Experience? Last time I made that optimization it was 100x faster, ballpark. I don't feel like benchmarking it right now, try yourself.
The latency comes from the fact you need to have the whole file. The use case I'm talking about is a JSON document you need to pull off the network because it doesn't exist on disk, might not fit there, and might not fit in memory.
> Experience? Last time I made that optimization it was 100x faster, ballpark. I don't feel like benchmarking it right now, try yourself.
I have. Many times. There's definitely not a 100x difference given that normal file I/O can easily saturate NVMe throughput. I'm sure it's possible to build a repro showing a 100x difference, but you have to be doing something intentionally to cause that (e.g. using a very small read buffer so that you're doing enough syscalls that it shows up in a profile).
> The latency comes from the fact you need to have the whole file
That's a whole other matter. But again, if you're pulling it off the network, you usually can't mmap it anyway unless you're using a remote-mounted filesystem (which will add more overhead than mmap vs buffered I/O).
The latency comes from the fact you need to have the whole file. The use case I'm talking about is a JSON document you need to pull off the network because it doesn't exist on disk, might not fit there, and might not fit in memory.