Hardware Benchmarking Topic Single Precision VS Double Precision

Introduction

The TUFLOW Classic/HPC program download includes both single and double precision versions of the executable:

• TUFLOW_iSP_w64.exe = Single Precision
• TUFLOW_iDP_w64.exe = Double Precision.
  

This page of the Wiki discusses the functional difference between both, when one should be used instead of the other and also the speed and memory performance differences.

Both TUFLOW Classic and TUFLOW HPC can run using either a single precision (SP) or double precision (DP). When storing floating point values on a computer, a certain number of bytes per value is needed. Single precision numbers use 4 bytes and double precision numbers use 8 bytes. This will yield from 6 to 9 digits of precision for single precision and 15 to 17 digits for double. This page discuss the relative difference in performance of the SP and DP versions of TUFLOW. This includes comparisons for TUFLOW Classic, TUFLOW HPC on CPU hardware and TUFLOW HPC on GPU hardware.

Benchmark Model

The benchmark model used for this testing is based on a “challenge” issued prior to the 2012 Flood Managers Association (FMA) Conference in Sacramento, USA. There is more information on the model setup and purpose in the FMA challenge model introduction. This hardware benchmark is based on the second challenge which involves a coastal river in flood with two ocean outlets. The model has been modified slightly (mainly in terms of the outputs). It is setup to use both the TUFLOW Classic (CPU) and TUFLOW HPC (on both CPU and GPU hardware) with 20m cell size and 181,981 2D cells. The model runs for three days of simulation time (72 hours) and outputs xmdf data every two hours.

Benchmark Results

TUFLOW Classic

The table below has runtimes for the benchmark model at 20m cell size. The same model has been run for both the SP and DP versions of TUFLOW using the Classic solution scheme on CPU hardware. This same test has been performed on a number of CPU chips.

CPU	SP Runtime (mins)	DP Runtime (mins)	% Change
AMD Ryzen Threadripper 2990WX 32-Core Processor	65.8	80.3	22.0
Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz	71.7	87.4	21.9
Intel(R) Core(TM) i7-6800K CPU @ 3.40 GHz	90.0	119.1	32.3
Intel(R) Core(TM) i7-6900K CPU @ 3.20 GHz	90.5	109.3	20.7
Intel(R) Core(TM) i7-4790K CPU @ 4.00 GHz	91.3	115.2	26.2
Intel(R) Core(TM) i7-5960X CPU @ 3.00 GHz	101.9	128.8	26.4
Intel(R) Core(TM) i7-5820K CPU @ 3.30GHz	121.4	158.1	30.2
Intel(R) Xeon(R) CPU E3-1240 V2 @ 3.40 GHz	158.0	127.2	24.2
Intel(R) Xeon(R) CPU X5680 @ 3.33 GHz	162.1	207.6	28.1

TUFLOW HPC on CPU Hardware

The table below has runtimes for the benchmark model at 20m cell size. The same model has been run for both the SP and DP versions of TUFLOW using the HPC solution scheme on CPU hardware. This same test has been performed on a number of CPU chips.
Note The GPU code has been compiled for CPU execution so users can trial the solver without access to an NVidia GPU if necessary, but the solver has been first and foremost designed for Highly Parallel Compute on GPU hardware.

CPU	SP Runtime (mins)	DP Runtime (mins)	% Change
Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz	216.8	230.9	6.5
Intel(R) Core(TM) i7-4790K CPU @ 4.00 GHz	221.8	254.3	14.7
Intel(R) Core(TM) i7-5960X CPU @ 3.00 GHz	236.9	260.3	9.9
Intel(R) Core(TM) i7-6800K CPU @ 3.40 GHz	278.3	291.4	4.7
AMD Ryzen Threadripper 2990WX 32-Core Processor	278.8	347.7	24.7
Intel(R) Core(TM) i7-6900K CPU @ 3.20 GHz	286.0	328.9	15.0
Intel(R) Core(TM) i7-5820K CPU @ 3.30GHz	298.2	322.6	8.2
Intel(R) Xeon(R) CPU E3-1240 V2 @ 3.40 GHz	307.2	350.6	12.4
Intel(R) Xeon(R) CPU X5680 @ 3.33 GHz	404.0	466.4	15.5

TUFLOW HPC on GPU Hardware

For GPU devices, the quoted performance of GPU devices can be very different for single and double precision calculations. The table below has runtimes for the benchmark model at 20m cell size. The same model has been run for both the SP and DP versions of TUFLOW using the HPC solution scheme on GPU hardware. This same test has been performed on a number of different GPU cards.
Note In some cases, output drive writing speed can noticeably affect runtimes, especially when writing intermediate results. For example the runs on MS Azure Cloud were three to seven times slower than in the below table when storage network drive was used instead of a local disk.

