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FORKRUN

forkrun is an extremely fast pure-bash function that leverages bash coprocs to efficiently run several commands simultaneously in parallel (i.e., it's a "loop parallelizer").

forkrun is used in much the same way that xargs or parallel are, but is faster (see the hyperfine_benchmark subdirectory for benchmarks) while still being full-featured and only requires having a fairly recent bash version (4.0+) to run<sup>1</sup>. forkrun:

<sup>1: bash 5.1+ is preffered and much better tested. A few basic filesystem operations (rm, mkdir) must also be available. fallocate and inotifywait are not required; but, if present, will be used to lower runtime resource usage. bash-completion is required to enable automatic completion (on <TAB> press) when typing the forkrun cmdline.</sup>

CURRENT VERSION: forkrun v1.4.0

PREVIOUS VERSION: forkrun v1.3.0

CHANGELOG

forkrun v1.4: 3 new features have been added:

  1. forkrun can now dynamically determine how many coprocs to spawn based on runtime conditions (specifically: CPU usage and whether or not coprocs are waiting in a read queue to read data from stdin). To use this functionality, pass the ;-j; flag a negative number (just passing -j - works too). See the help (run forkrun --help or forkrun --help=all) for additional info.
  2. forkrun can now read its input from a file descriptor other than stdin using the -u flag (which is standard in bash's read and mapfile commands). MINOR API CHANGE: the existing -u flag, which prevents escaping the commands given on forkrun's commandline, has been changed to use -U or --UNESCAPE (i.e., it is now uppercase instead of lowercase).
  3. on x86_64 platforms, forkrun will use a custom bash loadable builtin to call lseek, greatly improving the efficiency of reading data from stdin as forkrun runs. forkrun's "no-load" speed (i.e., stdin is all newlines and they are being passed to a : call) now exceeds 4 million lines per second on my system, highlighting how efficient forkrun's parallelization framework is. Using lseek also removes dd from the "required dependency" list, even when using NULL-delimited input.

forkrun v1.3: forkrun {-z|-0|--null} has been fixed and now works 100% reliably with NULL-delimited input! However, [only] when using NULL-delimited input dd is now a required dependency.

forkrun v1.2: forkrun now supports bash automatic completion. Pressing <TAB> while typing out the forkrun commandline will auto-complete (when possible) forkrun options, command names, and command options. If no unique auto-complete result is available, pressing <TAB> a second time will bring up a list of possibilities. The code required for this functionality is loaded (via the _forkrun_complete function) and registered (via the complete builtin) when forkrun.bash is sourced.

NOTE: forkrun uses mildly "fuzzy" option matching, so to make the automatic completion feature actually useful only the single most reasonable completion is shown for any given forkrun option. e.g., the completion for typing --pip will only show --pipe, but if you continue typing --pipe-r the completion will change to --pipe-read, since --pipe and --pipe-read are both aliases for the same option (-p).

forkrun v1.1: 2 new flags (-b <bytes> and -B <bytes>) that cause forkrun to split up stdin into blocks of <bytes> bytes. -B will wait and accululate blocks of exactly <bytes> bytes, -b will not. The -I flag has been expanded so that if -k (or -n) is also passed then a second susbstitution is made, swapping {IND} for the batch ordering index (the same thing that -n outputs at the start of each block) ({ID} will still be swapped for coproc ID). A handful of optimizations and bug-fixes have also been implemented (notably with how the coproc source code is dynamically generated). Lastly. the forkrun repo had some changes to how it is organized.

NOTE: for the -b and -B flags to have the sort of effeciency and speed that forkrun typically has, you need to have GNU dd available. If you dont, forkrun will try to use head -c (which is much slower), and if thats unavailable itll use the read builtin with either -n or -N (which is much slower still...You really want to use GNU dd here). Also, when using these flags the -S flag is automatically selected, meaning data is passed to the function being parallelized via its stdin. This is to avoid mangling binary data passed on stdin. This can be overruled by passing the+S flag, but all NULLs in stdin will be dropped.

