Intel Software Conference 2014 Brazil May 2014 Leonardo Borges

1. Intel Software Conference 2014 Brazil
May 2014
Leonardo Borges
Notes on NUMA architecture

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Non-Uniform Memory Access (NUMA)
FSB architecture
- All memory in one location
Starting with Nehalem
- Memory located in multiple
places
Latency to memory
dependent on location
Local memory
- Highest BW
- Lowest latency
Remote Memory
- Higher latency
Socket 0 Socket 1
QPI
Ensure software is NUMA-optimized for best performance
Notes for Intel Software Conference – Brazil, May 2014

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CPU1 DRAM
Node 1
Non-Uniform Memory Access (NUMA)
Locality matters
- Remote memory access latency ~1.7x than local memory
- Local memory bandwidth can be up to 2x greater than remote
Intel® QPI = Intel® QuickPath Interconnect
Remote Memory Access
Intel®
QPI
CPU0DRAM
Local Memory Access
Node 0
BIOS:
- NUMA mode (NUMA Enabled)
First Half of memory space on Node 0, second half on Node 1
Should be default on Nehalem (!)
- Non-NUMA (NUMA Disabled)
Even/Odd cache lines assigned to Nodes 0/1: Line interleaving
Notes for Intel Software Conference – Brazil, May 2014

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Local Memory Access Example
CPU0 requests cache line X, not present in any CPU0 cache
- CPU0 requests data from its DRAM
- CPU0 snoops CPU1 to check if data is present
Step 2:
- DRAM returns data
- CPU1 returns snoop response
Local memory latency is the maximum latency of the two responses
Nehalem optimized to keep key latencies close to each other
CPU0 CPU1
QPI
DRAMDRAM
Notes for Intel Software Conference – Brazil, May 2014

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Remote Memory Access Example
CPU0 requests cache line X, not present in any CPU0 cache
- CPU0 requests data from CPU1
- Request sent over QPI to CPU1
- CPU1’s IMC makes request to its DRAM
- CPU1 snoops internal caches
- Data returned to CPU0 over QPI
Remote memory latency a function of having a low latency
interconnect
CPU0 CPU1
QPI
DRAMDRAM
Notes for Intel Software Conference – Brazil, May 2014

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Non Uniform Memory Access and
Parallel Execution
Process-parallel execution:
- NUMA friendly- data belongs only to the process
- E.g. MPI
- Affinity pinning maximizes local memory access
- Standard for HPC
Shared-memory threading:
- More problematic: same thread may require data from multiple
NUMA nodes
- E.g. OpenMP, TBB , explicit threading
- OS scheduled thread migration can aggravate situation
- NUMA and non-NUMA should be compared
Notes for Intel Software Conference – Brazil, May 2014

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Operating System Differences
Operating systems allocate data differently
Linux*
- Malloc reserves the memory
- Assigns the physical page when data touched (first touch)
Many HPC code initialize memory by single ‘master’ thread !!
- A couple of extensions available via numactl and libnuma like
numactl --interleave=all /bin/program
numactl --cpunodebind=1 --membind=1 /bin/program
numactl --hardware
numa_run_on_node(3) // run thread on node 3
Microsoft Windows*
- Malloc assigns the physical page on allocation
- This default allocation policy is not NUMA friendly
- Microsoft Windows has NUMA Friendly API’s
VirtualAlloc reserves memory (like malloc on Linux*)
Physical pages assigned at first use
For more details:
http://kernel.org/pub/linux/kernel/people/christoph/pmig/numamemory.pdf
http://msdn.microsoft.com/en-us/library/aa363804.aspx
Notes for Intel Software Conference – Brazil, May 2014

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Other Ways to Set Process Affinity
taskset: sets or retrieves the CPU affinity
Intel MPI: using I_MPI_PIN and
I_MPI_PIN_PROCESSOR_LIST environment
variables
KMP_AFFINITY on Intel Compilers OpenMP
- Compact: binds the OpenMP thread n+1 as close as
possible to OpenMP thread n
- Scatter: distributes threads evenly across the entire
system. Scatter is the opposite of compact
Notes for Intel Software Conference – Brazil, May 2014

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NUMA Application Level Tuning:
Shared Memory Threading Example: TRIAD
Parallelized time consuming hotspot “TRIAD” (e.g.
of STREAM benchmark) using OpenMP
main() {
…
#pragma omp parallel
{
//Parallelized TRIAD loop…
#pragma omp parallel for private(j)
for (j=0; j<N; j++)
a[j] = b[j]+scalar*c[j];
} //end omp parallel
…
} //end main
Parallelizing hotspots may not be sufficient for NUMA
Notes for Intel Software Conference – Brazil, May 2014

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NUMA Shared Memory Threading
Example ( Linux* )
KMP_AFFINITY=compact,0,verbose
main() {
…
#pragma omp parallel
{
#pragma omp for private(i)
for(i=0;i<N;i++)
{ a[i] = 10.0; b[i] = 10.0; c[i] = 10.0;}
…
//Parallelized TRIAD loop…
#pragma omp parallel for private(j)
for (j=0; j<N; j++)
a[j] = b[j]+scalar*c[j];
} //end omp parallel …
} //end main
Each thread initializes its data
pinning the pages to local memory
Environment variable
to pin affinity
Same thread that initialized
data uses data
Notes for Intel Software Conference – Brazil, May 2014

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NUMA Optimization Summary
NUMA adds complexity to software parallelization
and optimization
Optimize for latency and for bandwidth
- In most cases goal to minimize latency
- Use local memory
- Keep memory near the thread it accesses
- Keep thread near memory it uses
Rely on quality middle-ware for CPU affinitization:
Example: Intel Compiler OpenMP or MPI environment
variables
Application level tuning may be required to
minimize NUMA first touch policy effects
Notes for Intel Software Conference – Brazil, May 2014

12. 12 12Notes for Intel Software Conference – Brazil, May 2014