Rust simd-csv crate

Full documentation

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The simd-csv crate provides specialized readers & writers of CSV data able to leverage SIMD instructions.

It has been designed to fit the xan command line tool’s requirements, but can be used by anyone to speed up CSV parsing.

Is is less flexible and user-friendly than the csv crate, so one should make sure the performance gain is worth it before going further.

This crate is not a port of simdjson branchless logic applied to CSV parsing. It uses a somewhat novel approach instead, mixing traditional state machine logic with memchr-like SIMD-accelerated string searching. See the design notes for more details.

Examples

Reading a CSV file while amortizing allocations

use std::fs::File;
use simd_csv::{Reader, ByteRecord};

let mut reader = Reader::from_reader(File::open("data.csv")?);
let mut record = ByteRecord::new();

while reader.read_byte_record(&mut record)? {
for cell in record.iter() {
dbg!(cell);
}
}

Using a builder to configure your reader

use std::fs::File;
use simd_csv::ReaderBuilder;

let mut reader = ReaderBuilder::new()
.delimiter(b'\t')
.buffer_capacity(16 * (1 << 10))
.from_reader(File::open("data.csv")?);

Using the zero-copy reader

use std::fs::File;
use simd_csv::ZeroCopyReader;

let mut reader = ZeroCopyReader::from_reader(File::open("data.csv")?);

while let Some(record) = reader.read_byte_record()? {
// Only unescaping third column:
dbg!(record.unescape(2));
}

Counting records as fast as possible using the splitter

use std::fs::File;
use simd_csv::Splitter;

let mut splitter = Splitter::from_reader(File::open("data.csv")?);

println!("{}", splitter.count_records()?);

Readers

From least to most performant. Also from most integrated to most barebone.

• [Reader], [ReaderBuilder]: a streaming copy reader, unescaping quoted data on the fly. This is the closest thing you will find to the csv crate Reader.
• [ZeroCopyReader], [ZeroCopyReaderBuilder]: a streaming zero-copy reader that only find cell delimiters and does not unescape quoted data.
• [Splitter], [SplitterBuilder]: a streaming zero-copy splitter that will only find record delimitations, but not cell delimiters at all.
• [LineReader]: a streaming zero-copy line splitter that does not handle quoting at all.

You can also find more exotic readers like:

• [TotalReader], [TotalReaderBuilder]: a reader optimized to work with uses-cases when CSV data is fully loaded into memory or with memory maps.
• [Seeker], [SeekerBuilder]: a reader able to find record start positions in a seekable CSV stream. This can be very useful for parallelization, or more creative uses like performing binary search in a sorted file.
• [ReverseReader], [ReaderBuilder]: a reader able to read a seekable CSV stream in reverse, in amortized linear time.

Writers

• [Writer], [WriterBuilder]: a typical CSV writer.

Supported targets

• On x86_64 targets, sse2 instructions are used. avx2 instructions will also be used if their availability is detected at runtime.
• On aarch64 targets, neon instructions are used.
• On wasm targets, simd128 instructions are used.
• Everywhere else, the library will fallback to SWAR techniques or scalar implementations.

Using RUSTFLAGS='-C target-cpu=native' should not be required when compiling this crate because it either uses SIMD instructions tied to your target_arch already and because it will rely on runtime detection to find better SIMD instructions (typically avx2).

Design notes

Regarding performance

This crate’s CSV parser has been cautiously designed to offer “reasonable” performance by combinining traditional state machine logic with SIMD-accelerated string searching.

I say “reasonable” because you cannot expect to parse 16/32 times faster than a state-of-the-art scalar implementation like the csv crate. What’s more, the throughput of the SIMD-accelerated parser remains very data-dependent. Sometimes you will go up to ~8 times faster, sometimes you will only go as fast as scalar code. (Remember also that CSV parsing is often an IO-bound task, even more so than with other data formats usually expected to fit into memory like JSON etc.)

As a rule of thumb, the larger your records and cells, the greater the performance boost vs. a scalar byte-by-byte implementation. This also means that for worst cases, this crate’s parser will just be on par with scalar code. I have made everything in my power to ensure this SIMD parser is never slower (I think one of the reasons why SIMD CSV parsers are not yet very prevalent is that they tend to suffer real-life cases where scalar code outperform them).

Also, note that this crate is geared towards parsing streams of CSV data only quoted when needed (e.g. not written with a QUOTE_ALWAYS policy).

