Guideline for building and setting-up a win32x64 PC computing system running NT-family Microsoft OS with PC GAMESS 6.x. by Serge V. Kovrigin, IPMS & Kiev National University, ©2005 Table of Contents: 1. Conventional vs. direct methods: tasks, where usage of conventional methods still reasonable. 2. Building a PC for PC GAMESS conventional calculations: consideration of general aspects and desires. 3. OS and PC GAMESS tuning procedure - achieving best performance. *This how-to doesn’t claim to be an official and/or finished paper, neither has it come from the authors of GAMESS or PC GAMESS software; hereby, you may consider these materials as a set of recommendations, obtained as a result of usage of PC GAMESS by experienced users. This experience now is to your attention. Why waste time for reading this? Authors of this how-to achieved almost 100% efficiency (means CPU time/wall-clock time ratio) and three times as short calculation times compared to classical set-ups. 1. Conventional vs. direct methods: tasks, where their usage still reasonable. PC GAMESS is being continuously developed and upgraded. As a result, in the last 6.5 build direct SCF code is almost as fast as conventional SCF code for basis-sets of medium complexity, and even faster in certain situations. Undoubtedly, future belongs to direct methods, due to their excellent scalability with parallel computing, higher reliability as a consequence of avoiding using HDD operations in most of the time and for same reason, lower costs for building a node in a cluster or workstation. However, at present time there are a lot of calculations, which can be performed much faster using conventional methods. Typically, all these calculations are similar in several aspects: these are big systems (more than a thousand of base functions), with basis set used up to 6-31G**. Benefit from using techniques described below should be noticeable for any disk-intensive tasks, i.e. MP2, but it is only estimation and was not completely tested. The root of all problems. To tell it simpler, with conventional methods the more 2e integrals needed for a specific task, the wider bandwidth of HD-storage system should be. With increasing 2e integrals quantity not only storage capacity demands rapidly grows, but a number of integrals needed to be read/written per second increases. At some point, HDD storage system just cannot provide necessary throughput for PC GAMESS, and CPU has to wait for information to be delivered. This situation can be easily observed with a Task Manager on a P4-3GHz class PC running a 6- 31G basis set job of about 800 basis functions in size, when CPU load drops to 60%. On the other hand, PC GAMESS doesn’t operate HDD directly (as almost any other program) but through OS. With increasing number of read/write requests OS overload grows and more CPU time is distributed to serve those requests. As a rule, OS use its own cache to speed- up read/write operations (this referred to as ‘read-ahead’ and ‘delayed-write’ techniques), which
in real slows down disk operations instead of speeding them up. This happens primarily because
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