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Thread-specific Storage (TSS)Thread-specific Storage (TSS)
• Storage/space (a variable) per thread.– A variable is associated with a thread.– The per-thread variable is never touched by other
threads
• java.lang.ThreadLocal
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Imagine this Scenario…Imagine this Scenario…
• Different threads– generate different data– store them in a result holder– read them from the result holder.
• Need to protect the result holder from threads.– A read-write lock to be implemented in the holder.
Resultholder
Thread 1
…
Thread 1’sresult
Thread 2’sresult
Thread 2
Thread 3
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When does a TSS Work?When does a TSS Work?
• If each element is paired with a thread and accessed only by the thread…– TSS works well.
• Easier-to-read code• Safer code
Resultholder
Thread 1
…
Thread 1’sresult
Thread 2’sresult
Thread 2
Thread 3
<--TSS
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TSLog.javaTSLog.java
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• Locking is encapsulated in ThreadLocal• No ways to access other threads’ TSS.
• No code to acquire and release a lock in TSLog– Shorter (easier-to-understand) code– No worry on race conditions and deadlock
TSLog
Thread 1
Thread 2
Thread 5
<-- TSS…setResult()/printResult()
IntegerThread name/idthread 1thread 2
… …
Thread 1’sresult
Thread 2’sresult
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An Architectural View of an OS An Architectural View of an OS
Inter-processcommunication
Process Scheduling
ProcessControl
Subsystem
File Subsystem
Device Drivers
Hardware Control
System Call Interface
Hardware levelHardware
Kernel level
User programs Libraries User level
Memory Mgt Subsystem
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Host/machine
Inter-Process CommunicationInter-Process Communication
• Same system calls/APIs program for both types of communication
Process(e.g. JVM,
web browser,DB client)
Process(e.g. JVM, web server,
DB)
Host/machine
Process(e.g. JVM,
web browser,DB client)
Process(e.g. JVM, web server,
DB)
Host/machine
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Protocol StackProtocol Stack
• Application layer– e.g., HTTP, POP, SMTP, SSH
• Session layer– e.g., SSL
• Transport layer– e.g., TCP, UDP
• Network layer– e.g., IP
• MAC (data link) and physical layer– e.g., Ethernet, FDDI, ATM, PPP
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Network ProtocolsNetwork Protocols
• A protocol allows multiple processes to talk with each other in an unambiguous way.
• Each protocol defines…– Communication primitives/commands– Pairs of request and response messages– Message format
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An Example: HTTPAn Example: HTTP• GET /index.html HTTP/1.0
• HTTP/1.0 200 OKServer: Apache…..Date: Wed, 11 April 2007 HH:MM:SS GMTContent-Type: text/html:charset=ISO…Set-cookie: XXXXX=ZZZZZ
<html><body><title>Welcome to my home page!</title>…
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Another Example: POPAnother Example: POP• USER jxs <-- client• +OK Password required for jxs <-- server• PASS mypasswd• +OK jxs has 2 messages (300 octets)• STAT• +OK 2 300• RETR 1• +OK 200 octets
email text included here• DELE 1• +OK message 1 deleted• …• QUIT• +OK POP server signing off
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Network-related System CallsNetwork-related System Calls• Socket interface
– A part of OS system call interface– A set of functions specific to networking– implements the transport layer
– socket()• Creates a socket
– bind()• Names a created socket
– connect()• Sends out a connection request
– listen()• Waits for connection requests
– accept()• Accepts a connection request
– select(), read(), write(), close()
programmer
program
Socket interface
sock
et()
conn
ect(
)
writ
e()
…
sock
et()
liste
n()
read
()
…
Network connection
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Java Networking APIJava Networking API
• A set of classes/methods in the java.net package– follows the the socket interface’s design.– implements TCP and UDP. – glues Java programs to the socket interface– makes it easier to implement network systems than
using socket system calls directly
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SocketSocket• Socket
– A communication channel to transmit TCP/UDP packets between processes
• on the same machine or on different machines – Google Desktop
» inter-process comm on the same machine– Remote server access (e.g., HTTP and POP)
» Inter-process comm on different machines.
Client process Server process
socket()
creates
socket()
creates
socket socket
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Client process Server process
connect() accept()
Connects
Client process Server process
write()read()
read()write()
Talk with each other(A TCP connection is full duplex.)
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File DescriptorFile Descriptor
User-level process
open(“foo.txt”)
creates
• How does a process reference and access its sockets?– Using a file descriptor
• File descriptors– Used to reference various data structures in the kernel
• e.g., files, directories, character devices, sockets, pipes, etc.
fd=10
foo.txtfinds andopens it
Kernel
fd
File system
pointer10
fd table
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fd=10
foo.txt
Kernel
fd File systempointer11
fd table
Client process
socket()creates
Socketfd=11
TC
P
Protocolstack
NICDevicedriver
Server process
socket()
creates
Socketfd=30
TC
P
Protocol stack
NICDevicedriver
Kernel
10
fd pointer30
fd table
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Process
fd=0: standard input (file)
Default file descriptors (special files)for every process
fd=1: standard output (file)
fd=2: standard error output (file)
Shell Java program
InputStream in = new InputStreamReader(System.in);in.read()
stdin: fd=0> Hi Java
System.out.println( “Hi shell” );
System.err.println(“I have a problem”);
stdout: fd=1
stderr: fd=2
Hi shell
I have a problem.
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IP and Port NumberIP and Port Number• How does a process identify and access a
remote process? – 10 to 100+ processes on a machine– A huge number of machines on the network (the
Internet)
• How about using process IDs? – Not good
• The same program uses different pids when running at different times.
– If a program is rebooted, it uses a different pid than the one it was using before the reboot.
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• A combination of an IP address and port number– An IP address uniquely identifies a particular
machine in the network• 158.121.105.85 (www.cs.umb.edu)
– A port number uniquely identifies a particular process on a machine.
• A program can use the same port number at different times.
– e.g., before and after a reboot.– http://www.cs.umb.edu:80
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Java SocketJava Socket• Server• ServerSocket serverSocket =
new ServerSocket( 9000 );
Socket socket = serverSocket.accept();
Scanner scanner = new Scanner( socket.getInputStream);
PrintWriter writer = new PrintWriter( socket.getOutputStream);
• Client• Socket socket =
new Socket( “localhost”, 9000);
Scanner scanner = new Scanner( socket.getInputStream);
PrintWriter writer = new PrintWriter( socket.getOutputStream);
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Sample CodeSample Code• Networked bank account
– A bank account at the server side– A client accesses the bank account through via TCP socket
• Simple Banking Protocol (SBP)– Commands from a client
• BALANCE– Get the current balance. The current balance is returned.
• DEPOSIT X– Deposit amount X. The current (updated) balance is returned.
• WITHDRAW X– Withdraw amount X. The current (updated) balance is returned.
• QUIT– Close a TCP connection
