How Not to Write a High-Performance N-body Code

Lehman Garrison (SCC, formerly CCA)

FWAM, High-Performance Computing Case Study

Oct. 28, 2022

 

slides.com/d/JYBnZPE/live

https://github.com/lemmingapex/N-bodyGravitySimulation

The Large-Scale Structure of the Universe

First 8 weeks of DESI Survey data (2021; Credit: D. Schlegel/Berkeley Lab)

Telescope surveys see that galaxies cluster together, forming a "cosmic web"

Simulating the Cosmic Web

  • Gravitational instability in an expanding universe explains the cosmic web
  • Can simulate by treating all matter as collisionless particles
    • Ignore gas dynamics; good enough for large scales
  • Discretize the matter into N particles, evolve equations of motion under Newtonian gravity in a GR background

Credit: Benedikt Diemer & Phil Mansfield

N-body: the computational task

  • Integrate equations of motion of each particle using leapfrog
  • Requires gravitational force at each time step: $$ \vec{F_i} \propto \sum_j^N \frac{\vec{r_j} - \vec{r_i}}{|\vec{r_j} - \vec{r_i}|^3} $$
    • Naive time complexity \( \mathcal{O}(N^2) \)
  • Must also respect periodic boundary conditions
    • Won't use \( \mathcal{O}(N^2) \) on large scales; might still on small scales

It gets worse...

  • Tiny galaxies shine across vast cosmic distances
    • Need high mass resolution, large simulation volume
  • Non-trivial gravitational coupling of large scales to small
  • Typical flagship N-body simulations have 1+ trillion particles in 2022
  • 1 trillion particles
    • Typically 32+ TB of particle state
    • All-to-all communication
    • Thousands of time steps
    • Tightly coupled

The Abacus N-body Code

  • New code for cosmological N-body simulations (Garrison et al., 2021)
  • Based on new multipole method with exact near-field/far-field split
  • Near field GPU-accelerated; far field CPU SIMD-accelerated
  • Fastest published per-node performance of any cosmological N-body code
  • Tiny force errors, typically \(10^{-5}\) (frac.)
  • Written in C++/CUDA, with MPI+OpenMP parallelization

Hearken back to 2013...

  • GPU computing was just entering mainstream
  • Beyoncé only had 17 Grammys
  • Few academic GPU clusters
  • A single node could house enough GPUs to run a simulation, but how do you hold it in memory? With disk!
  • Abacus "slab pipeline" designed to only need a thin slice of the simulation in memory
  • Rest of the simulation can be on disk (2013), on another node (2020), etc
  • Lots of IO optimization

Abacus cluster in the Harvard Astro Dept. basement

Lesson #0: Don't cook your hard drives

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

\(X\)-axis

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Far-field
force

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Far-field
force

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Near-field
force (GPU)

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Near-field
force (GPU)

Far-field
force

Kick/Drift/
Multipoles

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Near-field
force (GPU)

Far-field
force

Kick/Drift/
Multipoles

Write

Abacus Slab Pipeline

Slab 7

Slab 6

Slab 5

Slab 4

Slab 3

Slab 2

Slab 1

Read

Near-field
force (GPU)

Far-field
force

Kick/Drift/
Multipoles

Write

Abacus Slab Pipeline

  • Uses the exact decomposition of the multipole method to process the simulation volume slab-by-slab
  • FFT convolution of multipoles happens after pipeline, produces far-field for next step
  • Pipeline actually expressed as an event loop (code example)
  • Easily incorporates asynchronous activity, like IO, GPU, or MPI
void slab_pipeline(){
  while(!Finish.doneall()){
      if(LaunchRead.precondition()) LaunchRead.action();
      if(NearForce.precondition())  NearForce.action();
      if(FarForce.precondition())   FarForce.action();
      if(Kick.precondition())       Kick.action();
      if(Drift.precondition())      Drift.action();
      if(Finish.precondition())     Finish.action();
  }
}

class NearForce : Stage {
    int precondition(int slab){
        for(int i = -RADIUS; i <= RADIUS; i++){
            if(!Read.done(slab+i)) return 0;
        }
        return 1;
    }
    void action(int slab){
        SubmitToGPU(slab);
    }
};

From single-node to multi-node

  • From a node's perspective, it doesn't matter if the not-in-memory particles are on disk or on another node
  • In 2018, we added MPI parallelization, reusing the slab decomposition
  • This 1D parallelization had perfect scaling to 140+ nodes on Summit
    • Any larger and the domains become too thin for halo finding
  • Rolling decomposition: a node ends with a different domain than it starts with
  • No boundary approximations!

Custom OpenMP Scheduler

  • Modern systems have "Non-Uniform Memory Access" (NUMA)
    • Cores access memory on the same socket (NUMA node) faster
  • Abacus slabs are divided over sockets. Want the CPUs to only work on rows on that socket.

