thanks a lot guys for such a great explanation. On Tue, Sep 21, 2010 at 10:31 AM, Prabhu nath <[email protected]> wrote:
> Here are my thoughts. Plz correct me if I am wrong. > > In the given example 5000 is the offset within the file and not the > address. Given, the offset 5000, kernel anyway could allocate a physical > page and place the contents from the starting of the page and map it to the > virtual address. > For E.g. it could allocate a physical page of base address 0x2abd1000 and > copy the 1000 bytes of contents from the offset 5000 and map the physical > address to some virtual address say 0x08054000. > > Q. How does the offset not being page aligned affect the execution ? > > The 2nd point wrt fragmentation is convincing. > > Regards, > Prabhu > > > On Mon, Sep 20, 2010 at 10:27 PM, Mulyadi Santosa < > [email protected]> wrote: > >> hi.. >> >> On Mon, Sep 20, 2010 at 19:30, mohit verma <[email protected]> wrote: >> > thanks guys, >> > but the problem is : >> > what is the need of page aligned address? as we have given starting >> > address 5000 and the length 1000 ( in the example by prabhu) so it >> should >> > start from that location. >> >> I think it is due to two reasons: >> >> 1. on some architecture, non page aligned access could mean >> trap/exception. Well, x86 is immune in this case...but if you wanna >> make your code safely portable, I think it's a must. >> >> 2. By making it page aligned, at least you reduce fragmentation. >> >> How come? Let's assume you just have 8K address space. You allocate a >> memory starting at 3K as large as 4K. That means you occupy 3K up to >> 7K address. What's left? 0-1 K and 7-8 K, right? >> >> And what if you wanna allocate another 4K? Yes, a page can fit >> that...but not mapping. We can't split below page granularity, so at >> least we need two page. one is mapped to 0-1 K, the other is for 7-8K. >> It's 2 page vs 3 page case. >> >> Yes, there's slab...but if we can allocate straight in page size >> granularity, things would be simpler, right? >> >> -- >> regards, >> >> Mulyadi Santosa >> Freelance Linux trainer and consultant >> >> blog: the-hydra.blogspot.com >> training: mulyaditraining.blogspot.com >> >> -- >> To unsubscribe from this list: send an email with >> "unsubscribe kernelnewbies" to [email protected] >> Please read the FAQ at http://kernelnewbies.org/FAQ >> >> >
