Buffer space is created and managed in database systems in order to reduce accesses to the I/O devices for database information. In systems using virtual memory any increase in the buffer space may be accompanied by an increase in paging. The effects of these factors on system performance are quantified where system performance is a function of page faults and database accesses to I/O devices. This phenomenon is examined through the analysis of empirical data gathered in a multifactor experiment. The factors considered are memory size, size of buffer space, memory replacement algorithm, and buffer management algorithm. The improvement of system performance through an increase in the size of the buffer space is demonstrated. It is also shown that for certain values of the other factors an increase in the size of the buffer space can cause performance to deteriorate.

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Performance modeling and analysis of computer systems is often ignored during the project design phcse in favor of other techniques collectively known as structured design or software engineering. We describe benefits that can result from including performance analvsis an an integral part of the design process. Several different goals, timeframcs,ancl roles played by performance analysis during system design arc illustrated by three case, studies of current proJects at Los Alnmos.

The calculation of the fl exural strength of concrete T-beams has been extensively discussed in recent issues of the PCI JOURNAL. The debate centers on when T-beam behavior is assumed to begin. The AASHTO LRFD Bridge Design Specifi cations (LRFD) maintain that it begins when c (distance from extreme compression fi ber to neutral axis) exceeds the thickness of the fl ange. The AASHTO Standard Specifi cations for Highway Bridges (STD), and other references, contend that it begins when a (depth of equivalent rectangular stress block) exceeds the fl ange thickness. This paper examines the fundamentals of T-beam behavior at nominal fl exural strength, and compares the results of LRFD and STD with more rigorous analyses, including the PCI Bridge Design Manual (PCI BDM) method and a strain compatibility approach using nonlinear concrete compressive stress distributions. For pretensioned T-beams of uniform strength, a method consisting of a mixture of LRFD and STD is investigated. For T-beams with different concrete strengths in the fl ange and web, the PCI BDM method is compared with the nonlinear strain compatibility analysis. High strength concretes (HSC) up to 15,000 psi (103 MPa) are considered. The selection of appropriate φ factors and maximum reinforcement limits is also discussed. Comparisons with previous tests of T-beams are presented, and revisions to the relevant sections of LRFD are proposed.