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Advanced Operating Systems Lecture 5: Interprocess Communication University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Univ. of Tehran Distributed Operating Systems 1 How Processes communicate Improving process communication to make systems more modular, scalable and flexable. References “improving IPC by Kernel Design”, by Jochen Liedtke. Univ. of Tehran Distributed Operating Systems 2 Outline Why IPC? IPC mechanisms Pipe, Socket, Shared memory Message RPC (Remote Procedure call) Universal address space. How improve IPC through kernel. Univ. of Tehran Distributed Operating Systems 3 IPC Fundamentals What is IPC? Mechanisms to transfer data between processes To share resources Client/server paradigms Inherently distributed applications … Other good software engineering reasons Why is it needed? Not all important procedures can be easily built in a single process To make systems modular, flexible and scalable. Critical for micro-kernel OS and distributed systems. Univ. of Tehran Distributed Operating Systems 4 IPC from the OS Point of View Private address space Process A OS address space Private address space Process B •Each process has a private address space •Normally, no process can write to another process’s space •How to get important data from process A to process B? Univ. of Tehran Distributed Operating Systems 5 Solutions to IPC Fundamentally, two options 1. Support some form of shared address space Shared memory 2. Use OS mechanisms to transport data from one address space to another Shared memory Files, messages, pipes, RPC OS has job of setting it up and perhaps synchronizing But not transporting data Messages, etc OS involved in every IPC step OS transports data Univ. of Tehran Distributed Operating Systems 6 Ideal IPC Characteristics Fast Easy to use Well defined synchronization model Versatile (Flexible) Easy to implement Works remotely Univ. of Tehran Distributed Operating Systems 7 IPC and Synchronization Synchronization is a major concern for IPC Allowing sender to indicate when data is transmitted Allowing receiver to know when data is ready Allowing both to know when more IPC is possible Univ. of Tehran Distributed Operating Systems 8 IPC and Connections IPC mechanisms can be connectionless or require connection Connectionless IPC mechanisms require no preliminary setup Connection IPC mechanisms require negotiation and setup before data flows Sometimes much is concealed from user, though Univ. of Tehran Distributed Operating Systems 9 Connectionless IPC Data simply flows Typically, no permanent data structures shared in OS by sender and receiver + Good for quick, short communication + less long-term OS overhead - Less efficient for large, frequent communications - Each communication takes more OS resources per byte Univ. of Tehran Distributed Operating Systems 10 Connection-Oriented IPC Sender and receiver pre-arrange IPC delivery details OS typically saves IPC-related information for them Advantages/disadvantages pretty much the opposites of connectionless IPC Univ. of Tehran Distributed Operating Systems 11 IPC Through File System Sender writes to a file Receiver reads from it But when does the receiver do the read? Often synchronized with file locking or lock files Special types of files can make file-based IPC easier Process A Univ. of Tehran Data Distributed Operating Systems Process B 12 Message-Based IPC Sender formats data into a formal message With some form of address for receiver OS delivers message to receiver’s message input queue (might signal too) Receiver (when ready) reads a message from the queue Sender might or might not block Univ. of Tehran Distributed Operating Systems 13 Message-Based IPC Diagram OS Process A Univ. of Tehran Data sent from A to B B’s message queue Distributed Operating Systems Process B 14 Procedure Call IPC Interprocess communication uses same procedure call interface as intraprocess Data passed as parameters Information returned via return values Complicated since destination procedure is in a different address space Generally, calling procedure blocks till call returns Univ. of Tehran Distributed Operating Systems 15 Procedure IPC Diagram main () { . call(); . . . Data as parameters Data as return values . . . server(); . . . } Process B Process A Univ. of Tehran Distributed Operating Systems 16 Shared Memory IPC Processes share a common piece of memory either physically or virtually Communications via normal reads/writes May need semaphores or locks In or associated with the shared memory main () { . x = 