Experiments with sparse Cholesky using a sequential task-flow implementation

Iain Duff; Jonathan Hogg; Florent Lopez

doi:10.3934/naco.2018014

Article Contents

2018, Volume 8, Issue 2: 237-260. Doi: 10.3934/naco.2018014

This issue Previous Article Asymptotic properties of an infinite horizon partial cheap control problem for linear systems with known disturbances Next Article On the extension of an arc-search interior-point algorithm for semidefinite optimization

Experiments with sparse Cholesky using a sequential task-flow implementation

Scientific Computing Department, STFC Rutherford Appleton Laboratory, Harwell Campus, Oxfordshire, OX11 0QX, UK

* Corresponding author: florent.lopez@stfc.ac.uk
* Corresponding author: florent.lopez@stfc.ac.uk

Received: May 2017

Revised: September 2017

Published: May 2018

This work is supported by the NLAFET Project funded by the European Unions Horizon 2020 Research and Innovation Programme under Grant Agreement 671633

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Abstract

We describe the development of a prototype code for the solution of large sparse symmetric positive definite systems that is efficient on parallel architectures. We implement a DAG-based Cholesky factorization that offers good performance and scalability on multicore architectures. Our approach uses a runtime system to execute the DAG. The runtime system plays the role of a software layer between the application and the architecture and handles the management of task dependencies as well as the task scheduling. In this model, the application is expressed using a high-level API, independent of the hardware details, thus enabling portability across different architectures. Although widely used in dense linear algebra, this approach is nevertheless challenging for sparse algorithms because of the irregularity and variable granularity of the DAGs arising in these systems. We assess the ability of two different Sequential Task Flow implementations to address this challenge: one implemented with the OpenMP standard, and the other with the modern runtime system StarPU. We compare these implementations to the state-of-the-art solver HSL_MA87 and demonstrate comparable performance on a multicore architecture.

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Mathematics Subject Classification: 65F05, 65F50, 65Y05.

Citation:

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Figure 2.1. Simple assembly tree with three supernodes partitioned into square blocks of order $ \mathtt{nb} $

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Figure 2.2. DAG corresponding to the factorization of the tree in Figure 2.1

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Figure 2.3. Pseudo-code for the sequential version of the taskbased sparse Cholesky factorization

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Figure 3.1. Simple example of a sequential code

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Figure 3.2. STF code corresponding to Figure 3.1 example

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Figure 3.3. DAG corresponding to the sequential code presented in Figure 3.1

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Figure 4.1. Simple example of a parallel version of the sequential code in Figure 3.1 using a STF model with OpenMP

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Figure 4.2. Simple example of a parallel version of the sequential code in Figure 3.1 using a STF model with StarPU

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Figure 4.3. Illustration of the dynamic scheduling strategy of tasks in the runtime system

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Figure 5.1. Pseudo-code for the sparse Cholesky factorization using a STF model presented in Section 3

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Figure 5.2. Submission routine used for the solve tasks in the StarPU code

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Figure 5.3. Submission routine used for the solve tasks in the OpenMP code

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Figure 7.1. Performance results for both OpenMP and StarPU versions of the SpLLT and HSL_MA87 solvers on 28 cores for the test matrices presented in Table 8

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Figure 7.2. Performance results obtained with SpLLT, OpenMP and StarPU relative to the performance obtained with HSL_MA87

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Figure 7.3. Efficiency analysis for a subset of the test matrices including the parallel efficiency (top-left), the task efficiency (top-right), the runtime efficiency (bottom-left) and the pipeline efficiency (bottom-right)

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Figure 7.4. Main submission routine executed by the master thread. This performs both the initialisation and factorization tasks for the nodes in the assembly tree

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Figure 7.5. Kernel for the $ \mathtt{insert\_factorize\_node} $ task in charge of submitting the factorization tasks for a given node

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Figure 7.6. Illustration of the tree pruning strategy used by SpLLT

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Figure 7.7. Performance results for StarPU versions of the SpLLT and HSL_MA87 solvers on 28 cores for the test matrices presented in Table 8. We also show the impact of tree pruning on the performance of SpLLT

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Figure 7.8. Performance results for OpenMP versions of the SpLLT and HSL_MA87 solvers on 28 cores for the test matrices presented in Table 8. We also show the impact of tree pruning on the performance of SpLLT

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Table 7.1. Factorization times (seconds) obtained with MA87 and SpLLT with both OpenMP and StarPU versions. The factorizations were run with the block sizes $ \mathtt{nb = (256,384,512,768,1024)} $ on 28 cores and $ \mathtt{nemin = 32} $.

