bug#73721: grep perf docs barely mention mem usage

[mark.yagnatinsky]

This page: https://www.gnu.org/software/grep/manual/html_node/Performance.html
discusses how to get good performance out of grep.
That is nice, but the perf advice focuses almost entirely on speed.
I'm more interested in how to avoid excessive memory usage.
After a bit of research, it seems that once upon a time, grep used mmap where 
possible, but it no longer does this.
Thus, peak memory usage will be proportional to the length of the longest line 
in the file.
Thus, if use the "-z multiline hack" to search across lines, grep will read the 
whole file into memory.
Thus, if I try this on a huge file, I will likely have a bad time.
(e.g., a 5 gig file would fail in a 32-bit grep, and would increase memory 
pressure on the system on a 64-bit grep.)

Is the above about right?

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