10 . . . Univ. of Tehran } write variable x x: 10 read variable x . . . print(x); . . . Process B Process A Distributed Operating Systems 17 Synchronizing in IPC How do sending and receiving process synchronize their communications? Many possibilities, based on which process block when Blocking Send, Blocking Receive, Often called message rendezvous Non-Blocking Send, Blocking Receive, Essentially, receiver is message-driven Non-Blocking Send, Non-Blocking Receive, polling or interrupt for receiver Univ. of Tehran Distributed Operating Systems 18 Addressing in IPC How does the sender specify where the data goes? The mechanism makes it explicit (shared memory or RPC) Or, there are options: Direct Addressing Sender specifies name of the receiving process using some form of unique process name Receiver can either specify name of expected sender or take stuff from anyone Indirect Addressing: Data is sent to queues, mailboxes, or some other form of shared data structure: Much more flexible than direct addressing Univ. of Tehran Distributed Operating Systems 19 Duality in IPC Mechanisms Many aspects of IPC mechanisms are duals of each other Which implies that these mechanisms have the same power First recognized in context of messages vs. procedure calls At least, IPC mechanisms can be simulated by each other Depends on model of computation And on philosophy of user In particular cases, hardware or existing software may make one perform better Univ. of Tehran Distributed Operating Systems 20 UNIX IPC Mechanisms Different versions of UNIX introduced different IPC mechanisms Pipes Message queues Semaphores Shared memory Sockets RPC Univ. of Tehran Distributed Operating Systems 21 Pipes Only IPC mechanism in early UNIX systems (other than files) Uni-directional Unformatted Uninterpreted Interprocess byte streams Accessed in file-like way Univ. of Tehran Distributed Operating Systems 22 Pipe Details One process feeds bytes into pipe A second process reads the bytes from it Potentially blocking communication mechanism Requires close cooperation between processes to set up Named pipes allow more flexibility Univ. of Tehran Distributed Operating Systems 23 Pipes and Blocking Writing more bytes than pipe capacity blocks the sender Reading bytes when none are available blocks the receiver Until the receiver reads some of them Until the sender writes some Single pipe can’t cause deadlock Univ. of Tehran Distributed Operating Systems 24 UNIX Message Queues Introduced in System V Release 3 UNIX Like pipes, but data organized into messages Message component include: Type identifier Length Data Univ. of Tehran Distributed Operating Systems 25 Semaphores Also introduced in System V Release 3 UNIX Mostly for synchronization only Since they only communicate one bit of information Often used in conjunction with shared memory Univ. of Tehran Distributed Operating Systems 26 UNIX Shared Memory Also introduced in System V Release 3 Allows two or more processes to share some memory segments With some control over read/write permissions Often used to implement threads packages for UNIX Univ. of Tehran Distributed Operating Systems 27 Sockets Introduced in 4.3 BSD A socket is an IPC channel with generated endpoints Great flexibility in it characteristics Intended as building block for communication Endpoints established by the source and destination processes Univ. of Tehran Distributed Operating Systems 28 UNIX Remote Procedure Calls Procedure calls from one address space to another On the same or different machines Requires cooperation from both processes In UNIX, often built on sockets Often used in client/server computing Univ. of Tehran Distributed Operating Systems 29 Problems with Shared Memory Synchronization Protection Pointers Univ. of Tehran Distributed Operating Systems 30 Synchronization Shared memory itself does not provide synchronization of communications Except at the single-word level Typically, some other synchronization mechanism is used E.g., semaphore in UNIX Events, semaphores, or hardware locks in Windows NT Univ. of Tehran Distributed Operating Systems 31 Protection Who can access a segment? And in what ways? UNIX allows some read/write controls Windows NT has general security monitoring based on the object-status of shared memory Univ. of Tehran Distributed Operating Systems 32 A Troublesome Pointer a: __ z: __ b: __ y: 20 x: 10 w: 5 Process B Process A Univ. of Tehran Distributed Operating Systems 33 How share pointers? Copy-time translation Reference-time translation Encode pointers in shared memory segment as pointer