lapply() VS parallel::mclapply()

library(parallel)

benchmark(
  lapply(1000:9999, rnorm),
  mclapply(1000:9999, rnorm, mc.cores = 4)
)
                                      test replications elapsed relative 
1                 lapply(1000:9999, rnorm)          100 464.604    1.607 
2 mclapply(1000:9999, rnorm, mc.cores = 4)          100 289.162    1.000

반면에 input 이 상당히 적은경우 좋지 않은 성능

benchmark(
  lapply(10:99, rnorm),
  mclapply(10:99, rnorm, mc.cores = 4)
)
                                  test replications elapsed relative 
1                 lapply(10:99, rnorm)          100   0.118    1.000 
2 mclapply(10:99, rnorm, mc.cores = 4)          100   1.994   16.898

Filtering data.table, dplyr, data.frame

DF <- data.frame(x = runif(2.6e+07), y = rep(LETTERS, each = 10000))

DT <- as.data.table(DF)
setkey(DT, y) 

benchmark(
  x <- DF[DF$y == "C", ],
  x <- DT[J("C"), ],
  x <- DF %>% dplyr::filter(y == "C"),
  x <- DT %>% dplyr::filter(y == "C")
)
                                 test replications elapsed relative
2                   x <- DT[J("C"), ]          100   2.500    1.000
4 x <- DT %>% dplyr::filter(y == "C")          100  23.083    9.233
3 x <- DF %>% dplyr::filter(y == "C")          100  23.248    9.299
1              x <- DF[DF$y == "C", ]          100  38.298   15.319

동작환경

sessionInfo()
## R version 3.3.1 (2016-06-21)
## Platform: x86_64-apple-darwin13.4.0 (64-bit)
## Running under: OS X 10.12.3 (Sierra)
## 
## locale:
## [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
## 
## attached base packages:
## [1] parallel  stats     grDevices utils     datasets  graphics  methods  
## [8] base     
## 
## other attached packages:
## [1] rbenchmark_1.0.0            data.table_1.10.4          
## [3] ggplot2_2.2.1               dplyr_0.5.0                
## [5] knitr_1.15.1                useful.lovetoken_0.1.0.0090
## 
## loaded via a namespace (and not attached):
##  [1] Rcpp_0.12.9      magrittr_1.5     munsell_0.4.3    colorspace_1.3-2
##  [5] R6_2.2.0         stringr_1.2.0    plyr_1.8.4       tools_3.3.1     
##  [9] grid_3.3.1       gtable_0.2.0     pacman_0.4.1     DBI_0.5-1       
## [13] htmltools_0.3.5  yaml_2.1.14      lazyeval_0.2.0   assertthat_0.1  
## [17] digest_0.6.12    rprojroot_1.2    tibble_1.2       evaluate_0.10   
## [21] rmarkdown_1.3    stringi_1.1.2    scales_0.4.1     backports_1.0.5