USAGE

forkrun is invoked in much the same way as xargs: on the command-line, pass forkrun options, then the function/script/binary that you are parallelizing, then any initial constant arguments (in that order). The arguments to parallelize running are passed to forkrun on stdin. A typical forkrun invocation looks something like this:

printf '%s\n' "${inArgs[@]}" | forkrun [flags] [--] <parFunc> ["${args0[@]}"]
forkrun [flags] [--] <parFunc> ["${args0[@]}"] <inArgs

forkrun strives to automatically choose reasonable and near-optimal values for flags, so in most usage scenarios no flags will need to be set to attain maximum performance and speed.

NOTE: you'll need to source forkrun before using it

source /path/to/forkrun.bash

Alternately, if you dont have forkrun.bash saved locally but have internet access (or want to ensure you are using the latest version), you can run

source <(curl https://raw.githubusercontent.com/jkool702/forkrun/main/forkrun.bash)

Or, for those (understandably) concerned with directly sourcing unseen code from the internet, you can use

source <(echo 'shopt -s extglob'; ( cd /proc/self/fd; decfun='forkrun forkrun_displayHelp '; type -p cat &>/dev/null || decfun+='cat '; type -p mktemp &>/dev/null || decfun+='mktemp '; shopt -s extglob; curl="$(type -p curl)"; bash="$(type -p bash)"; PATH=''; { $curl https://raw.githubusercontent.com/jkool702/forkrun/main/forkrun.bash; echo 'declare -f '"$decfun"; } | $bash -r ) )

This monster of a one-liner will source the forkrun code in an extremely restricted shell that really cant do much else, then declare -f the required forkrun functions and finally the main shell sources those. This drops any non-forkrun-related code and ensures that nothing is actually run until the forkrun function is called, giving you a chance to review the code via declare -f (should you wish). This offers some protection against a bad actor maliciously changing the code (without your nor my knowledge) through some attack.

PARALLELIZING FUNCTIONS: one extremely powerful feature of forkrun is that it can parallelize arbitrarily complex tasks very efficiently by wrapping them in a function. This is done by doing something like the following:

myfun() {
    mapfile -t A < <(some_task "$@")
    some_other_task "${A[@]}"
    # ...
}
forkrun myfun <inputs

forkrun particularly excels at doing this since, unlike other loop parallelizers that have to spin up a whole new bash shell for every function invocation, forkrun simply calls this function directly. For simple functions (e.g., myfun() { :; }), simply running myfun from an already-running bash shell is ~200x faster and more efficient than running export -f myfun; bash -c 'myfun' (overhead from 10,000 calls is ~55 seconds using bash -c vs ~1/4 second with direct calling).

HELP: the forkrun.bash script, when sourced, will source a helper function (forkrun_displayHelp) to display help. This is activated by calling one of the following:

--usage              :  display brief usage info
-? | -h | --help     :  dispay standard help (includes brief descriptions + short names for flags)
--help=s[hort]       :  more detailed varient of '--usage'
--help=f[lags]       :  display detailed info about flags (longer descriptions, short + long names)
--help=a[ll]         :  display all help (includes detailed descriptions for flags)

NOTE: text inside of the brackets [...] is optional. NOTE: forkrun -? may not work unless you escape the ?. i.e., forkrun -\? or forkrun '-?'

SPEED: In my testing, forkrun was, on average (for problems where the efficiency of the parallelization framework actually makes a difference) ~20% faster to twice as fast versus xargs -P $(nproc); and ~2x to ~8x as fast versus parallel -m. To be clear: these are the "fast" invocations of xargs and parallel. If you were to compare the "1 line at a time" version of all 3 (forkrun -l1, xargs -P $(nproc) -L 1, parallel -j $(nprooc)), forkrun is 7-10x as fast as xargs and 20-30x as fast as parallel.

