Anomalia De Belady

Carrera: Ingeniería Informática y Ciencias de la Computación. | |
Resumen: (Máximo 250 palabras)
La anomalía de Belady es un efecto descubierto y demostrado por el científico de la computación húngaro Laszlo Belady, por el cual es posible tener más fallos de página al aumentar el número de marcos en la memoria física utilizando el método FIFO como algoritmo de reemplazo de páginas ensistemas de gestión de memoria virtual con paginación. Antes de esta fecha, se creía que incrementar el número de marcos físicos siempre llevaría a un descenso del número de fallos de página o, en el peor de los casos, a mantenerlos. Así, pues, antes del descubrimiento de la anomalía de Belady, el algoritmo FIFO era aceptable.
Abstrac: (la traducción al inglés del Resumen)
Belady's anomaly is an effectdiscovered and proved in 1969 by the Hungarian computer scientist Laszlo Belady, by which it is possible to have more page faults by increasing the number of frames in physical memory using the FIFO method as a replacement algorithm Pages in systems management virtual memory paging. Before this date, it was believed that increasing the number of physical frames always lead to a drop in the numberof page faults or, at worst, to keep them. Thus, before the discovery of the anomaly Belady, FIFO algorithm was acceptable.

For example, if the system had 512 kilobytes of RAM, 64 k could be allocated for the operating system, a partition larger than 64 k, another 128k and more than 256 k. This was very simple, but inflexible, as if there were any urgent work, such as 400k, had to wait foranother day or repartition, starting your team from scratch.
 The alternative, which emerged after, and need to improve the previous alternative was to create variable size contiguous partitions. For this, the system had to maintain and a data structure well enough to know where they had gaps in available RAM and from where to where they had occupied partitions running programs. Thus, when aprogram needed to be loaded into RAM, the system analyzes the gaps to see if there were any large enough for the program you wanted to go, if so, assigned the space. If not, trying to relocate the existing programs in order to make the entire space occupied contiguous and all free space and get a hole of sufficient size. If the program still did not fit, then it crashed and took another. The processthrough which holes are coupled or occupied spaces is called `compaction '. The reader may have already noticed several problems that arise with fixed partition schemes and partition variables or based on what criteria you choose the best partition size for a program?


For example, if the system has two gaps, one 18K and one for a 24 k 20 k process you want, or what is assigned? There areseveral algorithms to give answer to the question above, which are listed and described below.

First Set: Allocate the first hole that is larger than the size desired.
Better Fit: Allocate the space whose size exceeds the minimum quantity to the desired size.
Requires an exhaustive search.

Worst Fit: Allocate the hole whose size exceeds the maximum amount to the desired size.
Thefollowing adjustments: Same as the first 'fix' with the difference that makes a pointer to where the last gap was assigned to conduct the next search from it.
Quick Set: separate linked lists are maintained according to the sizes of the holes, so look for a hole the fastest processes in the queue.
Another problem that is seen from here is that, once assigned a place, for example, with "the worst fit,the process may require 12 kilobytes and the gap was 64 kilobytes allocated, so the process goes to waste a lot of memory inside its partition, which is called `internal fragmentation."
On the other hand, as the system moves forward in the day, ending and starting other processes, the memory is configured as a contiguous sequence of holes and assigned places, causing gaps exist, for example,...