventually) Current version is HTTP/1.1
RFC 2616, June, 1999.
Webclient
(browser)
http://www.w3.org/Protocols/rfc2616/rfc2616.html
IP
TCP
HTTP
Datagrams
Streams
Web content
Web Content Web servers return content to clients
content: a sequence of bytes with an associated MIME (Multipurpose Internet Mail Extensions) type
Example MIME types text/html HTML document text/plain Unformatted text application/postscript Postcript document image/gif Binary image encoded in GIF
format image/jpeg Binary image encoded in JPEG
format
Static and Dynamic Content
The content returned in HTTP responses can be either static or dynamic. Static content: content stored in files and retrieved in response to
an HTTP request Examples: HTML files, images, audio clips. Request identifies content file
Dynamic content: content produced on-the-fly in response to an HTTP request
Example: content produced by a program executed by the server on behalf of the client.
Request identifies file containing executable code Bottom line: All Web content is associated with a file that
is managed by the server.
URLs Each file managed by a server has a unique name called a URL
(Universal Resource Locator) URLs for static content:
http://reed.cs.depaul.edu:80/index.html http://reed.cs.depaul.edu/index.html http://reed.cs.depaul.edu
Identifies a file called index.html, managed by a Web server at reed.cs.depaul.edu that is listening on port 80.
URLs for dynamic content: http://riely373.cdm.depaul.edu:8000/cgi-bin/adder?15000&213
Identifies an executable file called adder, managed by a Web server at riely373.cdm.depaul.edu that is listening on port 8000, that should be called with two argument strings: 15000 and 213.
How Clients and Servers Use URLs Example URL: http://www.depaul.edu:80/index.html Clients use prefix (http://www.depaul.edu:80) to infer:
What kind of server to contact (Web server) Where the server is (www.depaul.edu) What port it is listening on (80)
Servers use suffix (/index.html) to: Determine if request is for static or dynamic content.
No hard and fast rules for this. Convention: executables reside in cgi-bin directory
Find file on file system. Initial “/” in suffix denotes home directory for requested content. Minimal suffix is “/”, which all servers expand to some default
home page (e.g., index.html).
Anatomy of an HTTP Transaction$ telnet reed.cs.depaul.edu 80Trying 140.192.39.42...Connected to reed.cti.depaul.edu.Escape character is '^]'.GET / HTTP/1.1host: reed.cs.depaul.edu
HTTP/1.1 200 OKServer: Apache-Coyote/1.1Accept-Ranges: bytesETag: W/"2285-1357855910000"Last-Modified: Thu, 10 Jan 2013 22:11:50 GMTContent-Type: text/htmlContent-Length: 2285Date: Mon, 04 Mar 2013 04:01:00 GMT
<html><head><META http-equiv="Content-Type" content="text/html; charset=UTF-8”>...
HTTP Requests
HTTP request is a request line, followed by zero or more request headers
Request line: <method> <uri> <version> <version> is HTTP version of request (HTTP/1.0 or HTTP/1.1)
<uri> is typically URL for proxies, URL suffix for servers. A URL is a type of URI (Uniform Resource Identifier) See http://www.ietf.org/rfc/rfc2396.txt
<method> is either GET, POST, OPTIONS, HEAD, PUT, DELETE, or TRACE.
HTTP Requests (cont) HTTP methods:
GET: Retrieve static or dynamic content Arguments for dynamic content are in URI Workhorse method (99% of requests)
POST: Retrieve dynamic content Arguments for dynamic content are in the request body
OPTIONS: Get server or file attributes HEAD: Like GET but no data in response body PUT: Write a file to the server! DELETE: Delete a file on the server! TRACE: Echo request in response body
Useful for debugging. Request headers: <header name>: <header data>
Provide additional information to the server.
HTTP Versions
Major differences between HTTP/1.1 and HTTP/1.0 HTTP/1.0 uses a new connection for each transaction. HTTP/1.1 also supports persistent connections
multiple transactions over the same connection Connection: Keep-Alive
HTTP/1.1 requires HOST header Host: www.depaul.edu Makes it possible to host multiple websites at single Internet
host HTTP/1.1 supports chunked encoding (described later)
Transfer-Encoding: chunked HTTP/1.1 adds additional support for caching
HTTP Responses HTTP response is a response line followed by zero or more
response headers. Response line: <version> <status code> <status msg>
<version> is HTTP version of the response. <status code> is numeric status. <status msg> is corresponding English text.