HOW IT WORKS

BASH COPROCS: forkrun parallelizes loops by running multiple inputs through a script/function in parallel using bash coprocs. forkrun is fundamentally different than most existing loop parallelization codes in the sense that individual function evaluations are not forked. Rather, initially, a number of persistent bash coprocs are forked, and then inputs (passed on stdin) are distributed to these coprocs without any additional forking (or reopening pipes/fd's, or...). In other words, you FORK [coprocs], then you RUN. This, combined with the (almost) exclusive use of bash builtins as well as being heavily optimized, is what makes forkrun so fast.

Coproc workers read data (via mapfile) using a shared file descriptor, keeping them all "in sync" and allowing workers to read data "on demand" without any buffers. This necessitates only allowing 1 worker to read data at a time, but has the benefit of avoiding the CPU overhead of, the potential bottleneck (especially on high-core count systems) from, and the need to buffer inputs due to having a separate process explicitly distribute stdin to the worker coprocs. Typically a "helper" coproc saves stdin to a tmpfs temp file (under /dev/shm) and workers read from the tmpfile (avoiding the "read 1 byte at a time from pipes" issue); but, depending on the flags passed to forkrun, the "workers" can also read data directly from the stdin pipe.

Compared to the traditional approach, forkrun's approach allows for much better use of multiple CPU cores in parallel, since nearly every task can be run in parallel. The traditional "forking every function call" method, on the other hand, can be implemented such that is takes slightly fewer total CPU cycles, but that comes at the cost of having most of the CPU cores waiting while the main thread is setting up for / cleaning up after the forked function call. To be clear here, I am referring to forking in a lower-level language like C...forking in bash is horribly slow and inefficient, and any pure-bash loop parallelizer that forks every function call will take orders of magnitude more time and CPU cycle to run.

To give a real-life data point: on my test rig that has a 14C/28T i9-7940x CPU, perf consistently tells me that for sufficiently large problems, forkrun is (averaged over its entire runtime) fully utilizing between 19-20 cores. Considering that the machine only has 14 "real" cores and that28 threads running on 14 hyperthreaded cores will never be as capable as 28 threads running on 28 "real" non-hyperthreaded cores, doing (on average) nearly 20 cores worth of work is about as good as it gets.

AUTOMATIC BATCH SIZE ADJUSTMENT: by default, forkrun will automatically dynamically adjust how many lines are passed to the function each time it is called (batch size). The batch size starts at 1, and is dynamically adjusted upwards (but never downwards) up to a maximum of 512 lines per batch (which is typically near-optimal in my personal trial-and-error testing). The logic used here involves:

  1. Calculating the average bytes/line by looking number of lines read and the number of bytes read (from /proc/self/fdinfo/$fd)
  2. Estimating the number of remaining lines left to read by getting the difference in the number of bytes read/written and dividing by the average bytes/line
  3. dividing the estimated number of remaining lines by the number of worker coprocs

NOTE: this is a "maximum lines per batch" (implemented via mapfile -n ${nLines})...if stdin is arriving slowly then fewer than this many lines will be used. What this serves to accomplish is to prevent a couple of coproc workers from claiming all the lines of input while the rest sit idle if the total number of lines is less than 512 * (# worker coprocs)

To overrule this logic and set a static batch size use the '-l' flag. Alternately, use the -L flag to keep the automatic batch size logic enabled but to change the initial and maximum number of lines per batch.

IPC: Forkrun distributes stdin to the worker coprocs by first saving them to a tmpfile (by default on a tmpfs -- in a directory under /dev/shm; customizable with the -t flag) using a forked coproc. The worker coprocs then read data from this file into an array (using mapfile) using a shared read-only file descriptor and an exclusive read lock.

