Hi Vaibhav, On Mon, Jul 25, 2011 at 9:40 PM, Vaibhav Jain <vjoss197@xxxxxxxxx> wrote: > Hi Dave, Santhosh, > > Thanks for the reply! I was talking about the following paragraph from the > reference you provided : > > The kernel (on the x86 architecture, in the default configuration) splits > the 4-GB virtual address space between user-space and the kernel; the same > set of mappings is used in both contexts. A typical split dedicates 3 GB to > user space, and 1 GB for kernel space.* The kernel’s code and data > structures must fit into that space, but the biggest consumer of kernel > address space is virtual mappings for physical memory. The > kernel cannot directly manipulate memory that is not mapped into the > kernel’s address space. The kernel, in other words, needs its own virtual > address for any memory it must touch directly. Thus, for many years, the > maximum amount of physical memory that could be handled by the kernel was > the amount that could be mapped into the kernel’s portion of the virtual > address space, minus the space needed for the kernel code itself. As a > result, x86-based Linux systems could work with a maximum of a little under > 1 GB of physical memory. > > I am still not clear about the sentences in bold. Why is the space needed > for kernel code subtracted from the amount that could be mapped > into kernel's portion of virtual address space ? Well the kernel code needs to run from the kernel virtual space. > Also , what difference does > the fact - "biggest consumer of kernel address space is virtual mappings for > physical memory" make in the amount of memory that can be handled by the > kernel. In order to access memory you need to have MMU tables. These MMU tables are what maps the virrtual to physical memory. MMU entries are needed for every single page. I don't quite think that they take up a huge amount of memory. There is also a page structure for every page of physical memory. -- Dave Hylands Shuswap, BC, Canada http://www.davehylands.com _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
