Network latency

Performance In general, a service performs more poorly than an in-process method call due to network latency and bandwidth constraints. To make things worse, a service may call other services to fulfill its responsibilities—a chain of services. Performance has to be a design consideration throughout the development cycle. 

It may sound like a paradox, but currently, in optimizing for a parallel computer most of the effort should be spent on making sure that the individual node performance is as good as possible. This is a consequence of the power of the individual node compared to the network latency and bandwidth. In short, the current parallel machines are of large grain type. The parallel algorithm used should make every effort to communicate as seldom as possible. For a best performance it often means that a particular calculated value, needed on several nodes, can actually be recalculated more quickly on each node, compared to communicating it to the nodes where it is needed. This is the parallel form of the classic optimization trade-off between memory and CPU cycles. 

By sending software to the system where data reside, as opposed to sending data to the system where software runs, this solution framework obviates the need to transfer large amount of data across the network, and consequently addresses the performance issues associated with the network latency and also improves the end-user productivity. In this solution framework data are captured once and kept at the source of collection, and consequently the system addresses issues associated with data quality, integrity, currency, security, privacy, and intellectual property rights. 

Thus the number of packets on the network scales as the square of the number of hosts. The amount information sent from each machine is very small - typically just a few bytes. But the real-time character of the interaction demands low-network latency. TTiis is discussed below. 

The real-time interactive character of the system pushes the limit of network latency more than bandwidth. For example, a half second may be required for a send and reply in such cases involving geo-synchronous satellites, a factor of 5 too great for reasonable animation. The future may bring low-orbit satellite systems or undersea optical fibre, which with faster gateways, would suffice. 

Hot Backup Site Hot backup site is the most efficient and expensive site. This site provides the access to the database even after the disaster occurs with minimal RTO and RPO. It can also have the largest impact on normal application performance since network latency between the two sites increases response times. Recovery through a hot backup site can take place within a few hours due to the fact that the hot backup contains a replica of the current data in the data center. 

Bartlette, C., D. Headlam, M. Bocko, and G. Velikic. 2006. Effect of network latency on interactive musical performance. Music Perception 24(1) 49-62. 

The REACH system in southern Georgia (United States) and the TEMPiS system in Germany reported decreased latency to rt-PA delivery on a larger scale. REACH system investigators reported 194 acute stroke consultations dehvered via telemedicine. The time from symptom onset to rt-PA delivery decreased from 143 minutes in the first 10 patients treated to 111 minutes in last 20 patients of 30 patients treated with rt-PA, 23% were treated in 90 minutes or less and 60% were treated within 2 hours without any incidence of post-treatment symptomatic intracerebral hemorrhage. In 2004, the second year of the TEMPiS system, 115 patients in telemedicine-networked community hospitals and 110 patients in stroke centers received rt-PA for acute ischemic stroke or TIA. Patients treated at networked community... 

Physical Hardware units, networks Processing and communication capabilities (speed, latency, resources), communication relations, topology, physical containment... 

The former construction is used to create mobile agents that present the carried content simultaneously on many devices after certain point in time. That feature depends, however, on availability of the time agent—if there is latency in the network the content might be presented on some devices a bit later than on the others. 

What is the maximum acceptable delay (latency) on the network ... 

As a first approximation, after the first word of data enters the network, the subsequent words are assumed to immediately follow as a steadily flowing stream of data. Thus, the time required to send data is the sum of the time needed for the first word of data to begin arriving at the destination (the latency, a) and the additional time that elapses until the last word arrives (the number of words times the inverse bandwidth, j8) ... 

The latency can be decomposed into several contributions the latency due to each endpoint in the communication, fendpoint the time needed to pass through each switching element, 4w and the time needed for the data to travel through the network links, funk thus, the latency can be expressed as... 

The endpoint latency, fendpoint, consists of both hardware and software contributions and is on the order of 1 /jls. The contribution due to each switching element, tsw, is on the order of 100 ns. The timk contribution is due to the finite signal speed in each of the nimk cables and is a function of the speed of light and the distance traveled it is on the order of 1-10 ns per link. The total time required to send data through the network depends on the route taken by the data through the network however, when we discuss performance modeling in chapter 5, we will use an idealized machine model where a and are constants that are obtained by measuring the performance of the computer of interest. 

Algorithms and cost analyses for a number of collective communication operations have been discussed in some detail by Grama et al. A comprehensive performance comparison of implementations of MPI on different network interconnects (InfiniBand, Myrinet , and Quadrics ), including both micro-level benchmarks (determination of latency and bandwidth) and application-level benchmarks, has been carried out by Liu et al. A discussion of the optimization of collective communication in MPICH, including performance analyses of many collective operations, has been given by Thakur et al. ... 

The network performance characteristics for a parallel computer may greatly influence the performance that can be obtained with a parallel application. The latency and bandwidth are among the most important performance characteristics because their values determine the communication overhead for a parallel program. Let us consider how to determine these parameters and how to use them in performance modeling. To model the communication time required for a parallel program, one first needs a model for the time required to send a message between two processes. For most purposes, this time can... 

Another network performance characteristic, related to the latency and bandwidth, is the effective bandwidth, which is defined as the message length divided by the total send time... 

The execution time for a parallel algorithm is a function of the number of processes, p, and the problem size, n. Additionally, the execution time depends parametrically on several machine-specific parameters that characterize the communication network and the computation speed the latency and the inverse of the bandwidth, a and p, respectively (both defined in section 5.1),... 

---