On Fri, 20 Aug 2010, JD wrote:
> On 08/20/2010 06:44 AM, Michael Hennebry wrote:
>> On Thu, 19 Aug 2010, JD wrote:
>>
>>> 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.
>> Perhaps I misunderstood.
>> Do both of the following necessarily require physically contiguous memory?
>> char fred[69000];
>> char *greg=malloc(96000);
>> Would they sometimes require physically contiguous memory?
>>
> It depends on what you want to achieve.
> If the target device you will write that buffer to can handle a
> contiguous physical space of, say ... a few pages, then you
> would want to ask the special driver of that device, via an ioctl,
> to give you those pages, and map them to user virtual space -
> i.e. you would not allocate them from the heap.
It makes sense that if a process insists on physically
contiguous memory and can't get it, the process would die,
but the above code does not tell the compiler what is to be achieved.
In the following, would fred or greg necessarily
refer to physically contiguous memory?
#include <stdlib.h>
extern void hank(char *);
int main(*args[], int argsNum)
{
char fred[69000];
char *greg=malloc(96000);
char *greg=malloc(96000);
hank(fred);
hank(greg);
return 0;
}
--
Michael hennebry@xxxxxxxxxxxxxxxxxxxxx
"Pessimist: The glass is half empty.
Optimist: The glass is half full.
Engineer: The glass is twice as big as it needs to be."
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