NO FUNCTION MODE: forkrun supports an additional mode of operation where parFunc and initialArgs are not given as function inputs, but instead are integrated into each line of args. In this mode, each line passed on stin will be run as-is (by saving groups of 512 lines to tmp files and then sourcing them). This allows you to easily run multiple different functions/scripts/binaries in paralel and still utalize forkrun's very quick and efficient parallelization method. To activate this mode, use flag -N and do not provide parFunc or initialArgs. This is implemented via source <(printf '%s\n' "${args[@]}")

DEPENDENCIES

forkrun strives to rely on as few external dependencies as possible. It is almost pure-bash, though does have a handful of [optional] external dependencies:

REQUIRED DEPENDENCIES

Bash 4+ : This is when coprocs were added. NOTE: forkrun will be much faster on bash 5.1+, since it healivy relies of arrays and the mapfile command which got a major overhaul in bash 5.1. The vast majority of testing has been done on bash 5.2 so while bash 4-5.0 should work it is not well tested.

rm and mkdir: For some basic filesystem operations. I couldnt figure out how to re-implement these in pure bash. Either the GNU or the busybox versions of these will both work.

dd: Only when using NULL-delimited input (flag -z or -0 or --null), dd is required. It is used to quickly seek to a specific byte locaion in the file caching stdin and check it that byte is a NULL to detect and correct for partial line reads.

OPTIONAL DEPENDENCIES

Bash 5.1+: For improved speed due to overhauled handling of arrays.

mktemp and cat: The code will provide pure-bash replacements for these if they arent available, but if external binaries for these are present they will be used

inotifywait: If available, this is used to monitor the tmpfile where stdin is saved before being read by the coprocs. This enables the coprocs to efficiently wait for input if stdin is arriving slowly (e.g., ping 1.1.1.1 | forkrun <...>)

fallocate: If available, this is used to deallocate already-processed data from the beginning of the tmpfile holding stdin. This enables forkrun to be used in long-running processes that consistently output data for days/weeks/months/... Without fallocate, this tmpfile will continually grow and will not be removed until forkrun exits

dd (GNU) -OR- head (GNU|busybox): When splitting up stdin by byte count (due to either the -b or -B flag being used), if either of these is available it will be used to read stdin instead of the builtin read -N. Note that one of these is required to split + process binary data without mangling it - otherwise bash will drop any NULL's. If both are available dd is preferred.

bash-completion: Required for bash automatic completion (on <TAB> press) to work as you are typing the forkrun commandline. This is strictly a "quality of life" feature to maqke typing the nforkrun commandline easier -- it has zero effect on forkrun's execution after it has been called.

WHY USE FORKRUN

There are 2 other common programs for parallelizing loops in the (bash) shell: xargs and parallel. I believe forkrun offers more than either of these programs can offer:

COMPARED TO PARALLEL

COMPARED TO XARGS

SUPPORTED OPTIONS / FLAGS

forkrun supports many of the same flags as xargs (excluding options intended for interactive use), plus several additional options that are present in parallel but not xargs. A quick summary will be provided here - for more info refer to the comment block at the top of the forkrun function, or source forkrun and then run forkrun --help[={flags,all}].

GENERAL NOTES: 1. Flags must be given separately (e.g., use -k -v and not -kv) 2. Flags must be given before the name of the function being parallelized (parFunc) -- any flags given after the function name will be assumed to be initial arguments for the function, not forkrun options. 3. There are also "long" versions of the flags (e.g., --insert is the same as -i). Run forkrun --help=all for a full list of long options/flags.

The following flags are supported:

FLAGS WITH ARGUMENTS

   (-j|-p) <#>  : num worker coprocs. set number of worker coprocs. Default is $(nproc).
    -l <#>      : num lines per function call (batch size). set static number of lines to pass to the function on each function call. Disables automatic dynamic batch size adjustment. if -l=1 then the "read from a pipe" mode (-p) flag is automatically activated (unless flag `+p` is also given). Default is to use the automatic batch size adjustment.
    -L <#[,#]>  : set initial (<#>) or initial+maximum (<#,#>) lines per batch while keeping the automatic batch size adjustment enabled. Default is '1,512'
    -t <path>   : set tmp directory. set the directory where the temp files containing lines from stdin will be kept. These files will be saved inside a new mktemp-generated directory created under the directory specified here. Default is '/dev/shm', or (if unavailable) '/tmp'
 -d <delimiter> : set the delimiter to something other than a newline (default) or NULL ((-z|-0) flag). must be a single character.