200 OK Request was handled without error 301 Moved Provide alternate URL 403 Forbidden Server lacks permission to access file 404 Not found Server couldn’t find the file.
Response headers: <header name>: <header data> Provide additional information about response Content-Type: MIME type of content in response body. Content-Length: Length of content in response body.
GET Request From Chrome Browser
GET / HTTP/1.1\r\nHost: reed.cs.depaul.edu\r\nConnection: keep-alive\r\nAccept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8\r\nUser-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.97 Safari/537.22\r\nAccept-Encoding: gzip,deflate,sdch\r\nAccept-Language: en-US,en;q=0.8\r\nAccept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3\r\nCookie:__utma=114012434.756988690.1360702406.1360702406.1360874291.2; __utmz=114012434.1360874291.2.2.utmcsr=cdm.depaul.edu|utmccn=(referral)|utmcmd=referral|utmcct=/academics/Pages/bs%20computerscience%20standard.aspx\r\n\r\n
URI is just the suffix, not the entire URL
GET Response From Apache Server
HTTP/1.1 200 OKServer: Apache-Coyote/1.1\r\nAccept-Ranges: bytes\r\nETag: W/”2285-1357855910000”\r\nLast-Modified: Thu, 10 Jan 2013 22:11:50 GMT\r\nContent-Type: test/html\r\nContent-Length: 2285\r\nDate: Mon, 04 Mar 2013 04:58:40 GMT\r\n\r\n<html>\n<head>\n...
Tiny Web Server Tiny Web server described in text
Tiny is a sequential Web server. Serves static and dynamic content to real browsers.
text files, HTML files, GIF and JPEG images. 226 lines of commented C code. Not as complete or robust as a real web server
Tiny Operation Read request from client Split into method / uri / version
If not GET, then return error If URI contains “cgi-bin” then serve dynamic content
Fork process to execute program Otherwise serve static content
Copy file to output
Tiny Serving Static Content
Serve file specified by filename Use file metadata to compose header “Read” file via mmap Write to output
/* Send response headers to client */ get_filetype(filename, filetype); sprintf(buf, "HTTP/1.0 200 OK\r\n"); sprintf(buf, "%sServer: Tiny Web Server\r\n", buf); sprintf(buf, "%sContent-length: %d\r\n", buf, filesize); sprintf(buf, "%sContent-type: %s\r\n\r\n", buf, filetype); Rio_writen(fd, buf, strlen(buf));
/* Send response body to client */ srcfd = Open(filename, O_RDONLY, 0); srcp = Mmap(0, filesize, PROT_READ, MAP_PRIVATE, srcfd, 0); Close(srcfd); Rio_writen(fd, srcp, filesize); Munmap(srcp, filesize);
From tiny.c
Serving Dynamic Content
Client Server
Client sends request to server.
If request URI contains the string “/cgi-bin”, then the server assumes that the request is for dynamic content.
GET /cgi-bin/env.pl HTTP/1.1
Serving Dynamic Content (cont)
Client Server The server creates a child
process and runs the program identified by the URI in that process
env.pl
fork/exec
Serving Dynamic Content (cont)
Client Server The child runs and generates
the dynamic content. The server captures the
content of the child and forwards it without modification to the client
env.pl
Content
Content
Issues in Serving Dynamic Content
How does the client pass program arguments to the server?
How does the server pass these arguments to the child?
How does the server pass other info relevant to the request to the child?
How does the server capture the content produced by the child?
These issues are addressed by the Common Gateway Interface (CGI) specification.
Client Server
Content
Content
Request
Create
env.pl
CGI
Because the children are written according to the CGI spec, they are often called CGI programs.
Because many CGI programs are written in Perl, they are often called CGI scripts.
However, CGI really defines a simple standard for transferring information between the client (browser), the server, and the child process.
The cdmlinux addition portalinput URL
Output page
host port CGI program args
Serving Dynamic Content With GET Question: How does the client pass arguments to the server? Answer: The arguments are appended to the URI Can be encoded directly in a URL typed to a browser or a URL
in an HTML link http://cdmlinux.cdm.depaul.edu/cgi-bin/adder?n1=4&n2=7
adder is the CGI program on the server that will do the addition. argument list starts with “?” arguments separated by “&” spaces represented by “+” or “%20”
URI often generated by an HTML form
<FORM METHOD=GET ACTION="cgi-bin/adder"><p>X <INPUT NAME="n1"><p>Y <INPUT NAME="n2"><p><INPUT TYPE=submit></FORM>
Serving Dynamic Content With GET
URL: cgi-bin/adder?4&7
Result displayed on browser:
Welcome to THE Internet addition portal.