#	Name	MA87		spLLT
		MA87		OpenMP (gnu)		StarPU
		nb	factor. (s)	nb	factor. (s)	nb	factor. (s)
1	Schmid/thermal2	1024	0.391	1024	1.742	512	2.215
2	Rothberg/gearbox	256	0.253	384	0.236	512	0.339
3	DNVS/m_t1	256	0.206	256	0.208	1024	0.298
4	Boeing/pwtk	768	0.249	768	0.262	768	0.396
5	Chen/pkustk13	256	0.218	256	0.242	384	0.344
6	GHS_psdef/crankseg_1	256	0.233	256	0.234	384	0.281
7	Rothberg/cfd2	256	0.242	256	0.256	384	0.386
8	DNVS/thread	256	0.236	256	0.223	384	0.222
9	DNVS/shipsec8	256	0.253	256	0.234	384	0.380
10	DNVS/shipsec1	256	0.245	256	0.270	384	0.398
11	GHS_psdef/crankseg_2	256	0.267	256	0.268	384	0.326
12	DNVS/fcondp2	256	0.294	384	0.347	384	0.479
13	Schenk_AFE/af _shell3	256	0.437	512	0.550	512	0.906
14	DNVS/troll	256	0.381	384	0.434	512	0.599
15	AMD/G3_circuit	256	0.607	768	2.534	768	3.561
16	GHS_psdef/bmwcra_1	256	0.339	256	0.356	512	0.466
17	DNVS/halfb	256	0.370	256	0.442	384	0.631
18	Um/2cubes_sphere	256	0.321	384	0.451	512	0.634
19	GHS_psdef/ldoor	256	0.620	512	1.058	768	1.726
20	DNVS/ship_003	256	0.361	384	0.457	512	0.554
21	DNVS/fullb	256	0.445	256	0.469	384	0.651
22	GHS_psdef/inline_1	256	0.659	384	0.711	768	0.995
23	Chen/pkustk14	256	0.557	256	0.576	512	0.768
24	GHS_psdef/apache2	256	0.710	768	1.448	512	2.053
25	Koutsovasilis/F1	384	0.776	384	0.829	768	0.981
26	Oberwolfach/boneS10	256	1.102	256	1.185	768	1.702
27	ND/nd12k	384	1.452	384	1.479	768	1.611
28	JGD_Trefethen/Trefethen_20000	768	4.233	512	3.700	768	3.843
29	ND/nd24k	384	5.350	512	5.570	768	5.485
30	Janna/Flan_1565	384	7.722	512	7.921	768	8.419
31	Oberwolfach/bone010	384	7.117	768	7.350	768	7.525
32	Janna/StocF-1465	768	8.337	768	9.455	768	9.869
33	GHS_psdef/audikw_1	384	10.419	768	10.630	768	10.680
34	Janna/Fault_639	384	14.392	768	14.350	768	14.260
35	Janna/Hook_1498	512	17.019	384	16.860	768	16.960
36	Janna/Emilia_923	768	23.221	384	22.620	768	23.060
37	Janna/Geo_1438	768	29.654	768	29.250	1024	29.380
38	Janna/Serena	768	52.830	384	51.170	768	51.900

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Table 7.2. DAG information along with the times (seconds) spent by the master thread to submit all the tasks to the runtime system on a subset of our test matrices

#	Name	DAG stats		SpLLT (StarPU)
#	Name	# task	time/task avg. (ms)	task insert (s)	factor. time (s)
1	Schmid/thermal2	166695	0.071	2.213	2.215
2	Rothberg/gearbox	19392	0.417	0.326	0.339
8	DNVS/thread	7481	0.746	0.155	0.222
13	Schenk_AFE/af _shell3	65195	0.250	0.906	0.906
15	AMD/G3_circuit	290235	0.154	3.558	3.561
19	GHS_psdef/ldoor	126786	0.230	1.724	1.726
24	GHS_psdef/apache2	163804	0.264	2.052	2.053
32	Janna/Flan_1565	226486	1.007	3.452	8.419
35	Janna/Hook_1498	639060	0.867	8.441	16.960
38	Janna/Serena	795367	1.857	10.920	51.900

