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REAL

REAL. An Engineering Approach to Computer Networking. Basics. REAL: globals and simulation engine. thread. UNIX. Threads. Thread = infinite loop for (;;) Share a single address space can read and write all globals but not another thread’s stack

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REAL

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  1. REAL An Engineering Approach to Computer Networking

  2. Basics • REAL: globals and simulation engine thread UNIX

  3. Threads • Thread = infinite loop • for (;;) • Share a single address space • can read and write all globals • but not another thread’s stack • Communication is through sendm(dest, key, packet) and recvm(&dest, &key, &pkt) • Only pointers to packets are exchanged • sender must not free a packet - receivers do this

  4. REAL functions • sendm(dest, key, pkt) • recvm(&dest, &key, &pkt) • blocking • get_node_id() • runtime() • struct timeval { long tv_sec, long tv_usec} • abs_advance() • make_pkt() • pkt type is DATA • make_float() • pr_error()

  5. A simple node function #include "../kernel/real.h" ecn_dummy() { PKT_PTR pkt; int node, num_pkts_sent = 0; ident destn, sender, sink; long key; timev now; int seq_no = 0; node = get_node_id(); /* find out local id */ sink = assigned_sink[node]; /* find out sink (from .l file ) */ abs_advance(node_start_time[node]); /* advance time to start time */ now = runtime();

  6. Contd. /* send the request packet */ if(node is 1) { make_pkt(pkt); pkt->dest = sink; /* use num_pkts field to store command (0 or 1) */ pkt->data[0] = num_pkts[node]; sendm(sink, 0, pkt); printf("Node 1 sent request packet\n"); }

  7. Contd. for (ever) { sender = recvm(&destn, &key, &pkt); now = runtime(); switch (pkt->type){ case ACK: /* just free acks for now */ free(pkt); break; case INT: /* when the line goes free */ free(pkt); break; case DATA: /* convert the request packet to reply packet and send * it right back */ pkt->type = ACK; pkt->dest = pkt->source; pkt->source= node; sendm(pkt->dest, 0, pkt); break;

  8. How it works • Simulator simulates a topology given to it in an input file • described in NetLanguage • parameters, nodes, edges • defaults • node function = C function implementing a thread is part of node description • Simulator parses the file and starts the threads • Same C function may run at multiple nodes • At each instant, run the thread which has the earliest packet to serve • Preemption only during recvm() • Details in sim/src

  9. Things to watch out for • Globals can be modified by any thread • need to synchronize access • For non-shared global, index by node ID (from get_node_id()) • Debugging: can land up at the same statement many times! • Threads are serial, but programming abstraction is that they run in parallel • I/O by a thread blocks all others • Nodes exchange pointers to packets

  10. Adding a new function • Write the function • Modify sim/sources/makefile and sim/makefile

  11. Adding a new parameter • Change lang/lang.lex to add the name • Change lang/lang.yacc to read the value into a variable • Change kernel/parameters.h to make it a global • Recompile

  12. Useful files • sim/kernel/types.h • the packet type • sim/ kernel/config.h • configuration parameters • sim/ kernel/parameters.h • globals • sim/ lang/lang.lex; sim/ lang/lang.yacc • NetLanguage scanner and parser • src/defs.h • additions to C syntax • src/nest.a • simulation engine

  13. RealEdit • Java based GUI for creating input files and simulating them on a server • http://real.cs.cornell.edu:80

  14. More information • http://www.cs.cornell.edu/home/skeshav/real/overview.html • Also contains online manual pages • Searchable archive of REAL mailing list is at http://minnie.cs.odfa.oz.au/cgi-bin/real.cgi

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