  • Abacus used to use static scheduling, but started choking on load balancing in big sims

Socket 1

Socket 2

CPU 1

CPU 1

CPU 2

CPU 3

CPU 3

CPU 4

void KickSlab(int slab){
    #pragma omp parallel for schedule(???)
    for(int j = 0; j < nrow; j++){
        KickRow(slab,j);
    }
}

CPU 2

CPU 4

Custom OpenMP Scheduler

  • OpenMP 4 doesn't have a "dynamic-within-socket" schedule
  • Nor does it let you write a custom scheduler
  • But we can hack one together...
    • Use OpenMP tasks
    • Atomic addition to grab next loop iteration
  • 25% improvement in performance of far-field calculation
  • But can't use "reduction" clauses, have to implement manually...
#define NUMA_FOR(N)
    _next_iter[0] = 0;   // node 0
    _last_iter[0] = N/2;
    
    _next_iter[1] = N/2; // node 1
    _last_iter[1] = N;
    
    #pragma parallel
    for(int j; ; ){
        #pragma atomic capture
        j = _next_iter[NUMA_NODE]++;
        if(j >= _next_iter[NUMA_NODE+1])
            break;

#pragma omp parallel for
for(int j = 0; j < nrows; j++){
    KickRow(slab,j);
}

NUMA_FOR(nrow)
    KickRow(slab,j);
}

Given the following code:

Rewrite as:

Lesson #1: don't let your threads access memory on the wrong NUMA node

Lesson #1b: Don't write a custom OpenMP scheduler

Lesson #1a: don't let your threads access memory on the wrong NUMA node

Loop Unrolling & SIMD Metacode

void multipoles(const double *pos, const int N, double *multipole){
    for(int i = 0; i < N; i++){
        double mx = 1.;
        for(int a = 0; a <= ORDER; a++){
            double mxy = mx;
            for(int b = 0; b <= ORDER - a; b++){
                double mxyz = mxy;
                for(int c = 0; c <= ORDER - a - b; c++){
                    multipole[a,b,c] += mxyz;
                    mxyz *= pos[2,i];
                }
                mxy *= pos[1,i];
            }
            mx *= pos[0,i];
        }
      }
   }
}
M_{a,b,c} = \sum_q^N x_q^a y_q^b z_q^c \begin{cases} 0 \le a \le p \\ 0\le b \le p-a \\ 0\le c \le p-a-b \end{cases}

Performance: \(7\times10^6\) particles/sec (1 core)

print('double mx = 1.;')
for a in range(ORDER + 1):
    print('double mxy = mx;')
    for b in range(ORDER - a + 1):
        print('double mxyz = mxy;')
        for c in range(ORDER - a - b + 1):
            print(f'multipoles[{a},{b},{c}] += mxyz;')
            print('mxyz *= pos[2,i];')
        print('mxy *= pos[1,i];')
    print('mx *= pos[0,i];')

\(19\times10^6\) particles/sec

print('__m512d mx = _mm512_set1_pd(1.);')
for a in range(ORDER + 1):
    print('__m512d mxy = mx;')
    for b in range(ORDER - a + 1):
        print('__m512d mxyz = mxy;')
        for c in range(ORDER - a - b + 1):
            print(f'multipoles[{a},{b},{c}] = '\
              f'_mm512_add_pd(multipoles[{a},{b},{c}],mxyz);')
            print('mxyz = _mm512_mul_pd(mxyz,pos[2,i]);')
        print('mxy = _mm512_mul_pd(mxy,pos[1,i]);')
    print('mx = _mm512_mul_pd(mx,pos[0,i]);')

\(54\times10^6\) particles/sec

Loop Unrolling & SIMD Metacode

Lesson #2: don't trust the compiler to optimize complicated, performance-critical loops!

Other tricks: interleaving vectors, FMA, remainder iterations

PTimer Cache Line Contention

  • Abacus PTimer (“parallel timer”) is used to profile performance-intensive, multi-threaded loops
  • Each thread has a binary flag that it sets when it is active
  • All flags were in a contiguous array
  • 20 threads fighting for the same two cache lines inside tight nested loops
  • Resolving this gave an immediate 50% CPU performance boost

  • Typical cache line size 64 bytes

    • Keep threads away from each other by at least this much

Lesson #3: don't share cache lines, especially in tight loops

GPU Data Model: Cells vs. Pencils

  • Old version of GPU code processed cell pairs
  • NVIDIA A100 GPU: 1500 GB/s memory bandwidth
    • 125 Gparticles/sec
  • 20 TFLOPS compute
    • 770 Gforces/sec
  • 770/125 = 6 forces per load
    • Each particle should be used at least 6 times per load to shared mem
  • 25 GB/s host to device PCIe bandwidth
    • 2 Gparticles/sec
    • 770/2 = 385 forces per host-device transfer
  • Pencil-on-pencil
    • Each particle acts on 5 sink pencils, 5 cells each
    • Need at least 15 particle per cell—not bad!

Lesson #4: don't send data to the GPU unless you have a lot of floating-point work to do on it

What did we do with all this?

  • 60 trillion particles
    • More particles than all N-body simulations ever run, combined
  • Performance of 300+ trillion directs/sec
  • 7+ PB of raw outputs, processed and compressed to 2 PB
  • 60% of theoretical peak usage of Summit GPUs
  • Primary N-body simulation suite used by the DESI galaxy survey (Stage-IV DOE Dark Energy Experiment)
  • Public data release: abacusnbody.org

Conclusions

  • Many lessons about HPC learned from mistakes made in the Abacus code
    • Don't cook hard drives
    • Don't use static scheduling as a replacement for NUMA-awareness
    • Don't trust compiler unrolling of complicated loops
    • Don't let threads work within the same cache line
    • Don't send data to the GPU unless you have lots of work to do on it
    • Event loops are a powerful simulation programming model
  • If you have a better approach for one of these problems, I'm eager to talk about it!
  • Many other optimizations I didn't get to talk about:
    • Thread pinning
    • Memory allocators (tcmalloc)
    • GPU memory pinning
    • Ramdisk/mmap
    • MPI point-to-point vs all-to-all

  • Checksumming

  • Compression/file formats

  • Representation of particle positions

  • SSD TRIM

  • GPU intermediate accumulators

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