disk.frame

Introduction

The disk.frame package aims to be the answer to the question: how do I manipulate structured tabular data that doesn’t fit into Random Access Memory (RAM)?

In a nutshell, disk.frame makes use of two simple ideas

1. split up a larger-than-RAM dataset into chunks and store each chunk in a separate file inside a folder and
2. provide a convenient API to manipulate these chunks

disk.frame performs a similar role to distributed systems such as Apache Spark, Python’s Dask, and Julia’s JuliaDB.jl for medium data which are datasets that are too large for RAM but not quite large enough to qualify as big data that require distributing processing over many computers to be effective.

Installation

You can install the released version of disk.frame from CRAN with:

install.packages("disk.frame")

And the development version from GitHub with:

# install.packages("devtools")
devtools::install_github("xiaodaigh/disk.frame")

Vignette

Please see this vignette Introduction to disk.frame which replicates the sparklyr vignette for manipulating the nycflights13 flights data.

Common questions

a) What is disk.frame and why create it?

disk.frame is an R package that provides a framework for manipulating larger-than-RAM structured tabular data on disk efficiently. The reason one would want to manipulate data on disk is that it allows arbitrarily large datasets to be processed by R. In other words, we go from “R can only deal with data that fits in RAM” to “R can deal with any data that fits on disk”. See the next section.

b) How is it different to data.frame and data.table?

A data.frame in R is an in-memory data structure, which means that R must load the data in its entirety into RAM. A corollary of this is that only data that can fit into RAM can be processed using data.frames. This places significant restrictions on what R can process with minimal hassle.

In contrast, disk.frame provides a framework to store and manipulate data on the hard drive. It does this by loading only a small part of the data, called a chunk, into RAM; process the chunk, write out the results and repeat with the next chunk. This chunking strategy is widely applied in other packages to enable processing large amounts of data in R, for example, see chunkded arkdb, and iotools.

Furthermore, there is a row-limit of 2^31 for data.frames in R; hence an alternate approach is needed to apply R to these large datasets. The chunking mechanism in disk.frame provides such an avenue to enable data manipulation beyond the 2^31 row limit.

c) How is disk.frame different to previous “big” data solutions for R?

R has many packages that can deal with larger-than-RAM datasets, including ff and bigmemory. However, ff and bigmemory restrict the user to primitive data types such as double, which means they do not support character (string) and factor types. In contrast, disk.frame makes use of data.table::data.table and data.frame directly, so all data types are supported. Also, disk.frame strives to provide an API that is as similar to data.frame’s where possible. disk.frame supports many dplyr verbs for manipulating disk.frames.

Additionally, disk.frame supports parallel data operations using infrastructures provided by the excellent future package to take advantage of multi-core CPUs. Further, disk.frame uses state-of-the-art data storage techniques such as fast data compression, and random access to rows and columns provided by the fst package to provide superior data manipulation speeds.

d) How does disk.frame work?

disk.frame works by breaking large datasets into smaller individual chunks and storing the chunks in fst files inside a folder. Each chunk is a fst file containing a data.frame/data.table. One can construct the original large dataset by loading all the chunks into RAM and row-bind all the chunks into one large data.frame. Of course, in practice this isn’t always possible; hence why we store them as smaller individual chunks.

disk.frame makes it easy to manipulate the underlying chunks by implementing dplyr functions/verbs and other convenient functions (e.g. the map.disk.frame(a.disk.frame, fn, lazy = F) function which applies the function fn to each chunk of a.disk.frame in parallel). So that disk.frame can be manipulated in a similar fashion to in-memory data.frames.

e) How is disk.frame different from Spark, Dask, and JuliaDB.jl?

Spark is primarily a distributed system that also works on a single machine. Dask is a Python package that is most similar to disk.frame, and JuliaDB.jl is a Julia package. All three can distribute work over a cluster of computers. However, disk.frame currently cannot distribute data processes over many computers, and is, therefore, single machine focused.

In R, one can access Spark via sparklyr, but that requires a Spark cluster to be set up. On the other hand disk.frame requires zero-setup apart from running install.packages("disk.frame") or devtools::install_github("xiaodaigh/disk.frame").

Finally, Spark can only apply functions that are implemented for Spark, whereas disk.frame can use any function in R including user-defined functions.

f) How is disk.frame different from multidplyr, partools and distributedR?

The packages multidplyr doesn’t seem to be disk-focussed and hence does not allow arbitrarily large dataset to be manipulated; the focus on parallel processing is similar to disk.frame though. For partools [https://matloff.wordpress.com/2015/08/05/partools-a-sensible-r-package-for-large-data-sets/], it seems to use it’s own verbs for aggregating data instead of relying on existing verbs provided by data.table and dplyr. The package distributedR hasn’t been updated for a few years and also seems to require its own functions and verbs.

Set-up disk.frame

disk.frame works best if it can process multiple data chunks in parallel. The best way to set-up disk.frame so that each CPU core runs a background worker is by using

setup_disk.frame()

# this allows large datasets to be transferred between sessions
options(future.globals.maxSize = Inf)

The setup_disk.frame() sets up background workers equal to the number of CPU cores; please note that, by default, hyper-threaded cores are counted as one not two.

Alternatively, one may specify the number of workers using setup_disk.frame(workers = n).