The answer is: 4+7=11
Thanks for visiting!
Serving Dynamic Content With GET Question: How does the server pass these arguments to
the child? Answer: In environment variable QUERY_STRING
A single string containing everything after the “?” For add: QUERY_STRING = “4&7”
Additional CGI Environment Variables General
SERVER_SOFTWARE SERVER_NAME GATEWAY_INTERFACE (CGI version)
Request-specific SERVER_PORT REQUEST_METHOD (GET, POST, etc) QUERY_STRING (contains GET args) REMOTE_HOST (domain name of client) REMOTE_ADDR (IP address of client) CONTENT_TYPE (for POST, type of data in message body, e.g., text/html)
CONTENT_LENGTH (length in bytes)
Even More CGI Environment Variables
In addition, the value of each header of type type received from the client is placed in environment variable HTTP_type Examples (any “-” is changed to “_”) :
HTTP_ACCEPT HTTP_HOST HTTP_USER_AGENT
Serving Dynamic Content With GET Question: How does the server capture the content produced by the child? Answer: The child generates its output on stdout. Server uses dup2 to
redirect stdout to its connected socket. Notice that only the child knows the type and size of the content. Thus the child
(not the server) must generate the corresponding headers.
/* Make the response body */ sprintf(content, "Welcome to add.com: "); sprintf(content, "%sTHE Internet addition portal.\r\n<p>", content); sprintf(content, "%sThe answer is: %s\r\n<p>",
content, msg); sprintf(content, "%sThanks for visiting!\r\n", content); /* Generate the HTTP response */ printf("Content-length: %u\r\n", (unsigned) strlen(content)); printf("Content-type: text/html\r\n\r\n"); printf("%s", content);
From adder.c
Serving Dynamic Content With GET
HTTP request sent by client
HTTP response generated by the server
HTTP response generated bythe CGI program
$ telnet riely373.cdm.depaul.edu 8000Trying 140.192.39.11...Connected to riely373.cdm.depaul.edu.Escape character is '^]'.GET /cgi-bin/adder?4&7 HTTP/1.0
HTTP/1.0 200 OKServer: Tiny Web ServerContent-length: 97Content-type: text/html
Welcome to THE Internet addition portal.<p>The answer is: 4 + 7 = 11<p>Thanks for visiting!Connection closed by foreign host.$
Tiny Serving Dynamic Content
Fork child to execute CGI program Change stdout to be connection to client Execute CGI program with execve
/* Return first part of HTTP response */ sprintf(buf, "HTTP/1.0 200 OK\r\n"); Rio_writen(fd, buf, strlen(buf)); sprintf(buf, "Server: Tiny Web Server\r\n"); Rio_writen(fd, buf, strlen(buf)); if (Fork() == 0) { /* child */
/* Real server would set all CGI vars here */setenv("QUERY_STRING", cgiargs, 1); Dup2(fd, STDOUT_FILENO); /* Redirect stdout to client */Execve(filename, emptylist, environ);/* Run CGI prog */
} Wait(NULL); /* Parent waits for and reaps child */
From tiny.c
Proxies A proxy is an intermediary between a client and an origin
server. To the client, the proxy acts like a server. To the server, the proxy acts like a client.
Client ProxyOriginServer
1. Client request 2. Proxy request
3. Server response4. Proxy response
Why Proxies? Can perform useful functions as requests and responses pass
by Examples: Caching, logging, anonymization, filtering, transcoding
ClientA
Proxycache
OriginServer
Request foo.html
Request foo.html
foo.html
foo.html
ClientB
Request foo.html
foo.html
Fast inexpensive local network
Slower more expensiveglobal network
For More Information Study the Tiny Web server described in your text
Tiny is a sequential Web server. Serves static and dynamic content to real browsers.
text files, HTML files, GIF and JPEG images. 220 lines of commented C code. Also comes with an implementation of the CGI script for the add.com
addition portal.