FLAGS WITHOUT ARGUMENTS: for each of these passing -<FLAG> enables the feasture, and passing +<FLAG> disables the feature. Unless otherwise noted, all features are, by default, disabled. If a given flag is passed multiple times both enabling -<FLAG> and disabling +<FLAG> some option, the last one passed is used.

SYNTAX NOTE: for each of these passing `-<FLAG>` enables the feasture, and passing `+<FLAG>` disables the feature. Unless otherwise noted, all features are, by default, disabled. If a given flag is passed multiple times both enabling `-<FLAG>` and disabling `+<FLAG>` some option, the last one passed is used.

    -i          : insert {}. replace `{}` with the inputs passed on stdin (instead of placing them at the end)
    -I          : insert {id}. replace `{id}` with an index (0, 1, ...) describing which coproc the process ran on. 
    -k          : ordered output. retain input order in output. The 1st output will correspond to the 1st input, 2nd output to 2nd input, etc. 
    -n          : add ordering info to output. pre-pend each output group with an index describing its input order, demoted via `$'\n'\n$'\034'$INDEX$'\035'$'\n'`. This requires and will automatically enable the `-k` output ordering flag.
    (-0|-z)     : NULL-seperated stdin. stdin is NULL-separated, not newline separated. WARNING: this flag (by necessity) disables a check that prevents lines from occasionally being split into two separate lines, which can happen if `parFunc` evaluates very quickly. In general a delimiter other than NULL is recommended, especially when `parFunc` evaluates very fast and/or there are many items (passed on stdin) to evaluate.
    -s          : run in subshell. run each evaluation of `parFunc` in a subshell. This adds some overhead but ensures that running `parFunc` does not alter the coproc's environment and affect future evaluations of `parFunc`.
    -S          : pass via function's stdin. pass stdin to the function being parallelized via stdin ( $parFunc < /tmpdir/fileWithLinesFromStdin ) instead of via function inputs  ( $parFunc $(</tmpdir/fileWithLinesFromStdin) )
    -p          : pipe read. dont use a tmpfile and have coprocs read (via shared file descriptor) directly from stdin. Enabled by default only when `-l 1` is passed.
    -D          : delete tmpdir. Remove the tmp dir used by `forkrun` when `forkrun` exits. NOTE: the `-D` flag is enabled by default...disable with flag `+D`.
    -N          : enable no func mode. Only has an effect when `parFunc` and `initialArgs` are not given. If `-N` is not passed and `parFunc` and `initialArgs` are missing, `forkrun` will silently set `parFunc` to `printf '%s\n'`, which will basically just copy stdin to stdout.
    -u          : unescape redirects/pipes/forks/logical operators. Typically `parFunc` and `initialArgs` are run through `printf '%q'` making things like `<` , `<<` , `<<<` , `>` , `>>` , `|` , `&&` , and `||` appear as literal characters. This flag skips the `printf '%q'` call, meaning that these operators can be used to allow for piping, redirection, forking, logical comparison, etc. to occur *inside the coproc*. 
    --          : end of forkrun options indicator. indicate that all remaining arguments are for the function being parallelized and are not forkrun inputs. This allows using a `parFunc` that begins with a `-`. NOTE: there is no `+<FLAG>` equivalent for `--`.
    -v          : increase verbosity level by 1. This can be passed up to 4 times for progressively more verbose output. +v decreases the verbosity level by 1.
    (-h|-?)     : display help text. use `--help=f[lags]` or `--help=a[ll]` for more details about flags that `forkrun` supports. NOTE: you must escape the `?` otherwise the shell can interpret it before passing it to forkrun.