# Increasing RxODE speed by multi-subject parallel solving

RxODE originally developed as an ODE solver that allowed an ODE solve for a single subject. This flexibility is still supported.

The original code from the RxODE tutorial is below:


library(RxODE)
library(microbenchmark)
library(mvnfast)

mod1 <- RxODE({
C2 = centr/V2;
C3 = peri/V3;
d/dt(depot) = -KA*depot;
d/dt(centr) = KA*depot - CL*C2 - Q*C2 + Q*C3;
d/dt(peri) = Q*C2 - Q*C3;
d/dt(eff) = Kin - Kout*(1-C2/(EC50+C2))*eff;
eff(0) = 1
})

## Create an event table

ev <- et() %>%

nsub <- 100 # 100 subproblems
sigma <- matrix(c(0.09,0.08,0.08,0.25),2,2) # IIV covariance matrix
mv <- rmvn(n=nsub, rep(0,2), sigma) # Sample from covariance matrix
CL <- 7*exp(mv[,1])
V2 <- 40*exp(mv[,2])
params.all <- cbind(KA=0.3, CL=CL, V2=V2, Q=10, V3=300,
Kin=0.2, Kout=0.2, EC50=8)

## For Loop

The slowest way to code this is to use a for loop. In this example we will enclose it in a function to compare timing.

runFor <- function(){
res <- NULL
for (i in 1:nsub) {
params <- params.all[i,]
x <- mod1$solve(params, ev) ##Store results for effect compartment res <- cbind(res, x[, "eff"]) } return(res) } ## Running with apply In general for R, the apply types of functions perform better than a for loop, so the tutorial also suggests this speed enhancement runSapply <- function(){ res <- apply(params.all, 1, function(theta) mod1$run(theta, ev)[, "eff"])
}

## Run using a single-threaded solve

You can also have RxODE solve all the subject simultaneously without collecting the results in R, using a single threaded solve.

The data output is slightly different here, but still gives the same information:

runSingleThread <- function(){
solve(mod1, params.all, ev, cores=1)[,c("sim.id", "time", "eff")]
}

## Run a 2 threaded solve

RxODE supports multi-threaded solves, so another option is to have 2 threads (called cores in the solve options, you can see the options in rxControl() or rxSolve()).

run2Thread <- function(){
solve(mod1, params.all, ev, cores=2)[,c("sim.id", "time", "eff")]
}

## Compare the times between all the methods

Now the moment of truth, the timings:

bench <- microbenchmark(runFor(), runSapply(), runSingleThread(),run2Thread())
print(bench)
#> Unit: milliseconds
#>               expr       min        lq      mean    median        uq       max
#>           runFor() 393.56040 415.46669 456.35749 435.61480 484.87974 644.28239
#>        runSapply() 385.91672 413.22457 444.73684 426.61598 457.66543 584.12210
#>  runSingleThread()  78.60839  87.12926  88.38308  88.12848  89.37643  96.85980
#>       run2Thread()  46.18088  47.15559  49.12363  48.02319  50.34936  73.49148
#>  neval
#>    100
#>    100
#>    100
#>    100
plot(bench) It is clear that the largest jump in performance when using the solve method and providing all the parameters to RxODE to solve without looping over each subject with either a for or a sapply. The number of cores/threads applied to the solve also plays a role in the solving.

We can explore the number of threads further with the following code:

runThread <- function(n){
solve(mod1, params.all, ev, cores=n)[,c("sim.id", "time", "eff")]
}

