[Beowulf] Again about NUMA (numactl and taskset)

Mikhail Kuzminsky kus at free.net
Mon Jun 23 10:01:29 EDT 2008

I'm testing my 1st dual-socket quad-core Opteron 2350-based server.
Let me assume that the RAM used by kernel and system processes is 
zero, there is no physical RAM fragmentation, and the affinity of 
processes to CPU cores is maintained. I assume also that both the 
nodes are populated w/equal number of the same DIMMs.

If I run thread- parallelized (for example, w/OpenMP) application w/8 
threads (8 = number of server CPU cores), the ideal case for all the 
("equal") threads is: the shared memory used by each of 2 CPUs (by 
each of 2 processes "quads") should be divided equally between 2 
nodes, and the local memory used by each process should be mapped 

Theoretically like ideal case may be realized if my application (8 
threads) uses practically all the RAM and uses only shared memory (I 
assume here also that all the RAM addresses have the same load, and 
the size of program codes is zero :-) ).

The questions are
1) Is there some way to distribute analogously the local memory of 
threads (I assume that it have the same size for each thread) using 
"reasonable" NUMA allocation ?

2) Is it right that using of numactl for applications may gives 
improvements of performance for the following case:
the number of application processes is equal to the number of cores of 
one CPU *AND* the necessary (for application) RAM amount may be placed 
on one node DIMMs (I assume that RAM is allocated "continously").

What will be w/performance (at numactl using) for the case if RAM size 
required is higher than RAM available per one node, and therefore the 
program will not use the possibility of (load balanced) simultaneous 
using of memory controllers on both CPUs ? (I also assume also that 
RAM is allocated continously).

3) Is there some reason to use things like
mpirun -np N /usr/bin/numactl <numactl_parameters>  my_application   ?

4) If I use malloc()  and don't use numactl, how to understand - from 
which node Linux will begin the real memory allocation ? (I remember 
that I assume that all the RAM is free) And how to understand  where 
are placed the DIMMs which will corresponds to higher RAM addresses or 
lower RAM addresses ?

5) In which cases is it reasonable to switch on "Node memory 
interleaving" (in BIOS) for the application which uses more memory 
than is presented on the node ?  

And BTW: if I use 
taskset -c CPU1,CPU2, ... <program_file>
and the program_file creates some new processes, will all this 
processes run only on the same CPUs defined in taskset command ?

Mikhail Kuzminsky
Computer Assistance to Chemical Research Center,
Zelinsky Institute of Organic Chemistry


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