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. -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: https://admin.fedoraproject.org/mailman/listinfo/users Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines
