Q. Compare the utilization of networking sockets with the use of shared memory as a mechanism for communicating data between processes on a single computer. What are the merits of each method? When might each one be preferred?
Answer: Utilizing network sockets rather than shared memory for local communication has a number of advantages. The major advantage is that the socket programming interface features a rich set of synchronization features. A process can effortlessly determine when new data has arrived on a socket connection how much data is present and who sent it. Processes are able to block until new data arrives on a socket or they can request that a signal be delivered when data arrives. A socket as well manages separate connections. A process with a socket open for receive is able to accept multiple connections to that socket and will be told when new processes try to connect or when old processes drop their connections.
Shared memory provided none of these features. There is no means for a process to determine whether another process has delivered or else changed data in shared memory other than by going to look at the contents of that memory. It is unfeasible for a process to block and request a wakeup when shared memory is delivered as well as there is no standard mechanism for other processes to establish a shared memory link to an existing process.
Nevertheless shared memory has the advantage that it is extremely much faster than socket communications in many cases. When data is transmit over a socket it is typically copied from memory to memory multiple times. Shared memory updates necessitate no data copies- if one process updates a data structure in shared memory that update is instantaneously visible to all other processes sharing that memory. Sending or else receiving data over a socket requires that a kernel system service call be made to initiate the transfer but shared memory communication can be performed completely in user mode with no transfer of control required.
Socket communication is usually preferred when connection management is significant or when there is a requirement to synchronize the sender and receiver. For instance server processes will typically establish a listening socket to which clients can connect when they want to use that service. Once the socket is conventional individual requests are as well sent using the socket as a result that the server can easily determine when a new request arrives and who it arrived from. In some cases nevertheless shared memory is preferred. Shared memory is frequently a better solution when either large amounts of data are to be transferred or when two processes require random access to a large common data set. In this case nevertheless the communicating processes may still require an extra mechanism in addition to shared memory to achieve synchronization among them. The X Window System a graphical display environment for UNIX is a good illustration of this most graphic requests are sent over sockets however shared memory is offered as an additional transport in special cases where large bitmaps are to be displayed on the screen. In this situation a request to display the bitmap will still be sent over the socket however the bulk data of the bitmap itself will be sent via shared memory.