See the HTTP/1.1 standard: http://www.w3.org/Protocols/rfc2616/rfc2616.html
2. Concurrency and threads
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
Iterative Servers Iterative servers process one request at a time
client 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
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
Concurrent Servers: Multiple Processes Spawn separate process for each client
client 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
...
Review: Iterative Echo Serverint 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);}
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);
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
Process-Based Concurrent Server(cont)
void sigchld_handler(int sig) { while (waitpid(-1, 0, WNOHANG) > 0)
; return;}
Reap all zombie children
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
Client 1Server
Process
Client 2Server
Process
ListeningServer
Process
Connection Requests
Client 1 data Client 2 data
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)
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
Pros and Cons of Process-Based Designs
+ Handle multiple connections concurrently + Clean sharing model
descriptors (no) file tables (yes) global variables (no)
+ Simple and straightforward – Additional overhead for process control – Nontrivial to share data between processes
Requires IPC (interprocess communication) mechanisms FIFO’s (named pipes), System V shared memory and semaphores
Approach #2: Multiple Threads
Very similar to approach #1 (multiple processes) but, with threads instead of processes
Traditional View of a Process Process = process context + code, data, and stack
shared libraries
run-time heap
0
read/write data
Program context: Data registers Condition codes Stack pointer (SP) Program counter (PC)Kernel context: VM structures Descriptor table brk pointer
Code, data, and stack
read-only code/data
stackSP
PC
brk
Process context
Alternate View of a Process Process = thread + code, data, and kernel context
shared libraries
run-time heap
0
read/write dataThread context:
Data registers Condition codes Stack pointer (SP) Program counter (PC)
Code and Data
read-only code/data
stackSP
PC
brk
Thread (main thread)
Kernel context: VM structures Descriptor table brk pointer
A Process With Multiple Threads Multiple threads can be associated with a process
Each thread has its own logical control flow Each thread shares the same code, data, and kernel context
Share common virtual address space (inc. stacks) Each thread has its own thread id (TID)
shared libraries
run-time heap
0
read/write dataThread 1 context: Data registers Condition codes SP1 PC1
Shared code and data
read-only code/data
stack 1
Thread 1 (main thread)
Kernel context: VM structures Descriptor table brk pointer
Thread 2 context: Data registers Condition codes SP2 PC2
stack 2
Thread 2 (peer thread)
Logical View of Threads Threads associated with process form a pool of peers
Unlike processes which form a tree hierarchy
P0
P1
sh sh sh
foo
bar
T1
Process hierarchyThreads associated with process foo
T2T4
T5 T3
shared code, dataand kernel context
Thread Execution
Single Core Processor Simulate concurrency
by time slicing
Multi-Core Processor Can have true
concurrency
Time
Thread A Thread B Thread C Thread A Thread B Thread C
Run 3 threads on 2 cores
Threads vs. Processes How threads and processes are similar
Each has its own logical control flow Each can run concurrently with others (possibly on different cores) Each is context switched
How threads and processes are different Threads share code and some data
Processes (typically) do not Threads are somewhat less expensive than processes
Process control (creating and reaping) is twice as expensive as thread control
Linux numbers:– ~20K cycles to create and reap a process– ~10K cycles (or less) to create and reap a thread
Posix Threads (Pthreads) Interface Pthreads: Standard interface for ~60 functions that
manipulate threads from C programs Creating and reaping threads
pthread_create() pthread_join()
Determining your thread ID pthread_self()
Terminating threads pthread_cancel() pthread_exit() exit() [terminates all threads] , RET [terminates current thread]
Synchronizing access to shared variables pthread_mutex_init pthread_mutex_[un]lock pthread_cond_init pthread_cond_[timed]wait
/* thread routine */void *thread(void *vargp) { printf("Hello, world!\n"); return NULL;}
The Pthreads "hello, world" Program/* * hello.c - Pthreads "hello, world" program */#include "csapp.h"
void *thread(void *vargp);
int main() { pthread_t tid;
Pthread_create(&tid, NULL, thread, NULL); Pthread_join(tid, NULL); exit(0);}
Thread attributes (usually NULL)
Thread arguments(void *p)
return value(void **p)
Execution of Threaded“hello, world”main thread
peer thread
return NULL;main thread waits for peer thread to terminate
exit() terminates
main thread and any peer threads
call Pthread_create()
call Pthread_join()
Pthread_join() returns
printf()
(peer threadterminates)
Pthread_create() returns
Thread-Based Concurrent Echo Serverint main(int argc, char **argv) { int port = atoi(argv[1]); struct sockaddr_in clientaddr; int clientlen=sizeof(clientaddr); pthread_t tid;
int listenfd = Open_listenfd(port); while (1) {
int *connfdp = Malloc(sizeof(int));*connfdp = Accept(listenfd,
(SA *) &clientaddr, &clientlen);Pthread_create(&tid, NULL, echo_thread, connfdp);
}}
Spawn new thread for each client Pass it copy of connection file descriptor Note use of Malloc()!