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Table A.1. Test matrices and their characteristics without node amalgamation. $n$ is the matrix order, $nz(A)$ represent the number entries in the matrix $A$, $nz(L)$ represent the number of entries the factor $L$ and Flops correspond to the operation count for the matrix factorization

#	Name	n (10³)	nz(A) (10⁶)	nz(L) (10⁶)	Flops (10⁹)	Application/Description
1	Schmid/thermal2	1228	4.9	51.6	14.6	Unstructured thermal FEM
2	Rothberg/gearbox	154	4.6	37.1	20.6	Aircraft flap actuator
3	DNVS/m_t1	97.6	4.9	34.2	21.9	Tubular joint
4	Boeing/pwtk	218	5.9	48.6	22.4	Pressurised wind tunnel
5	Chen/pkustk13	94.9	3.4	30.4	25.9	Machine element
6	GHS_psdef/crankseg_1	52.8	5.3	33.4	32.3	Linear static analysis
7	Rothberg/cfd2	123	1.6	38.3	32.7	CFD pressure matrix
8	DNVS/thread	29.7	2.2	24.1	34.9	Threaded connector
9	DNVS/shipsec8	115	3.4	35.9	38.1	Ship section
10	DNVS/shipsec1	141	4.0	39.4	38.1	Ship section
11	GHS_psdef/crankseg_2	63.8	7.1	43.8	46.7	Linear static analysis
12	DNVS/fcondp2	202	5.7	52.0	48.2	Oil production platform
13	Schenk_AFE/af_shell3	505	9.0	93.6	52.2	Sheet metal forming
14	DNVS/troll	214	6.1	64.2	55.9	Structural analysis
15	AMD/G3_circuit	1586	4.6	97.8	57.0	Circuit simulation
16	GHS_psdef/bmwcra_1	149	5.4	69.8	60.8	Automotive crankshaft
17	DNVS/halfb	225	6.3	65.9	70.4	Half-breadth barge
18	Um/2cubes_sphere	102	0.9	45.0	74.9	Electromagnetics
19	GHS_psdef/ldoor	952	23.7	144.6	78.3	Large door
20	DNVS/ship_003	122	4.1	60.2	81.0	Ship structure
21	DNVS/fullb	199	6.0	74.5	100.2	Full-breadth barge
22	GHS_psdef/inline_1	504	18.7	172.9	144.4	Inline skater
23	Chen/pkustk14	152	7.5	106.8	146.4	Tall building
24	GHS_psdef/apache2	715	2.8	134.7	174.3	3D structural problem
25	Koutsovasilis/F1	344	13.6	173.7	218.8	AUDI engine crankshaft
26	Oberwolfach/boneS10	915	28.2	278.0	281.6	Bone micro-FEM
27	ND/nd12k	36.0	7.1	116.5	505.0	3D mesh problem
28	JGD_Trefethen/Trefethen_20000	20.0	0.3	90.7	652.6	Integer matrix
29	ND/nd24k	72.0	14.4	321.6	2054.4	3D mesh problem
30	Janna/Flan_1565	1565	59.5	1477.9	3859.8	3D mechanical problem
31	Oberwolfach/bone010	987	36.3	1076.4	3876.2	Bone micro-FEM
32	Janna/StocF-1465	1465	11.2	1126.1	4386.6	Underground aquifer
33	GHS_psdef/audikw_1	944	39.3	1242.3	5804.1	Automotive crankshaft
34	Janna/Fault_639	639	14.6	1144.7	8283.9	Gas reservoir
35	Janna/Hook_1498	1498	31.2	1532.9	8891.3	Steel hook
36	Janna/Emilia_923	923	21.0	1729.9	13661.1	Gas reservoir
37	Janna/Geo_1438	1438	32.3	2467.4	18058.1	Underground deformation
38	Janna/Serena	1391	33.0	2761.7	30048.9	Gas reservoir

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