Messages in this thread | | | Date | Fri, 18 Dec 1998 13:05:59 -0500 (GMT) | From | Prasun Kapoor <> | Subject | Re: swap cache |
| |
> > Yes, it's right. If fork() takes care of COW, then why swap-cache > > should be used?. > > Fork obviously does _not_ take care of COW. Fork just creates a COW > sharing in the first place. The actual "taking care of" such COW pages > comes later, when processes start to take page faults on them or try to > swap them out. That's why we have the swap cache. It lets us treat > these COW anonymous pages as in-memory copies of the swap-entry on disk, > so that the separate processes paging that data can do so independently. > > The real benefit of the swap cache is this: say we have a shared COW > page on swap, and one process reads in that page. All other processes > sharing that page still have references to the original entry on disk in > their page tables. However, the swap cache lets us record the fact that > the on-disk entry is now located in physical memory. As long as it > remains in core, any other process accessing that same page will take a > soft page fault and will find the existing page in memory rather than > trying to read the page in from disk again. In other words, the two > processes which originally shared the swap page on disk now share the > swap cache page in memory. > > The swap cache is therefore *critical* to the maintainance of the COW > pages created by fork.
Yes, I fully agree with the above description. But there has to be a similar arragement for sharing pages of other mmap()ed files. And why is the same arrangement not used for SWAP instead of having a seperate swap cache.
prasun > > --Stephen > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.rutgers.edu > Please read the FAQ at http://www.tux.org/lkml/ >
_____________________________________________________________________________ "If seven maids with seven mops, Swept it for half a Prasun Kapoor year, do you suppose" the Walrus said, "That they Wipro Infotech Ltd. could get it clear"."I doubt it" said the Carpenter, Ph. 2241730 Extn. 3312 and shed a bitter tear "Through the looking glass and what alice found there" - Alice in Wonderland -
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
| |