Is swap really needed when RAM's aplenty

[JD]

  On 08/19/2010 02:15 PM, Michael Hennebry wrote:
> On Thu, 19 Aug 2010, JD wrote:
>
>> Problem comes as Michael explains, that when a process needs a large
>> "physically contiguous" chunk of memory, it might not be available.
>> That said, usually, requests for physically contiguous memory is only
>> needed when wanting to map very large number of DMA pages for
>> doing direct physical I/O.
>> Otherwise, a process itself does not need to have physically contiguous
>> pages. Only the virtual space allocated to that "malloc" or large buffer
>> declaration in a program, is contiguous.
> Why would malloc or a large buffer declaration
> require physically contiguous memory?
It is done in a driver on the process' behalf when doing direct physical
IO .
typically, such blocks of physically contiguous chunks memory are set
aside during boot.
I have also seen special embedded linux drivers that provide an ioctl
to let the process get a set of physically contiguous pages and map the 
space
to user virtual space. This is for performance reasons to reduce copying
from user space to kernel space when large amounts of data need to be
moved.
This is not a new  idea. it has been around for many years. I first
saw it in Linux back in 1998/1999.