How to profile the grep engine despite the "/dev/null optimization" ?

[alexandre . ferrieux]

Hello,

In some circumstances, it is useful to profile, or do perf anaylsis on, the grep 
engine.
This happens for example when you need to tune your regexps for performance.
In the old times, we just did:

    time grep PATTERN < FILE > /dev/null

But nowadays, grep *detects* /dev/null and behaves differently in that case: it 
sets the done_on_match flag, and accordingly exits very quickly, on the first 
matching line of your hundreds-of-gigabytes test file....

It's not immediately obvious why grep tries to be "smarter than the human", as 
"-q" is an explicit way for the human to request this behavior.
Anyway, I realize I'm pretty late to complain, as this happened 7 years ago:

    af6af28 Paul Eggert     Sun May 1 22:56:39 2016 -0700           grep: 
/dev/null output speedup

So, what is today's recommended idiom to do the same ?

Thanks in advance,

-Alex


PS: Note "grep ... | cat > /dev/null" is a VERY poor approximation, as the 
scheduler's backpressure hits pretty bad. Even with enlarged pipe buffers, grep 
runs slower with a pipe than with a redirection to a RAMdisk file 
(/dev/shm/foo), which unfortunately is not scalable to hundreds of gigabytes on 
most machines.

PS2: I am aware I can fool grep's detection method, which is to compare inodes, 
by creating a "/dev/null2" with same device number (but different inode). 
However, I dearly hope one doesn't need to resort to such horrendous hacks for 
simple perf tuning...
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