bench <- eval(parse(text=sprintf("microbenchmark(%s)",
collapse=","))))
print(bench)
#> Unit: milliseconds
#>           expr      min       lq     mean   median       uq      max neval
#>   runThread(1) 85.93964 89.42870 90.41736 90.25587 91.11118 97.16488   100
#>   runThread(2) 46.74005 47.74302 49.00056 48.32068 48.88359 97.39354   100
#>   runThread(3) 33.17851 33.99224 35.20155 34.70399 35.45566 43.65722   100
#>   runThread(4) 26.47490 27.36947 28.29384 27.90727 28.43507 34.32530   100
#>   runThread(5) 22.59812 23.40820 25.47006 24.13421 26.91797 38.77258   100
#>   runThread(6) 20.05099 21.06165 22.86566 21.94901 23.84196 34.94874   100
#>   runThread(7) 26.18783 27.62378 29.31943 28.91787 31.17966 34.97496   100
#>   runThread(8) 24.21677 26.14518 27.30895 27.38362 28.24314 31.91728   100
#>   runThread(9) 22.87060 25.06679 26.06804 26.27812 26.96363 31.02734   100
#>  runThread(10) 20.89669 23.90599 24.83284 25.03631 25.92178 30.16645   100
#>  runThread(11) 20.85084 22.89568 23.84426 23.71278 24.72666 28.18737   100
#>  runThread(12) 21.15476 22.73106 24.88891 23.64608 25.59363 43.43209   100
plot(bench) There is a suite spot in speed vs number or cores. The system type, complexity of the ODE solving and the number of subjects may affect this arbitrary number of threads. 4 threads is a good number to use without any prior knowledge because most systems these days have at least 4 threads (or 2 processors with 4 threads).

# A real life example

Before some of the parallel solving was implemented, the fastest way to run RxODE was with lapply. This is how Rik Schoemaker created the data-set for nlmixr comparisons:

library(RxODE)
library(data.table)
#Define the RxODE model
ode1 <- "
d/dt(abs)    = -KA*abs;
d/dt(centr)  =  KA*abs-(CL/V)*centr;
C2=centr/V;
"

#Create the RxODE simulation object
mod1 <- RxODE(model = ode1)

#Population parameter values on log-scale
paramsl <- c(CL = log(4),
V = log(70),
KA = log(1))
#make 10,000 subjects to sample from:
nsubg <- 2500 # subjects per dose
doses <- c(10, 30, 60, 120)
nsub <- nsubg * length(doses)
#IIV of 30% for each parameter
omega <- diag(c(0.09, 0.09, 0.09))# IIV covariance matrix
sigma <- 0.2
#Sample from the multivariate normal
set.seed(98176247)
library(MASS)
mv <-
mvrnorm(nsub, rep(0, dim(omega)), omega) # Sample from covariance matrix
#Combine population parameters with IIV
params.all <-
data.table(
"ID" = seq(1:nsub),
"CL" = exp(paramsl['CL'] + mv[, 1]),
"V" = exp(paramsl['V'] + mv[, 2]),
"KA" = exp(paramsl['KA'] + mv[, 3])
)
#set the doses (looping through the 4 doses)
params.all[, AMT := rep(1000 * doses,nsubg)]

Startlapply <- Sys.time()

#Run the simulations using lapply for speed
s = lapply(1:nsub, function(i) {
#selects the parameters associated with the subject to be simulated
params <- params.all[i]
#creates an eventTable with 7 doses every 24 hours
ev <- eventTable()
ev$add.dosing( dose = params$AMT,
nbr.doses = 1,
dosing.to = 1,
rate = NULL,
start.time = 0
)
#generates 4 random samples in a 24 hour period
ev$add.sampling(c(0, sort(round(sample(runif(600, 0, 1440), 4) / 60, 2)))) #runs the RxODE simulation x <- as.data.table(mod1$run(params, ev))
#merges the parameters and ID number to the simulation output
x[, names(params) := params]
})

#runs the entire sequence of 10000 subjects and binds the results to the object res
res = as.data.table(do.call("rbind", s))

Stoplapply <- Sys.time()

print(Stoplapply - Startlapply)
#> Time difference of 1.282049 mins

By applying some of the new parallel solving concepts you can simply run the same simulation both with less code and faster:

rx <- RxODE({
CL =  log(4)
V = log(70)
KA = log(1)
CL = exp(CL + eta.CL)
V = exp(V + eta.V)
KA = exp(KA + eta.KA)
d/dt(abs)    = -KA*abs;
d/dt(centr)  =  KA*abs-(CL/V)*centr;
C2=centr/V;
})

omega <- lotri(eta.CL ~ 0.09,
eta.V ~ 0.09,
eta.KA ~ 0.09)

doses <- c(10, 30, 60, 120)

