davidfetter / wc2

Investigates optimizing 'wc', the Unix word count program

wc2 - optimizing wc with asynchronous state machine parsing

There have been multiple articles lately implementing the classic wc program in various programming languages, to prove they can be "just as fast" as C.

In this project, instead of a different language we choose a different algorithm. We implement it in both C and JavaScript, both of which result in a program that's faster than the built-in wc program.

The algorithm is known as an "asynchronous state-mchine parser". The code is written for correct parsing of UTF-8, though the concept can be extended to support any character-set without changing the fundamental algorithm or benchmark speed.

This projects contains three versions:

• wc2o.c is a simplified 25 line version highlighting the idea
• wc2.c is the full version in C
• wc2.js is the version in JavaScript

There are some additional bits of code:

• wctool to generate large test files
• wcdiff to find difference between two implementatins of wc
• wcstream to fragment iput files (demonstrates a bug in macOS's wc)

The basic algorithm

The algorithm reads input and passes each byte one at a time to a state-machine. It looks something like:

length = fread(buf, 1, sizeof(buf), fp);
for (i=0; i<length; i++) {
    c = buf[i];
    state = table[state][c];
    counts[state]++;
}

No, you aren't suppose to be able to see how the word-count works by looking at this code. The complexity happens elsewhere, setting up the state-machine. Elswhere, we've created a large state-machine for parsing UTF-8 characters, which takes about 250 lines of code.

How wc works

None of those recent re-implementations of wc are doing what they claim. They are writing a simple algortihm for counting the single-byte ASCII character-sets. The real wc program supports arbitrary character sets, such as UTF-8. The real programs spend most of their time in functions like mbrtowc() to parse multi-byte characters and iswspace() to test if they are spaces -- which re-implementations of wc skip.

Therefore, to start with, I benchmark a bunch of different input files against existing versions of wc under macOS and Linux on my laptop (MacBook Air 2017).

The files are:

• pocorgtfo18.pdf a large 92-million byte PDF file that contains binary/illegal characters
• ascii.txt a file the same size containing random words, ASCII-only
• utf8.txt a file containing random UTF-8 sequences of 1, 2, 3, and 4 bytes
• word.txt a file containing 92-million 'x' characters
• space.txt a file containing 92-million ' ' (space) characters

Before running the benchmarks, the character-set is configured as:

$ export LC_CTYPE=en_US.UTF-8

When running wc, either -lwc is configured for single-byte text, or -lwm is configured for multibyte text.

The numbers reported come from the time command, the number of seconds for user time (not elapsed or system time).

Command	Input File	macOS	Linux
wc -lwc	pocorgtfo18.pdf	0.709	5.591
wc -lwm	pocorgtfo18.pdf	0.693	5.419
wc -lwc	ascii.txt	0.296	2.509
wc -lwm	utf8.txt	0.532	1.840
wc -lwc	space.txt	0.296	0.284
wc -lwm	space.txt	0.295	0.298
wc -lwc	word.txt	0.302	1.268
wc -lwm	word.txt	0.294	1.337

These results tell us:

• Illegal characters (in pocorgtfo18.pdf) slows things down a lot, twice as slow on macOS, 10x slower on Linux.
• Text that randomly switches between spaces and words is much slower than text containing all the same character.
• On Linux, the code path that reads all spaces is significantly faster.
• The macOS program is in general much faster than the Linux version.
• Processing Unicode (the file utf8.txt with the -m option) is slower than processing ASCII (the file ascii.txt with the -c option).

Our benchmarks

The time for our algorithm, in C and JavaScript, are the following.

Program	Input File	macOS	Linux
wc2.c	(all)	0.206	0.278
wc2.js	(all)	0.281	0.488

These results tell us:

• This state machine approach always results in the same speed, regardless of input.
• This state machine approach is faster than the built-in programs.
• Even written in JavaScript, the state machine approach is competitive in speed.
• The difference in macOS and Linux speed is actually the difference in clang and gcc speed. The LLVM clang compiler is doing better optimizations for x86 processors here.
• I don't know why Node.js behaves differently on macOS and Linux, it's probably just due to different versions.
• A JIT (like NodeJS) works well with simple compute algorithms. This tells us little about it's relative performance in larger programs. All languages that have a JIT should compile this sort of algorithm to roughly the same speed.

Asynchronous

The legacy way of parsing couples receiving input with parsing. What we are doing here is decoupling the two.

In other words, a common way of doing this would be to write a function like getline() or getword() that reads input inside the parser to the next line or word boundary. In our case, we read in 64k chunks at a time, regardless of where boundaries might exist, and pass each chunk to the parser.

There's really no benefit doing it this way for wc, but there is a huge benefit for network applications. The older Apache web-server does things in the legacy, non-asynchronous way, but is steadily being replaced by asynchronous servers like nginx and Lighthttpd that use asynchronous techniques.

State machine parsers

The minimalistic wc2o.c program is shown below in its entirety. We've hard-coded the state-machine here.

#include <stdio.h> int main(void) { static const unsigned char table[4][4] = { {2,0,1,0,}, {2,0,1,0,}, {3,0,1,0,}, {3,0,1,0,} }; static const unsigned char column[256] = { 0,0,0,0,0,0,0,0,0,1,2,1,1,1,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0, }; unsigned long counts[4] = {0,0,0,0}; int state = 0; int c;

while ((c = getchar()) != EOF) {
    state = table[state][column[c]];
    counts[state]++;
}

printf("%lu %lu %lu\n", counts[1], counts[2], 
            counts[0] + counts[1] + counts[2] + counts[3]);
return 0;

}

The key part that does all the word counting is in the two lines inside:

while ((c = getchar()) != EOF) {
    state = table[state][column[c]];
    counts[state]++;
}

This is only defined for ASCII, so you can see the state-machine on a single-line in the code (table).

The program wc2.c has the same logic, the difference being that it generates a larger state-machine for parsing UTF-8.

Pointer arithmetic

C has a peculiar idiom called "pointer arithmetic", where pointers can be incremented. Looping through a buffer is done with an expression like *buf++ instead of buf[i++]. Many programmers think pointer-arithmetic is faster.

To test this, the wc2.c program has an option -P that makes this small change, to test the difference in speed.