Yet another Hacker News

Reading and writing are less symmetric than you (probably) think

46 points luu 3 days ago 12 comments

vatsachak 2 hours ago | parent

This is true even at the CPU cache level so it's probably going to trickle down all the way.

For example Jon G gave a talk at Jane Street about how he had a mysterious performance bug that was later attributed to cores fighting over the same cache line

https://youtu.be/tND-wBBZ8RY?is=624w3YoDN2QYBWuD

rwmj 51 minutes ago | parent

False sharing is pretty well known isn't it? I learned about it at university. It's the whole reason why the Linux "dwarves" tools were written, so you can check struct field alignment / grouping against cache line size and make sure you don't accidentally have two adjacent fields used by different cores.

1317 2 hours ago | parent

oh i thought this would be about language and how the brain works, that article sounds more interesting to be honest sorry

prmph 1 hour ago | parent

Indeed, that's what I also thought this would be about; What a bummer.

Someone should post that article instead, lol. I'd love to discuss why I am able to comprehend and read a language almost fully but cannot speak it.

todowhatexactly 1 hour ago | parent

Production is different from reception.

Compare the ease with which you recognize a bicycle and the difficulty of drawing one.

It’s not the mechanics of the pencil alone although that plays a role a well. But even if you could somehow imprint perfect lines like you imagine them, the bicycle and the lightening etc will not be realistic unless you are a trained artist.

chrisweekly 1 hour ago | parent

Search for "Broca and Wernicke" to learn more about the distinct areas of the brain responsible for producing vs processing language.

tbrownaw 1 hour ago | parent

Or even something about how writing usefully on a topic requires higher expertise than reading and understanding what was written.

verdverm 58 minutes ago | parent

or how if you want to write better, read more

tbrownaw 1 hour ago | parent

Yep, reading something doesn't invalidate your caches. Or everyone else's caches.

mightyham 58 minutes ago | parent

> the data may have made it to storage, but that doesn’t necessarily mean it’s actually visible, because the metadata necessary to reach it might not have been written yet.

The author should clarify here that this fsync wrinkle only affects newly created files, not updates to existing ones.

rwmj 52 minutes ago | parent

In any discussion of read, write & sync, I'd want to talk about write amplification (read/modify/write cycles), and also in Linux the sync_file_range syscall which is a useful way to sync part of a file.

Pannoniae 52 minutes ago | parent

Great article :D The rabbithole actually goes even deeper because buffering on the read side is almost always good, but reads are often way more latency-sensitive than writes. Which leads to the paradox that increasing the buffer size / prefetching more with mmapping can lead to worse performance due to the added latency. And decreasing the buffer size can have the same effect due to issuing more calls, so in order to squeeze in the optimal throughput/latency ratio on reads you have to tune it per workload.... or just try a few different buffer sizes and see which one works better.

Writes are the opposite, they're usually not RAW (read-after-write), they're "fire and forget", so you don't really care about latency, but you do care a whole a lot about your writes actually arriving... again two competing parameters because the more you batch the writes, the faster they are but the bigger chance of a torn write or data loss. And unlike reads, writes are not idempotent on the application level so ideally you'd want some logic to atomically write a meaningful amount of data so it won't get torn, even if it crashes.

Now in practice just about no one actually does any of this but it's a fun thought :D