https://github.com/artjomsimon/go-bots

1. Scalability comparison: Traditional fork-join-based
parallelism vs. Goroutines
Porting the Barcelona OpenMP Tasks Suite to Go
Artjom Simon
https://github.com/artjomsimon/go-bots
Know Your Gophers
2015-05-12

2. Traditional approach in C
Cilk:
cilk_spawn task();
[...]
cilk_sync;
OpenMP:
#pragma omp parallel
{
#pragma omp task
[...]
#pragma omp taskwait
[...]
}

3. Go: Parallel For Loop Pattern1
queue := make(chan int)
done := make(chan bool)
NP := runtime.GOMAXPROCS(0)
go func() {
for i := 0; i < n; i++ { queue <- i }
close(queue)
}()
for i := 0; i < NP; i++ {
go func() {
for i := range queue { work(i) }
done<-true
}()
}
for i := 0; i < NP; i++ { <-done }
1
Benchmarking Usability and Performance of Multicore Languages, PDF:
http://arxiv.org/pdf/1302.2837v2

4. Barcelona OpenMP Tasks Suite2
2
https://github.com/alcides/bots

5. ...used in academic publications3
3
http://www.sarc-ip.org/files/null/Workshop/1234128788173_
_TSchedStrat-iwomp08.pdf

6. Micro benchmarks
1 8 16 32 48
1
8
16
32
48
OMP_NUM_THREADS
Speeduprel.toseq.
spc (opteron)
n=1000µs
n=100µs
n=10µs
Figure: Speedup spc (icc), 10 000 Tasks

7. Task pools: Variations
• notaskpool
Start Goroutines as needed, no limitation, uses WaitGroup for
synchronization
• simple-queue
Buffered channel of func()s holds task queue. n goroutines
receive the func()s and execute them
• goroutines-dispatcher
Dispatcher function, executing tasks in Goroutine only if a
global counter of running goroutines is < n
• const-goroutines
n goroutines remove tasks from a double-linked list

8. Micro benchmarks
1 8 16 32 48
1
8
16
32
48
OMP_NUM_THREADS
Speeduprel.zusequentiell
spc (opteron)
gcc
icc
clang
go-notaskpool
go-simple-queue
go-const-goroutines
go-goroutine-dispatch
Figure: Speedup spc, n=100µs, 10 000 Tasks

9. BOTS: nqueens
• N-Queens problem with n=12
• Recursive backtracking search
• No cut-off when creating tasks

10. Ergebnisse: BOTS (nqueens)
1 8 16 32 48
0
5
10
CPU cores
Speedup nqueens (opteron)
gcc
icc
clang
go-const-goroutines
go-dispatch
go-notaskpool
gccgo-const-goroutines
gccgo-dispatch
gccgo-notaskpool
Figure: Speedup for nqueens -n 12, parallel

11. BOTS: sparselu
• LU factorization of a sparse block matrix
• 50x50-Matrix, 100x100 sub block matrices

12. Results: BOTS (sparselu)
1 8 16 32 48
0
10
20
30
CPU cores
Speedup sparselu (opteron)
gcc
icc
clang
go-const-goroutines
go-dispatch
go-notaskpool
go-simplequeue
gccgo-const-goroutines
gccgo-dispatch
gccgo-notaskpool
Figure: Speedup sparselu -n 50 -m 100, parallel

13. Problem: Recursion (dependencies!)

14. Memory
opteron
0
0.5
1
1.5
·105
RSS[Kbytes] spc-par 10000 1000, 4 Threads
gcc
icc
clang
go-notaskpool
go-const-goroutines
gccgo-notaskpool
gccgo-const-goroutines
Figure: Memory comparison (Resident Set Size), spc parallel

15. Side effect: Possible heap corruption bug in Go 1.4?

16. Questions?

17. Thank you!

18. Image credits
Icon N-Queens problem: Colin M.L. Burnett, Wikimedia Commons,
(GFDL & BSD & GPL)
http://commons.wikimedia.org/wiki/File:Chess_d45.svg
(2015-03-09)