(Part 1 of 7: Mastering Buffered I/O in Go)
Why Should You Care About bufio
?
Imagine you’re filling a swimming pool. You have two choices:
- The Spoon Method: Scoop water one spoonful at a time.
- The Bucket Method: Dump entire buckets in one go.
Which is faster? Obviously, the bucket.
Now, replace "water" with "data," and you’ve got the difference between unbuffered I/O and buffered I/O in Go.
Every time your Go program reads a file or writes to a network socket, it makes a system call—a request to the operating system. These calls are slow. Like, "waiting-for-your-coffee-to-brew" slow.
bufio
is your performance savior. It wraps I/O operations with an in-memory buffer, reducing system calls by batching data. Fewer calls = faster code.
In this article, we’ll explore:
✔ What buffered I/O really means (no jargon, promise).
✔ When to use bufio
(and when to skip it).
✔ A real benchmark proving it’s not just hype.
Let’s get started!
Buffered I/O: The "Bucket" for Your Data
The Problem with Unbuffered I/O
When you read a file in Go using os.ReadFile
or ioutil.ReadAll
, each Read()
call goes straight to the OS. If you’re reading a 1MB file byte-by-byte, that’s 1 million system calls!
🚫 Bad:
file, _ := os.Open("large.log")
data := make([]byte, 1)
for {
_, err := file.Read(data) // 1 byte per system call!
if err != nil {
break
}
}
(Don’t do this. Your CPU will cry.)
The bufio
Solution
bufio.Reader
acts like a middleman, reading big chunks upfront and serving them from memory:
✅ Good:
file, _ := os.Open("large.log")
reader := bufio.NewReader(file)
for {
line, err := reader.ReadString('\n') // Reads in chunks!
if err != nil {
break
}
fmt.Print(line)
}
(Fewer system calls, happier server.)
When Should You Use bufio
?
1. Reading Large Files
- Logs, CSVs, or any file bigger than a few KB.
-
Bonus:
bufio.Scanner
makes line-by-line reading trivial.
2. High-Frequency Small Writes (Like Logging)
Without buffering, writing 100 short log lines = 100 system calls. With bufio.Writer
, it might be just 1 call.
3. Network I/O (HTTP, TCP, etc.)
Ever parsed an HTTP request? bufio.Reader
is why Go’s net/http
is so efficient.
Benchmark Showdown: Unbuffered vs. bufio
(Real Numbers!)
Enough theory—let’s break out the stopwatch and see how bufio
actually performs. I ran four tests on a 10MB file (filled with random data to prevent OS optimizations):
- Unbuffered, 1-byte reads (The "Spoon Method")
- Buffered, 1-byte reads (The "Bucket Method")
- Unbuffered, 4KB reads (Chunking manually)
-
Buffered, 4KB reads (Letting
bufio
handle chunks)
Here’s the code (full repo here):
// BenchmarkUnbufferedRead: Painfully slow 1-byte reads
func BenchmarkUnbufferedRead(b *testing.B) {
file, _ := os.Open("10mb.log")
defer file.Close()
buf := make([]byte, 1)
for {
if _, err := file.Read(buf); err == io.EOF {
break
}
}
}
(Other benchmarks here.)
The Results (On My Intel Ultra 5 Laptop)
Method | Speed (ns/op) | Memory (B/op) | Allocs (per op) | Verdict |
---|---|---|---|---|
Unbuffered (1B) | 6,676,623,112 | 120 | 3 | 🐌 Snail pace |
Buffered (1B) | 35,121,689 | 4,216 | 5 | 🚀 190x faster! |
Unbuffered (4KB) | 2,197,041 | 120 | 3 | Decent, but manual work |
Buffered (4KB) | 2,292,039 | 4,224 | 4 | Similar, but bufio handles edge cases |
Key Takeaways
- 1-byte reads without buffering are a crime (6.6 seconds per run!).
-
bufio
makes 1-byte reads viable (35ms vs. 6.6s—yes, really). - For 4KB chunks, manual and
bufio
are close—butbufio
wins on:- Code simplicity (no manual chunk sizing).
- Edge cases (e.g., partial reads, EOF handling).
Why Is Buffered 1B So Much Faster?
Every file.Read(buf)
in the unbuffered version hits the OS. bufio.Reader
does this:
- Grabs a 4KB chunk from the OS upfront (default buffer size).
- Serves your tiny 1-byte reads from memory.
- Only goes back to the OS when the buffer is empty.
(Like filling a water glass from a pitcher instead of the tap each time.)
When to Skip bufio
If you’re already reading in large chunks (e.g., 4KB+), bufio
adds minor overhead. But for most cases—especially line-by-line reading (ReadString
/Scanner
)—it’s worth it.
Try it yourself:
git clone https://github.com/peymanahmadi/bufio-benchmark
go test -bench=. -benchmem
(Share your results! Did bufio
speed up your code? Let me know.)
What’s Next?
Now that you’re convinced bufio
is magic, let’s dig deeper.
Next article: "Mastering bufio.Reader
in Go" where we’ll cover:
- How to tweak buffer sizes.
- The difference between
Read()
,ReadString()
, andReadLine()
. - Handling edge cases (like partial reads).
Your Homework:
Replace one os.Read
call in your code with bufio
and measure the difference. Got a cool result? Tweet it at me! 🚀
Final Thought
Buffered I/O isn’t just an optimization—it’s how performant Go code is written. Whether you’re parsing logs or handling HTTP requests, bufio
is your silent speed booster.
Next time you see a slow I/O operation, ask yourself: "Am I using a spoon instead of a bucket?"
Stay tuned for Part 2!
💬 Discussion Prompt:
Have you ever been burned by slow I/O in Go? Share your war stories below!
Series Index
-
Introduction to
bufio
(You’re here!) - Mastering
bufio.Reader
-
bufio.Scanner
Deep Dive - Efficient Writing with
bufio.Writer
- Advanced
bufio
Tricks - Common
bufio
Pitfalls - Real-World
bufio
Use Cases
Top comments (0)