surrogates, typically, as offsets into some other segment in separate contexts. So each sharer can have its own copy of what is pointed to Slow, pointers in two formats Pointer Swizzling Locate each pointer within old version of the data Then translate pointers are required Requires both sides to traverse entire structure Not really feasible for shared memory cache results in the memory location, only first reference is expensive But each sharer must have his own copy Must “unswizzle” pointers to transfer data outside of local context Stale swizzled pointers can cause problems All involve somehow translating pointers at some point before they are used Univ. of Tehran Distributed Operating Systems 34 Shared Memory in a Wide Virtual Address Space When virtual memory was created, 16 or 32 bit addresses were available Reasonable size for one process But maybe not for all processes on a machine And certainly not for all processes ever on a machine. How a global address space, e.x 64 bit address space. (Opal system) Univ. of Tehran Distributed Operating Systems 35 Implications of Single Shared Address Space IPC is trivial Shared memory, RPC Separation of concepts of address space and protection domain Uniform address space Univ. of Tehran Distributed Operating Systems 36 Address Space and Protection Domain A process has a protection domain The data that cannot be touched by other processes And an address space The addresses it can generate and access In standard systems, these concepts are merged Univ. of Tehran Distributed Operating Systems 37 Uniform-Addressing Allows Easy Sharing Any process can issue any address So any data can be shared All that’s required is changing protection to permit desired sharing Suggests programming methods that make wider use of sharing Univ. of Tehran Distributed Operating Systems 38 Windows NT IPC Inter-thread communications Local procedure calls Within a single process Between processes on same machine Shared memory Univ. of Tehran Distributed Operating Systems 39 Windows NT and Client/Server Computing Windows NT strongly supports the client/server model of computing Various OS services are built as servers, rather than part of the kernel Windows NT needs facilities to support client/server operations Which guide users to building client/server solution Univ. of Tehran Distributed Operating Systems 40 Client/Server Computing and RPC In client/server computing, clients request services from servers Service can be requested in many ways But RPC is a typical way Windows NT uses a specialized service for single machine RPC Univ. of Tehran Distributed Operating Systems 41 Local Procedure Call (LPC) Similar in many ways to RPC, but optimized to only work on a single machine Primarily used to communicate with protected subsystems It also provides a true RPC facility Application calls routine in an application programming interface Which is usually in a dynamically linked library Which sends a message to the server through a messaging mechanism Univ. of Tehran Distributed Operating Systems 42 Windows NT LPC Messaging Mechanisms Messages between port objects Message pointers into shared memory Using dedicated shared memory segments Univ. of Tehran Distributed Operating Systems 43 Port Objects Windows NT is generally object-oriented Port objects support communications Two types: Connection ports Used to establish connections between clients and servers Named, so they can be located Only used to set up communication ports Communication ports Used to actually pass data Created in pairs, between given client and given server Private to those two processes Destroyed when communications end Univ. of Tehran Distributed Operating Systems 44 Windows NT Port Example Connection port Server process Client process Univ. of Tehran Distributed Operating Systems 45 Windows NT Port Example Connection port Server process Client process Univ. of Tehran Distributed Operating Systems 46 Windows NT Port Example Connection port Communication ports Server process Client process Univ. of Tehran Distributed Operating Systems 47 Windows NT Port Example Connection port Send request Communication ports Server process Client process Univ. of Tehran Distributed Operating Systems 48 Message Passing through Port Object Message Queues One of three methods in Windows NT to pass messages 1. Client submits message to OS 2. OS copies to receiver’s queue 3. Receiver copies from queue to its own address space Univ. of Tehran Distributed Operating Systems 49 Characteristics of Message Passing via Queues Two message copies required Fixed-sized, fairly short