Without corresponding Free()
Thread-Based Concurrent Server (cont)/* thread routine */void *echo_thread(void *vargp) { int connfd = *((int *)vargp); Pthread_detach(pthread_self()); Free(vargp); echo(connfd); Close(connfd); return NULL;}
Run thread in “detached” mode Runs independently of other threads Reaped when it terminates
Free storage allocated to hold clientfd “Producer-Consumer” model
Threaded Execution Model
Multiple threads within single process Some state between them
File descriptors
Client 1Server
Client 2Server
ListeningServer
Connection Requests
Client 1 data Client 2 data
Potential Form of Unintended Sharing
main thread
peer1
while (1) {int connfd = Accept(listenfd, (SA *) &clientaddr, &clientlen);Pthread_create(&tid, NULL, echo_thread, (void *) &connfd);
}}
connfd
Main thread stack
vargp
Peer1 stack
vargp
Peer2 stackpeer2
connfd = connfd1
connfd = *vargpconnfd = connfd2
connfd = *vargp
Race!
Why would both copies of vargp point to same location?
Could this race occur?
int i;for (i = 0; i < 100; i++) { Pthread_create(&tid, NULL, thread, &i);}
Race Test If no race, then each thread would get different value of i Set of saved values would consist of one copy each of 0 through 99.
Main
void *thread(void *vargp) { int i = *((int *)vargp); Pthread_detach(pthread_self()); save_value(i); return NULL;}
Thread
Experimental Results
The race can really happen!
No Race
Multicore server
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 990
1
2
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 990
2
4
6
8
10
12
14
Single core laptop
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 990
1
2
3
Issues With Thread-Based Servers Must run “detached” to avoid memory leak.
At any point in time, a thread is either joinable or detached. Joinable thread can be reaped and killed by other threads.
must be reaped (with pthread_join) to free memory resources. Detached thread cannot be reaped or killed by other threads.
resources are automatically reaped on termination. Default state is joinable.
use pthread_detach(pthread_self()) to make detached. Must be careful to avoid unintended sharing.
For example, passing pointer to main thread’s stack Pthread_create(&tid, NULL, thread, (void *)&connfd);
All functions called by a thread must be thread-safe Stay tuned
Pros and Cons of Thread-Based Designs
+ Easy to share data structures between threads e.g., logging information, file cache.
+ Threads are more efficient than processes.
– Unintentional sharing can introduce subtle and hard-to-reproduce errors! The ease with which data can be shared is both the greatest strength
and the greatest weakness of threads. Hard to know which data shared & which private Hard to detect by testing
Probability of bad race outcome very low But nonzero!
Future lectures
Event-Based Concurrent Servers Using I/O Multiplexing
Use library functions to construct scheduler within single process
Server maintains set of active connections Array of connfd’s
Repeat: Determine which connections have pending inputs If listenfd has input, then accept connection
Add new connfd to array Service all connfd’s with pending inputs
Details in book
I/O Multiplexed Event Processing
10
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
10
clientfd
7
4
-1
-1
12
5
-1
-1
-1
listenfd = 3
Active Descriptors Pending Inputs
Read
Pros and Cons of I/O Multiplexing
+ 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 concurrency E.g., our example will hang up with partial lines.
– Cannot take advantage of multi-core Single thread of control
Approaches to Concurrency
Processes Hard to share resources: Easy to avoid unintended sharing High overhead in adding/removing clients
Threads Easy to share resources: Perhaps too easy Medium overhead Not much control over scheduling policies Difficult to debug
Event orderings not repeatable I/O Multiplexing
Tedious and low level Total control over scheduling Very low overhead Cannot create as fine grained a level of concurrency Does not make use of multi-core
