GNU bug report logs - #73721
grep perf docs barely mention mem usage

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Message #19 received at 73721-done <at> debbugs.gnu.org (full text, mbox):

From: <mark.yagnatinsky <at> barclays.com>
To: <eggert <at> cs.ucla.edu>
Cc: 73721-done <at> debbugs.gnu.org
Subject: RE: bug#73721: grep perf docs barely mention mem usage
Date: Thu, 10 Oct 2024 16:17:48 +0000

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

 CAUTION:  This email originated from outside our organization - eggert <at> cs.ucla.edu  Do not click on links, open attachments, or respond unless you recognize the sender and can validate the content is safe. 

______________________________________________________________________
On 2024-10-09 14:12, mark.yagnatinsky <at> 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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