message Port objects stored in system memory ~256 bytes So always accessible to OS Fixed number of entries in message queue Univ. of Tehran Distributed Operating Systems 50 Message Passing Through Shared Memory Used for messages larger than 256 bytes Client must create section object Shared memory segment of arbitrary size Message goes into the section Pointer to message sent to receiver’s queue Univ. of Tehran Distributed Operating Systems 51 Setting up Section Objects Pre-arranged through OS calls Using virtual memory to map segment into both sender and receiver’s address space If replies are large, need another segment for the receiver to store responses OS doesn’t format section objects Univ. of Tehran Distributed Operating Systems 52 Characteristics of Message Passing via Shared Memory Capable of handling arbitrarily large transfers Sender and receiver can share a single copy of data i.e., data copied only once Requires pre-arrangement for section object Univ. of Tehran Distributed Operating Systems 53 Server Handling of Requests Windows NT servers expect requests from multiple clients Typically, they have multiple threads to handle requests Must be sufficiently general to handle many different ports and section objects Univ. of Tehran Distributed Operating Systems 54 Message Passing Through Quick LPC Third way to pass messages in Windows NT Used exclusively with Win32 subsystem Like shared memory, but with a key difference Dedicated resources Univ. of Tehran Distributed Operating Systems 55 Use of Dedicated Resources in Quick LPC To avoid overhead of copying Notification messages to port queue And thread switching overhead Client sets up dedicated server thread only for its use Also dedicated 64KB section object And event pair object for synchronization Univ. of Tehran Distributed Operating Systems 56 Characteristics of Message Passing via Quick LPC Transfers of limited size Very quick Minimal copying of anything Wasteful of OS resources Univ. of Tehran Distributed Operating Systems 57 Shared Memory in Windows NT Similar in most ways to other shared memory services Windows NT runs on multiprocessors, which complicates things Done through virtual memory Univ. of Tehran Distributed Operating Systems 58 Section Views Given process might not want to waste lots of its address space on big sections So a process can have a view into a shared memory section Different processes can have different views of same section Or multiple views for single process Univ. of Tehran Distributed Operating Systems 59 Shared Memory View Diagram view 1 view 2 Section view 3 Process B Process A Physical memory Univ. of Tehran Distributed Operating Systems 60 Improving IPC Experiments based on L3 operating system which is micro-kernel system. A null IPC Thread A (user mode) Kernel Thread B (user mod) Load ID of B Set Msg length to 0 Call Kernel Insepct received msg Access Thread B Switch Stack pointer Switch Address Space Load id of A Return to user On intel 486, 50 Mhz, time for null IPC is 172 cycles (3.5 µ) Entering and Leaving kernel 2 µ Our aim was 350 or 7 µ and achieved 250 or 5 µ Univ. of Tehran Distributed Operating Systems 61 Improving IPC System calls: simply entering and leaving kernel mode cost 2 µ Reduce the cost of system call. For one msg and reply we need 4 kernel crossing. Reduce it by implementing Reply and receive. Less Copies: User -> kernel -> user then, at least two copies. Copying n bytes cost 20 + 0.75n cycles plus additional TLB misses In L3 kernel determine the destination address and maps it into communication window and directly copy msg there. Univ. of Tehran Distributed Operating Systems 62 Improving IPC Combine Sequences of send operations into one message Reduce the cost of system call. Thread Control Blocks (TCB) are treated as objects and kept in a virtual array. Use 64 bit theard id to find TCB in the virtual array. Queues in Kernel implemented as double linked list for wakeup, etc. Lazy scheduling: instead of entering queue/delete change thread status. Univ. of Tehran Distributed Operating Systems 63 Improving IPC Use registers for IPC For passing parameter. Reduce cache misses by concentrating frequently used data in few cache lines. Put IPC related kernel codes in one page to reduce TLB misses Univ. of Tehran Distributed Operating Systems 64 Next Lecture Scheduling References Surplus Fair Scheduling: A ProportionalShare CPU Scheduling Algorithm for Symmetric Multiprocessors Scheduler Activations: Effective Kernel Support for User-Level Management of Parallelism", Univ. of Tehran Distributed Operating Systems 65