Sunday 20 February 2011

Techniques for Hardware with no Reference bit

Many of the techniques discussed above assume the presence of a reference bit associated with each page. Some hardware has no such bit, so its efficient use requires techniques that operate well without one.

One notable example is VAX hardware running HP OpenVMS. Its approach is known as Secondary Page Caching. Pages removed from working sets may be placed on one of two lists while remaining in physical memory for some time. A page whose backing store is still valid (it has not changed since the last load from disk, for example) may be placed on the tail of the Free Page List. A page that requires writing to backing store will be placed on a Modified Page List.

Pages may be selected for removal from a working set in an essentially random fashion, but since they're not overwritten immediately, a process may reference the page again, generating a soft fault. The Modified Page List provides an opportunity to selectively write pages out to backing store in groups of more than one page, increasing efficiency. These pages can then be placed on the Free Page List. The sequence of pages that works its way to the head of the Free Page List resembles the results of a LRU or NRU mechanism and the overall effect has similarities to the Second-Chance algorithm described earlier.

1 comment:

  1. Casinos Near Casinos Near Casinos & Gambling Near You - Mapyro
    › › Casinos Near Casinos › 김포 출장샵 › Casinos Near Casinos Find 하남 출장마사지 your nearest gambling and gambling 강원도 출장안마 location 당진 출장샵 in the 영천 출장안마 USA.

    ReplyDelete