GPU Card	SP Runtime (mins)	DP Runtime (mins)	% Change
NVIDIA Tesla V100 (MS Azure Cloud)	3.2	4.3	31.4
NVIDIA GeForce RTX 2080 Ti	5.1	11.3	123.1
NVIDIA TITAN Xp	5.7	10.6	87.6
NVIDIA GeForce RTX 2080 SUPER	7.0	14.0	100.5
NVIDIA GeForce RTX 2080	7.6	16.1	111.4
NVIDIA GeForce RTX 2070	8.9	18.4	107.3
NVIDIA GeForce GTX 1080 Ti	9.4	14.6	55.4
NVIDIA Tesla K80 (MS Azure Cloud)	10.8	15.4	42.3
NVIDIA GeForce GTX 1080	11.3	18.3	61.8
NVIDIA Quadro RTX 4000	17.6	18.2	3.4
NVIDIA GeForce GTX 980	17.7	29.8	68.0
NVIDIA GeForce GTX 750 Ti	28.6	72.8	60.8
NVIDIA GeForce 940MX (Laptop)	71.3	156.2	118.9
NVIDIA GeForce 840M (Laptop)	89.2	180.2	101.9

Conclusion

Simulation speed differences between single and double precision compute vary depending on the both the computational scheme and also the hardware being used for the simulation. Nevertheless, in general terms double precision calculations take slightly longer and require more memory for the field data. The memory requirement of DP is almost twice that of SP. There are a number of specific situations that will require DP compute for TUFLOW Classic (discussed in the following sections). With the exception of those particular cases, if the results of a model run in both SP and DP versions of TUFLOW prove to be similar (as is generally the case) the SP version of TUFLOW is recommended as it will be slightly faster and will enable larger models to be run within available CPU/GPU memory.

TUFLOW Classic

Under some situations TUFLOW Classic will require double precision compute to achieve an accurate solution. These situations include:

• Models with ground elevations greater than 100m or ft (depending on length unit used by your model); and
• Direct rainfall modelling.

TUFLOW Classic uses water level as the conserved variable in its implicit solution scheme. Due to this, some numerical precision can be lost under the above situations if single precision hardware is used. Loss of solution precision will be apparent by high mass balance error in the simulation log and result files. Single precision hardware can be used for all other situations without loss of accuracy or mass balance error issues. Running TUFLOW Classic (CPU hardware only) will on average increase simulation run times by approximately 20% when using double precision compared to single precision.

TUFLOW HPC

Unlike TUFLOW Classic, single precession compute will be suitable for the majority of applications when using TUFLOW HPC with no loss of accuracy. The calculation method in TUFLOW HPC uses depth as its conserved variable in the explicit solution scheme. As a result the precision issues associated with applying a very small rainfall volume in a single timestep, or alternatively modelling at high elevation are not applicable in HPC. Note that a reduced wet/dry depth of 0.0002m (0.0007ft) is still recommended for direct rainfall models. When TUFLOW HPC is used on CPU hardware the differences in simulation speed between single and double precision range from 5% to 25% depending on the processor specifications. Running TUFLOW HPC on GPU hardware shows even greater simulation speed differences between single and double precision. The precision solver that is required for running TUFLOW on GPU hardware will determine the type of GPU card that is best suited for the compute. For any given generation/architecture of cards, the “gaming” cards such as the GTX GeForce and RTX provide excellent single precision performance – typically comparable to that of the “scientific” cards such as the Tesla series. If double precision is required, the scientific cards are substantially faster, though it’s also noting that they are also significantly more expensive. The Quadro series of GPU card currently tend to represent a middle ground between the “gaming” and “scientific” both in terms of double precision performance and cost.