Regarding simdjson techniques

I have tried very hard to apply simdjson tricks to make this crate’s parser as branchless as possible but I couldn’t make it as fast as the state-machine/SIMD string searching hybrid.

PCLMULQDQ & shuffling tricks in this context only add more complexity and overhead to the SIMD sections of the code, all while making it less “democratic” since you need specific SIMD instructions that are not available everywhere, if you don’t want to fallback to slower instructions.

Said differently, those techniques seem overkill in practice for CSV parsing. But it is also possible I am not competent enough to make them work properly and I won’t hesitate to move towards them if proven wrong.

Hybrid design

This crate’s CSV parser follows a hybrid approach where we maintain a traditional state machine, but search for structural characters in the byte stream using SIMD string searching techniques like the ones implemented in the excellent memchr crate:

The idea is to compare 16/32 bytes of data at once with splats of structural characters like \n, " or ,. We then extract a “move mask” that will be handled as a bit string so we can find whether and where some character was found using typical bit-twiddling.

This ultimately means that branching happens on each structural character rather than on each byte, which is very good. But this is also the reason why CSV data with a very high density of structural characters will not get parsed much faster than with the equivalent scalar code.

Two-speed SIMD branches

This crate’s CSV parser actually uses two different modes of SIMD string searching:

1. when reading unquoted CSV data, the parser uses an amortized variant of the memchr routines where move masks containing more than a single match are kept and consumed progressively on subsequent calls, instead of restarting a search from the character just next to an earlier match, as the memchr_iter routine does.
2. when reading quoted CSV data, the parser uses the optmized & unrolled functions of the memchr crate directly to find the next quote as fast as possible.

This might seem weird but this seems to be the best tradeoff for performance. Counter-intuitively, using larger SIMD registers like avx2 for 1. actually hurts overall performance. Similarly, using the amortized routine to scan quoted data is actually slower than using the unrolled functions of memchr.

This actually makes sense if you consider that the longer a field is, the more probable it is to contain a character requiring the field to be quoted. What’s more the density of quotes to be found in a quoted field is usually lower that structural characters in an unquoted CSV stream. So if you use larger SIMD registers in the unquoted stream you will end up 1. throttling the SIMD part of the code too much because of the inner branching (when hitting a delimiter or a newline) and 2. you will often discard too much work when hitting a record end or a quoted field.

Copy amortization

Copying tiny amounts of data often is quite detrimental to overall performance. As such, and to make sure the copying [Reader] remains as fast as possible, I decided to change the design of the [ByteRecord] to save fields as fully-fledged ranges over the underlying byte slice instead of only delimiting them implicitly by the offsets separating them as it is done in the csv crate.

This means I am able to copy large swathes of unquoted data at once instead of copying fields one by one. This also means I keep delimiter characters and sometimes inconsequential double quotes in the underlying byte slice (but don’t worry, the user will never actually see them), so that copies remain as vectorized as possible.

Caveats

“Nonsensical” CSV data

To remain as fast as possible, “nonsensical” CSV data is handled by this crate differently than it might traditionally be done.

For instance, this crate’s CSV parser has no concept of “beginning of field”, which means opening quotes in the middle of a field might corrupt the output. (I would say this is immoral to do so in the first place but traditional parsers tend to deal with this case more graciously).

For instance, given the following CSV data:

name,surname\njoh"n,landis\nbéatrice,babka

Cautious parsers would produce the following result:

name	surname
joh“n	landis
béatrice	babka

While this crate’s parser would produce the following unaligned result:

name	surname
joh“n,landis\nbéatrice,babka	<eof>

Keep also in mind that fields opening and closing quotes multiple times might lose some characters here & there (especially whitespace) because the parser’s state machine is not geared towards this at all.

Rest assured that morally valid & sensical CSV data will still be parsed correctly ;)

Regarding line terminators

To avoid needless branching and SIMD overhead, this crate’s CSV parser expect line terminators to be either CRLF or single LF, but not single CR.

Also, to avoid state machine overhead related to CRLF at buffer boundaries when streaming and to make sure we skip empty lines of the file (we don’t parse them as empty records), one edge case has been deemed an acceptable loss: leading CR characters will be trimmed from the beginning of records.

For instance, given the following CSV data:

name,surname\n\rjohn,landis\r\nbéatrice,babka

A morally correct parser recognizing CRLF or LF line terminators should return:

name	surname
\rjohn	landis
béatrice	babka

While the hereby crate returns:

name	surname
john	landis
béatrice	babka