Operating Systems and Networks Recitation Session 4

Transcription

1 Systems Group Department of Computer Science ETH Zürich Operating Systems and Networks Recitation Session 4 25/03/2011 Simon Gerber Group 5

2 Checksum 5/10/13 2

3 1 s complement sum st value nd value Produced a carry-out + \ > rd value No carry to swing around th value Produced a carry-out + \ > 's complement sum Example from

4 Queuing Theory (see Lecture 4 slides for more details) 5/10/13 4

5 A queue JOBS SERVICE - Very useful model for many computer systems

6 Interarrival time τ1 τ2 τ3 τ4 τ5 Arrival Arrival Arrival Arrival Arrival Arrival TIME The interarrival times are assumed to form a sequence of Independent and Identically Distributed (IID) random variables Common assumption is a Poisson distribution

7 Mean arrival rate Mean interarrival time = E[τ] Mean arrival rate: λ = 1 / E[τ] λ is not a random variable! Examples; a single client submits a query every 200 ms, then λ is 5 queries/second 10 clients submit a query each every 500 ms, then λ is 20 queries/second These are the queries submitted to the system

8 Service time per job The time it takes to process a job (only the time it takes to process it, not including the time it has been waiting in the queue) = s Mean service rate: µ = 1/E[s] µ is not a random variable! Example: Printer takes on average 20 seconds per job, then µ = 0.05 jobs/second = 3 jobs/minute A 1kbit message is send over a 1mbps link, then µ = 1000 jobs/second = 60,000 jobs / minute

9 Utilization Utilization: probability that there is one or more jobs in the system U i = X i S i Throughput Mean Service Time The utilization law is simply based on the observation that the service time indicates how long the server needs to complete a job. Hence, the utilization of the server is the number of jobs completed multiplied the time that it takes to complete each one of them

10 Queues: The M/M/1 model Memoryless distribution for arrival and service Single server (state 0) Infinite buffers and FCFS Mean arrival rate: λ Mean service rate: µ λ λ λ λ λ i µ µ µ µ µ

11 M/M/1 behavior ρn is the probability that a job is in a state n (Markov Process) In M/M/1, if a job is not in p0(the originator), then it is in the system, hence the following equation for the utilization of the system: U = 1 p0 = ρ

12 M/M/1 behavior The mean number of jobs E[n] in the system is E[ n] = n p = n(1 ρ) ρ n = n 1 n= 1 n= 1 Applying Little s Law we get the response time E[w] 1/ = 1 µ ρ ρ ρ

13 TCP Congestion Control

14 4 TCP congestion control Probe for usable bandwidth ideally: transmit as fast as possible without loss increase rate until loss (congestion) loss: decrease rate then begin probing (increasing) again Congestion window In bytes! Keeps track of current sending rate NOT the same as the receiver window! Actual window used is minimum of the two.

15 5 TCP Slow start Slow start algorithm Host A Host B initialize: CWND = 1 for (each segment ACKed) CWND++ until (loss event OR CWND > threshold) RTT one segment two segments four segments exponential increase (per RTT) in window size (not so slow!) loss event: timeout (TCP Tahoe) and/or three duplicate ACKs (TCP Reno) time

16 TCP Slow start Limitation: TCP supports windows sizes of up to 64 KB TCP Segment uses 16-bits for window sizes What if we could extend it to 32-bits? Assignment 4b

17 Assignment 2 Post-Discussion

18 byte in Java (1/2) signed ( ) byte b = 0xFF; // kompiliert so nicht int ib = (int)b; System.out.println(ib); Resultat: -1

19 byte in Java (2/2) InputStream in; byte[] val = new byte[4]; in.read(val, 0, 4); return ((int)(val[0]) (int)(val[1]<<8) (int)(val[2]<<16) (int)(val[3]<<24)) Falls byte[0] == 0xFF Returnwert == -1.

20 Error-Checks für syscalls Nötig um zu erkennen ob Port schon belegt, etc. Gut für Codequalität. Datei auslesen: fgets() maximal eine Zeile fread() vgl Übungsslides von Woche 2.

21 Receiving 4byte integers in C static int recv_uint32(const int csock, uint8_t *buffer, int readlen, uint32_t *datalen) { int retval = recv(csock, buffer + readlen, 4 - readlen, 0); if (retval < 0) { fprintf(stderr, "Error - could not read the data length\n"); return retval; } readlen += retval; if (readlen == 4) { *datalen = buffer[3] << 24; *datalen = buffer[2] << 16; *datalen = buffer[1] << 8; *datalen = buffer[0]; printf("received %u bytes, representing value %i\n", 4, *datalen); } return readlen; } recv_uint32(socket, int32_bytes, bytes_read, &datalen);