The NCSA (National Center for Supercomputing Applications) has determined that a WWW (World Wide Web) surfer waits an average of 5 minutes between page requests. During that time, many other things can be happening. If you're on a slow connection (e.g., a 14.4-Kbps modem), for instance, you might be transmitting the file. Or you might be reading the page or linking to pages on another site. More commonly, you're turning through cached pages that you've already seen. Even if we assume twice that access rate, a single real-world client requests a file every 2-1/2 minutes, which translates into a processing rate of 0.4 transactions per minute.
A typical Alpha system can process many requests from many typical WWW user
s. But unless all your users are connected over a high-speed network (e.g., fast Ethernet or T3), your network connection becomes saturated long before your server does. An underloaded server is able to absorb new tasks without suffering a significant amount of decay in its transaction-processing rate. Such a server is able to, for example, transfer 100 files almost as quickly as it can transfer 50 files. On an unloaded server, the transaction-processing rate increases almost as quickly as the load does. When a server becomes saturated, however, doubling the load will double the amount of time it needs to perform its task. As the load gets larger, the server becomes supersaturated, and the transaction-processing rate drops sharply.
The figure
"Balance of Power: Network/Processor Loads"
shows the effective network utilization and processor time of the S.A.G. SFT Alpha server on three of our tests. A heavy FTP load simultaneously stresses the CPU and the network link. The HTTP f
igures indicate a much lower network load, although the processor-utilization rate remains relatively high.
Another factor to consider is the rate at which clients request information; a high transaction rate (e.g., HTTP 32) indicates that clients will spend much time connecting and disconnecting for data transfer operations. A low transaction rate (e.g., FTP 16) indicates that the server will spend more time transferring data and less time opening and closing connections.
Under the FTP test, the processor and the network have a balanced load. In the HTTP test, the processor is loaded more heavily than the network, indicating that opening and closing connections represents a significant amount of the work.
The WAIS test represents an intermediate point. During times of almost no disk activity (e.g., when the WAIS database is cached), significant processor utilization, or low network use, the chief constraint is the processor, and its time is spent in WAIS searching and network-connection
processing. So, for WAIS and HTTP activity, processing power becomes an important criterion when selecting an Internet server.
illustration_link (13 Kbytes)
Under heavy FTP activity, the server's work is balanced between network-servicing and processing chores. But as the server tends to HTTP and WAIS activity, the processor becomes the chief performance constraint.
A Heavy Load To Carry