Example usage

library(dplyr)
library(disk.frame)
library(data.table)
library(fst)

# you need to run this to make multiple workers
# this will setup disk.frame's parallel backend with number of workers equal to the number of CPU cores (hyper-threaded cores are counted as one not two)
setup_disk.frame()
# this allows large datasets to be transferred between sessions
options(future.globals.maxSize = Inf)

rows_per_chunk = 1e7
# generate synthetic data
tmpdir = "tmpfst"
if(fs::dir_exists(tmpdir)) {
  fs::dir_delete(tmpdir)
}

# write out nworkers chunks
pt = proc.time()

df = disk.frame(tmpdir)

sapply(1:(nworkers*2), function(ii) {
  system.time(ab <- data.table(a = runif(rows_per_chunk), b = runif(rows_per_chunk)) ) #102 seconds
  add_chunk(df, ab, ii)
  NULL
})
cat("Generating data took: ", timetaken(pt), "\n")


# read and output the disk.frame as it to assess "sequential" read-write performance
pt = proc.time()
df2 <- map(df, ~.x, outdir = "tmpfst2", lazy = F, overwrite = T)
cat("Read and write took: ", timetaken(pt), "\n")

# get first few rows
head(df)

# get last few rows
tail(df)

# number of rows
nrow(df)

# number of columns
ncol(df)

Example: dplyr verbs

df = disk.frame(tmpdir)

df %>%
  summarise(suma = sum(a)) %>% # this does a count per chunk
  collect(parallel = T)

# need a 2nd stage to finalise summing
df %>%
  summarise(suma = sum(a)) %>% # this does a count per chunk
  collect(parallel = T) %>% 
  summarise(suma = sum(suma)) 

# filter
pt = proc.time()
system.time(df_filtered <- df %>% 
              filter(a < 0.1))
cat("filtering a < 0.1 took: ", timetaken(pt), "\n")
nrow(df_filtered)

Group by

Group-by in disk.frame are performed within each chunk, hence a two-stage group by is required to obtain the correct group by results. The two-stage approach is preferred for performance reasons too.

pt = proc.time()
res1 <- df %>% 
  filter(b < 0.1) %>% 
  mutate(blt005 = b < 0.05) %>% 
  group_by(blt005) %>% 
  summarise(suma = sum(a), n = n()) %>% 
  collect %>%
  group_by(blt005) %>% 
  summarise(suma = sum(suma), n = sum(n)) %>% 
cat("group by took: ", data.table::timetaken(pt), "\n")

However, a one-stage group_by is possible with a hard_group_by to first rechunk the disk.frame. This not recommended for performance reasons, as it can quite slow.

pt = proc.time()
res1 <- df %>% 
  filter(b < 0.1) %>% 
  mutate(blt005 = b < 0.05) %>% 
  hard_group_by(blt005) %>% # hard group_by is MUCH SLOWER but avoid a 2nd stage aggregation
  summarise(suma = sum(a), n = n()) %>% 
  collect(parallel = T)
cat("group by took: ", timetaken(pt), "\n")

Other dplyr verbs

# keep only one var is faster
pt = proc.time()
res1 <- df %>% 
  srckeep("a") %>% #keeping only the column `a` from the input
  summarise(suma = sum(a), n = n()) %>% 
  collect(parallel = T)
cat("summarise keeping only one column ", timetaken(pt), "\n")

# same operation without keeping
pt = proc.time()
res1 <- df %>% 
  summarise(suma = sum(a), n = n()) %>% 
  collect(parallel = T)
cat("summarise without keeping", timetaken(pt), "\n")

Example: data.table syntax

library(data.table)

# count by chunks
system.time(df_cnt_by_chunk <- df[,.N])
pt = proc.time()
system.time(sum(df[,.N])) # need a 2nd stage of summary
cat("sum(df[,.N]) took: ", timetaken(pt), "\n")

# filter
pt = proc.time()
system.time(df_filtered <- df[a < 0.1,])
cat("df[a < 0.1,] took: ", timetaken(pt), "\n")
nrow(df_filtered)

# group by
pt = proc.time()
system.time(res1 <- df[b < 0.1,.(sum_a = sum(a), .N), by = b < 0.05])
cat("df[b < 0.1,.(sum_a = sum(a), .N), by = b < 0.05] took: ", timetaken(pt), "\n")
# res1 has performed group by for each of the 4 chunks need to further summarise
system.time(res2 <- res1[, .(sum(sum_a), sum(N)),b][, .(mean_a = V1/V2), b])
res2 # abit painful to create mean, but currently only this low level interface; will do a dplyr on top later

# keep only one var is faster
pt = proc.time()
system.time(df[,.(sum(a)), keep = "a"][,sum(V1)]) # 1.17
cat("df[,.(sum(a)), keep = 'a'] took: ", timetaken(pt), "\n")

# same operation without keeping
pt = proc.time()
system.time(df[,.(sum(a))][,sum(V1)]) #2.95
cat("df[,.(sum(a))] took: ", timetaken(pt), "\n")

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Contributors

This project exists thanks to all the people who contribute.

Open Collective

If you like disk.frame and want to speed up its development or perhaps you have a feature request? Please considering sponsoring me on Open Collective

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