startParallel <- Sys.time()
ev <- do.call("rbind",
lapply(seq_along(doses), function(i){
et() %>%
et(amt=doses[i]) %>% # Add single dose
et(0) %>% # Add 0 observation
## Generate 4 samples in 24 hour period
et(lapply(1:4, function(...){c(0, 24)})) %>%
et(id=seq(1, nsubg) + (i - 1) * nsubg) %>%
## Convert to data frame to skip sorting the data
## When binding the data together
as.data.frame
}))
## To better compare, use the same output, that is data.table
res <- rxSolve(rx, ev, omega=omega, returnType="data.table")
#> [====|====|====|====|====|====|====|====|====|====] 0:00:01
endParallel <- Sys.time()
print(endParallel - startParallel)
#> Time difference of 5.054943 secs

You can see a striking time difference between the two methods; A few things to keep in mind:

• RxODE uses mvnfast for simulation of eta values. This uses the thread safe sumo random number generator. Therefore the results are expected to be different (also the random samples are taken in a different order which would be different)

• This prior simulation was run in R 3.5, which has a different random number generator so the results in this simulation will be different from the actual nlmixr comparison when using the slower simulation.

• This speed comparison used data.table. RxODE uses data.table internally (when available) try to speed up sorting, so this would be different than installations where data.table is not installed. You can force RxODE to use order() when sorting by using forderForceBase(TRUE). In this case there is little difference between the two, though in other examples data.table’s presence leads to a speed increase (and less likely it could lead to a slowdown).

## Want more ways to run multi-subject simulations

The version since the tutorial has even more ways to run multi-subject simulations, including adding variability in sampling and dosing times with et() (see RxODE events for more information), ability to supply both an omega and sigma matrix as well as adding as a thetaMat to R to simulate with uncertainty in the omega, sigma and theta matrices; see RxODE simulation vignette.

## Session Information

The session information:

sessionInfo()
#> R version 3.6.0 (2019-04-26)
#> Platform: x86_64-pc-linux-gnu (64-bit)
#> Running under: Gentoo/Linux
#>
#> Matrix products: default
#> BLAS:   /usr/lib64/libblas.so.3.8.0
#> LAPACK: /usr/lib64/liblapack.so.3.8.0
#>
#> locale:
#>   LC_CTYPE=en_US.utf8       LC_NUMERIC=C
#>   LC_TIME=en_US.utf8        LC_COLLATE=en_US.utf8
#>   LC_MONETARY=en_US.utf8    LC_MESSAGES=en_US.utf8
#>   LC_PAPER=en_US.utf8       LC_NAME=C
#>  LC_MEASUREMENT=en_US.utf8 LC_IDENTIFICATION=C
#>
#> attached base packages:
#>  stats     graphics  grDevices utils     datasets  methods   base
#>
#> other attached packages:
#>  MASS_7.3-51.4        data.table_1.12.8    mvnfast_0.2.5
#>  microbenchmark_1.4-7 RxODE_0.9.2-0
#>
#> loaded via a namespace (and not attached):
#>   Rcpp_1.0.4.6     compiler_3.6.0   pillar_1.4.3     PreciseSums_0.3
#>   tools_3.6.0      digest_0.6.25    lattice_0.20-38  evaluate_0.14
#>   memoise_1.1.0    lifecycle_0.2.0  tibble_3.0.0     gtable_0.3.0
#>  pkgconfig_2.0.3  rlang_0.4.5      Matrix_1.2-17    cli_2.0.2
#>  yaml_2.2.1       pkgdown_1.4.1    xfun_0.12        dplyr_0.8.5
#>  stringr_1.4.0    knitr_1.28       desc_1.2.0       fs_1.3.2
#>  vctrs_0.2.4      tidyselect_1.0.0 rprojroot_1.3-2  grid_3.6.0
#>  glue_1.4.0       R6_2.4.1         fansi_0.4.1      rmarkdown_2.1
#>  dparser_0.1.8    purrr_0.3.3      ggplot2_3.3.0    magrittr_1.5
#>  units_0.6-6      backports_1.1.6  scales_1.1.0     htmltools_0.4.0
#>  ellipsis_0.3.0   assertthat_0.2.1 mime_0.9         colorspace_1.4-1
#>  lotri_0.2.2      stringi_1.4.6    munsell_0.5.0    markdown_1.1
#>  crayon_1.3.4