| 1 |
On Fri, Dec 14, 2012 at 10:16 PM, Grant <emailgrant@×××××.com> wrote: |
| 2 |
>> > >> > So if I have 2 physical CPU's with 4 cores each and I enable SMP, |
| 3 |
>> > >> > I'm |
| 4 |
>> > >> > using |
| 5 |
>> > >> > 8 cores? Can NUMA be either enabled or disabled when using more |
| 6 |
>> > >> > than |
| 7 |
>> > >> > one |
| 8 |
>> > >> > physical CPU, or is it required? |
| 9 |
>> > >> |
| 10 |
>> > >> |
| 11 |
>> > >> NUMA is a hardware architecture. It's how you access memory on a |
| 12 |
>> > >> hardware level: NUMA = Non Uniform Memory Access vs a UMA |
| 13 |
>> > >> architecture |
| 14 |
>> > >> of typical (old/legacy) SMP systems (UMA = Uniform Memory Access). |
| 15 |
>> > >> |
| 16 |
>> > >> In a UMA system, all the memory belongs to all the sockets. In a NUMA |
| 17 |
>> > >> system, each socket has it's "own" local memory. In modern (x86-64) |
| 18 |
>> > >> processors, each socket has it's own memory controller so each socket |
| 19 |
>> > >> controls its own local memory. If one socket runs out of memory it |
| 20 |
>> > >> can |
| 21 |
>> > >> ask another socket to lend him some memory. In a UMA system, no |
| 22 |
>> > >> socket |
| 23 |
>> > >> has to ask since memory is global and belongs to all sockets so if |
| 24 |
>> > >> one |
| 25 |
>> > >> socket uses up all the memory ... the rest "starve". In NUMA, there's |
| 26 |
>> > >> more control over who uses what (be it cores or RAM). |
| 27 |
>> > >> |
| 28 |
>> > >> If you have a modern dual or quad (or higher #) socket system ... |
| 29 |
>> > >> you've got NUMA architecture and you can't get rid of it, it's |
| 30 |
>> > >> hardware, not software. |
| 31 |
>> > > |
| 32 |
>> > > So I must enable CONFIG_NUMA for more than one physical CPU, and |
| 33 |
>> > > disable it |
| 34 |
>> > > for only one physical CPU? |
| 35 |
>> > |
| 36 |
>> > |
| 37 |
>> > Yup. But ... Why would you want to disable a socket (CPU)? If you |
| 38 |
>> > disable a socket (CPU) ... you lose the memory attached to that socket |
| 39 |
>> > (CPU) not to mention you lose those cores ;) |
| 40 |
>> |
| 41 |
>> Sure but it sounds like if my system only has one CPU socket, CONFIG_NUMA |
| 42 |
>> should be disabled. |
| 43 |
> |
| 44 |
> I read this in make menuconfig: |
| 45 |
> |
| 46 |
> "The kernel will try to allocate memory used by a CPU on the local memory |
| 47 |
> controller of the CPU and add some more NUMA awareness to the kernel. For |
| 48 |
> 64-bit this is recommended if the system is Intel Core i7 (or later), AMD |
| 49 |
> Opteron, or EM64T NUMA." |
| 50 |
> |
| 51 |
> To be sure I have this right, I should disable CONFIG_NUMA on any system |
| 52 |
> with a single physical CPU, even if it's an AMD Opteron? |
| 53 |
|
| 54 |
No harm done if you enable NUMA on a system where it's not necessary. |
| 55 |
|
| 56 |
-- |
| 57 |
:wq |
Archives