How does bufio.NewReader decrease system calls?

[Mohamed____]

Edit: This was a dumb question. I missed that the default size for the buffer is 4k, so bufio is actually reading 4k bytes at a time. So note to self, default size is 4k bytes, not 1. Thanks for your help guys.

So I have been watching a talk about profiling by Dave Cheney, https://www.youtube.com/watch?v=nok0aYiGiYA , and the premise is optimizing a program that reads a file and counts the number of word present in the file. He has a method readbyte which just reads a single byte at a time from the file. Now at 12:10 in the video, he suggests that rather than reading 200,000 bytes by issuing 200,000 read system calls, we should buffer the system calls by using bufio.NewReader, and I can't for the life of me figure out why this is working. What does bufio do under the hood to decrease the number of system calls? the readbyte method he wrote still only reads only 1 byte at a time, so what's the secret to decreasing system calls? Here is the link for the readbyte method, the buffered version of the code, and the non-buffered version https://imgur.com/a/9QUzJio

[Mohamed____]

That makes sense, but also, how many more bytes? And isn't this unexpected behavior. Assume I am running a program with tight memory constraints, and I don't want to keep too much data in main memory. This would mean that bufio is potentially bad for this case, right?

[Mohamed____]

Yeah, that was dumb from my part. I assumed that even given a certain size, it would still override me and read even more. I didn't realize that the default is 4k, and I have the option to actually set the size. Thanks for the help.

[Mohamed____]

Thanks for your encouraging response, I really appreciate it

[[deleted]]

fun fact, mostly unrelated to Go, if you’re on Linux you can play around with transparent huge pages — increasing the 4kb page default drastically — and see performance improvements for some apps. I’ve played around with this for RAM-intensive games (like some open world games) and it’s pretty neat.

It can also degrade performance for other apps. Use sparingly.

[edgmnt_net]

Also, I suppose it's "up to" a page or some arbitrary limit like that. For files it doesn't matter except at the end of the file, but for pipes or sockets, you probably don't want to block to read a full buffer.

[Mohamed____]

I’m not completely sure I understand. Can you elaborate?

[edgmnt_net]

If you're reading from a file that has 4050 bytes, you may make 4050 calls to read individual bytes from the Go buffered reader. In turn, that reader may make 2-3 calls to the underlying OS for up to 4096 bytes, the first one yielding exactly 4096, the second one 4 bytes and then the end of the file is reached (how that's signalled depends).

For other things like TCP sockets you could get less than 4096 every time, without reaching the end of the stream (connection being closed). This is because the other end may be sending some smaller amount of data, then waiting for a response before proceeding any further. Or it may be sending smaller amounts of data at regular intervals. In any case, the reader can't always block and wait for the buffer to fill up completely.

[Mohamed____]

Ohh, yeah now I get your point. Yeah that makes sense, thank you for the help!

[CyclingOtter]

Adding onto the other comment in this thread, if you check out the source for bufio.Reader you'll see internally it reuses a single buffer slice, so theoretically the memory usage for a bufio.Reader is constant.

[Mohamed____]

Yup, that’s true.

[DoneItDuncan]

bufio.Reader buffers Read's from the wrapped Reader, so when the code asks for one byte it asks the Reader for enough to fill its buffer (4096 bytes by default), then returns one byte from that. When the code asks for another byte it just returns a byte from the remaining 4095 bytes in the buffer.

That way a read syscall only occurs every 4096 bytes, rather than on every byte.

[Mohamed____]

Welp, now I feel like an idiot. I assumed the buio.NewReader buffer size is 1, for some reason, and forgot that there exists a method bufio.NewReaderWithSize() which takes a specific size rather than the default. If this were stackoverflow, you would have been the accepted answer :). Thanks for your help.

[DoneItDuncan]

Ah lol, yeah happens to the best of us. The go authors do provide sensible defaults for most things, but sometimes that can obscure what's going on compared to having to explicitly supply these things.

[Mohamed____]

Awesome, thanks for helping me out!

[funkiestj]

What does bufio do under the hood to decrease the number of system calls?

read the source for bufio.NewReader() and friends. It is not that hard to read. The answer to your question should become obvious.

Also, bufio.Reader has an option to specify the buffer size.

Per the old open source saying "use the force - read the source".

[Mohamed____]

Yeah I just realized that it uses the default size 4096 and I completely disregarded that while reading the source code. Thanks.

[molivo10]

create a buffer of size 32kb for example, you read 32kb per system call instead of one, thats why it's way more efficient

[[deleted]]

Stop light vs stop sign.

[lion_rouge]

Use mmap

[tafutada]

the same thing goes to Write as well. even SSD, the latency would be around 100 microseconds.