Regarding this bit: "Roll your own unbuffered input using raw system calls. / A small toy project I wrote last year was a modification of GNU grep that did the opposite -- it aggressively prefetched ahead of the file it was currently reading. "
I think you mis-understood. "Roll your own unbuffered input [....]" does not mean "do not read ahead" it means "do not use stdio". Stdio buffers have a lot of overhead (relatively speaking) in support of features that grep does not need.
I think you mis-understood. "Roll your own unbuffered input [....]" does not mean "do not read ahead" it means "do not use stdio". Stdio buffers have a lot of overhead (relatively speaking) in support of features that grep does not need.