Query regarding physically discontiguous memory
Hi, I just started reading Memory Managment chapter from book Professional Linux Kernel Architecture. At very start of chapter I came across terms FLATMEM, DISCONTIGMEM and SPARSEMEM. Out of these DISCONTIGMEM and SPARSEMEM are available to NUMA systens only. Book also says UMA systems can also choose DISCONTIGMEM option if there address space contains large holes. Some googling says DISCONTIGMEM is prefered when we have physical discontiguous memory. This is what I don't understand what does it mean by containing large holes? physical discontiguous memory? Is it like when I put 2 RAM chips in a system then memory for them will be discontiguous? Would /proc/iomem give me details I wanted? Per my understanding RAM memory will ALWAYS be contiguous whether its 512 MB or 4 GB. Please give me graceful example of discontiguous physical memory That will really help me out.
On Wed, 25 Dec 2013 14:45:01 +0530, Pritam Bankar said:
This is what I don't understand what does it mean by
containing large holes?
physical discontiguous memory?
Is it like when I put 2 RAM chips in a system then memory for them will be discontiguous?
Per my understanding RAM memory will ALWAYS be contiguous whether its 512 MB or 4 GB.
Your understanding is wrong. That may be true on most x86-based desktops and laptops and the like, but it's often very wrong on other architectures. In fact, given that the x86 architecture is just one of like 27 archs that Linux supports, it's a good bet that there's more archs that *don't* enforce contiguous memory than do. Counting through the defconfigs to get a number is left as an exercise for the student. :) A common case for embedded systems is "256M of RAM at address 0, and 256M of RAM at address 0x80000000" (or whatever the halfway mark for the architecture is). NUMA boxes are sometimes "each node's RAM is at <node number>*4G" or similar, which means lots of holes if each node is partly populated with only 2G.
On Wed, Dec 25, 2013 at 9:07 PM, <Valdis.Kletnieks@vt.edu> wrote:
On Wed, 25 Dec 2013 14:45:01 +0530, Pritam Bankar said:
This is what I don't understand what does it mean by
containing large holes?
physical discontiguous memory?
Is it like when I put 2 RAM chips in a system then memory for them will be discontiguous?
Per my understanding RAM memory will ALWAYS be contiguous whether its 512 MB or 4 GB.
Your understanding is wrong. That may be true on most x86-based desktops and laptops and the like, but it's often very wrong on other architectures. In fact, given that the x86 architecture is just one of like 27 archs that Linux supports, it's a good bet that there's more archs that *don't* enforce contiguous memory than do. Counting through the defconfigs to get a number is left as an exercise for the student. :)
A common case for embedded systems is "256M of RAM at address 0, and 256M of RAM at address 0x80000000" (or whatever the halfway mark for the architecture is).
NUMA boxes are sometimes "each node's RAM is at <node number>*4G" or similar, which means lots of holes if each node is partly populated with only 2G.
Thanks a lot Saptarshi and Valdis. Your comments cleared my doubt but while doing some more analysis I have come with following points. Following is a output of /proc/iomem on my system ***************************************************************************** [pritam@linux-box-1 ~]$ cat /proc/iomem 00000000-0009fbff : *System RAM* 00000000-00000000 : Crash kernel 0009fc00-0009ffff : reserved 000a0000-000bffff : Video RAM area 000c0000-000c7fff : Video ROM 000c8000-000c97ff : Adapter ROM 000c9800-000cafff : Adapter ROM 000f0000-000fffff : System ROM 00100000-3ffeffff : *System RAM* 00400000-005fed40 : Kernel code 005fed41-006d292f : Kernel data 3fff0000-3fffefff : ACPI Tables 3ffff000-3fffffff : ACPI Non-volatile Storage fc900000-fc9fffff : PCI Bus #01 fc9b0000-fc9bffff : 0000:01:05.1 fc9b0000-fc9bffff : tg3 fc9e0000-fc9effff : 0000:01:05.0 fc9e0000-fc9effff : tg3 fca00000-feafffff : PCI Bus #03 fd000000-fdffffff : 0000:03:04.0 feac0000-feadffff : 0000:03:04.0 feafd000-feafdfff : 0000:03:00.1 feafd000-feafdfff : ohci_hcd feafe000-feafefff : 0000:03:00.0 feafe000-feafefff : ohci_hcd feaff000-feafffff : 0000:03:04.0 febfe000-febfefff : 0000:00:0a.1 febff000-febfffff : 0000:00:0b.1 ff500000-ff5fffff : PCI Bus #01 ff780000-ffffffff : reserved ****************************************************************************** PCMIIW, 1. From above output it is clear that System RAM is discontinuous. I calculated addresses in both sections assigned to System RAM (highlighted in above output) and they are of 1024 MB which is exactly my systems physical RAM is. 2. But for other IO devices lets say Video RAM, area is 000a0000-000bffff Now who has assigned these addresses? Do they remain same forever ? How kernel comes to know that there is no RAM at 000a0000-000bffff but its Video RAM? Thanks, Pritam
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Pritam Bankar -
Valdis.Kletnieks＠vt.edu