r - Different runtime for svm and ranger using the same task

Question

I've bench-marked the runtime of the two learners and also took two screenshots of the {htop} while {ranger} and {svm} was training to make my point more clearer. As stated in the title of this post, my question is the reason Why train/predict in svm is so slow compare to other learners (in this case ranger)? Is it related to the underlying structure of the learners? Or I am making a mistake in the code? Or...? Any help is appreciated.

htop when ranger is training; all threads are used.

htop when svm is training; only 2 threads are used.

code:

library(mlr3verse)

library(future.apply)
#> Loading required package: future
library(future)
library(lgr)
library(data.table)

file <- 'https://drive.google.com/file/d/1Y3ao-SOFH/view?usp=sharing' 
#you can download the file using this link.
destfile = '/../Downloads/sample.RData'
download.file(file, destfile)


# pre-processing
sample$clc3_red <- as.factor(sample$clc3_red)
sample$X <- NULL
#> Warning in set(x, j = name, value = value): Column 'X' does not exist to remove
sample$confidence <- NULL
sample$ecoregion_id <- NULL
sample$gsw_occurrence_1984_2019 <- NULL

tsk_clf <- mlr3::TaskClassif$new(id = 'sample', backend = sample, target = "clc3_red")
tsk_clf$col_roles$group = 'tile_id' #spatial CV
tsk_clf$col_roles$feature = setdiff(tsk_clf$col_roles$feature ,  'tile_id')
tsk_clf$col_roles$feature = setdiff(tsk_clf$col_roles$feature ,  'x')
tsk_clf$col_roles$feature = setdiff(tsk_clf$col_roles$feature ,  'y')

# 2 learners for benchmarking
svm <- lrn("classif.svm", type = "C-classification", kernel = "radial", predict_type = "response")
ranger <- lrn("classif.ranger", predict_type = "response", importance = "permutation")

# ranger parallel
plan(multicore)
time <- Sys.time()
ranger$
  train(tsk_clf)$
  predict(tsk_clf)$
  score()
#> Warning: Dropped unused factor level(s) in dependent variable: 333, 335, 521,
#> 522.
#> classif.ce 
#>     0.0116
Sys.time() - time
#> Time difference of 20.12981 secs

# svm parallel
plan(multicore)
time <- Sys.time()
svm$
  train(tsk_clf)$
  predict(tsk_clf)$
  score()
#> classif.ce 
#>     0.4361
Sys.time() - time
#> Time difference of 55.13694 secs

^{Created on 2021-01-12 by the reprex package (v0.3.0)}

sessionInfo()
#> R version 4.0.2 (2020-06-22)
#> Platform: x86_64-pc-linux-gnu (64-bit)
#> Running under: Ubuntu 20.04.1 LTS
#> 
#> Matrix products: default
#> BLAS:   /opt/microsoft/ropen/4.0.2/lib64/R/lib/libRblas.so
#> LAPACK: /opt/microsoft/ropen/4.0.2/lib64/R/lib/libRlapack.so
#> 
#> locale:
#>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
#>  [3] LC_TIME=en_GB.UTF-8        LC_COLLATE=en_US.UTF-8    
#>  [5] LC_MONETARY=en_GB.UTF-8    LC_MESSAGES=en_US.UTF-8   
#>  [7] LC_PAPER=en_GB.UTF-8       LC_NAME=C                 
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#> [11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C       
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#>  [1] data.table_1.12.8   lgr_0.3.4           future.apply_1.6.0 
#>  [4] future_1.18.0       mlr3verse_0.1.3     paradox_0.3.0      
#>  [7] mlr3viz_0.1.1       mlr3tuning_0.1.2    mlr3pipelines_0.1.3
#> [10] mlr3learners_0.2.0  mlr3filters_0.2.0   mlr3_0.3.0         
#> 
#> loaded via a namespace (and not attached):
#>  [1] Rcpp_1.0.5         compiler_4.0.2     pillar_1.4.6       highr_0.8         
#>  [5] class_7.3-17       mlr3misc_0.3.0     tools_4.0.2        digest_0.6.25     
#>  [9] uuid_0.1-4         lattice_0.20-41    evaluate_0.14      lifecycle_0.2.0   
#> [13] tibble_3.0.3       checkmate_2.0.0    gtable_0.3.0       pkgconfig_2.0.3   
#> [17] rlang_0.4.7        Matrix_1.2-18      parallel_4.0.2     yaml_2.2.1        
#> [21] xfun_0.15          e1071_1.7-3        ranger_0.12.1      withr_2.2.0       
#> [25] stringr_1.4.0      knitr_1.29         globals_0.12.5     vctrs_0.3.2       
#> [29] grid_4.0.2         glue_1.4.1         listenv_0.8.0      R6_2.4.1          
#> [33] rmarkdown_2.3      ggplot2_3.3.2      magrittr_1.5       mlr3measures_0.2.0
#> [37] codetools_0.2-16   backports_1.1.8    scales_1.1.1       htmltools_0.5.0   
#> [41] ellipsis_0.3.1     colorspace_1.4-1   stringi_1.4.6      munsell_0.5.0     
#> [45] crayon_1.3.4

Answer 1

It is totally expected that an SVM runs longer than a random forest since SVMs scale badly with increasing number of observations and number of features. Things that make SVM more expensive:

• Multiclass: here, for each class one SVM is trained, because SVM natively can just solve binary classification
• Many categorical variables inflate the number of features as they are converted to numerical features through dummy encoding

There are approximate SVMs that are faster, but it's usually not worth to investigate further since they are often outperformed by tree based approaches (random forest) or boosting approaches (xgboost).

See also: Why does my SVM take so long to run? and How much time does take train SVM classifier?