bug#73721: grep perf docs barely mention mem usage

[mark.yagnatinsky]

Okay, that's a surprise.  I had assumed the buffer would be mapped read-only.
If indeed it needs to modify the buffer then even the memory gains start to 
look marginal.
Thanks for explaining where my assumptions were wrong, and thanks also for 
confirming the parts that were right.  I now know that the use-case I have in 
mind is likely better served by sed and not by grep.

-----Original Message-----
From: Paul Eggert <eggert@cs.ucla.edu> 
Sent: Thursday, October 10, 2024 12:11 AM
To: Yagnatinsky, Mark : IT (NYK) <mark.yagnatinsky@barclays.com>
Cc: 73721-done@debbugs.gnu.org
Subject: Re: bug#73721: grep perf docs barely mention mem usage

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On 2024-10-09 14:12, mark.yagnatinsky@barclays.com wrote:
> With mmap(), the kernel just needs to set up a bit of book-keeping

Yes, I'm sure than in some cases mmap would be a win. However, all in all it 
was found not to be, and I expect that the hassle of maintaining the mmap 
variant wasn't worth whatever benefits it provided. As I recall 'grep' wants to 
modify its input buffers for other efficiency reasons, and mmap CoW likely 
won't be a performance win over page-aligned read.

We could be proved wrong by someone taking the time to resurrect --mmap and 
doing performance measurements of realistic benchmarks. I don't have the time 
for that, myself.

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