Subject: bug#55636: 27.2; etags performance fix when working with very big TAGS files

From: VAN VLIERBERGHE Stef <[email-masked]>
Date: Thu, 26 May 2022 13:50:32 +0000

You are right, I tried an strace and only see the tags files being read.

I also can't get the 10 sec behaviour any more, maybe some other factors were involved that made the emacs tag file processing slow.

What I did at the time to analyze was to enable debugger on Ctrl-G and then I notices emacs was busy here :

Debugger entered--Lisp error: (quit)
  expand-file-name(#("/cm/ot/TOOL/GTK!27.0.0.1/build_G!27.IP.L7/sources/..." 0 88 (charset iso-8859-1)))
  tags-table-including("/tmp/vvl.Tstatus.out" t)
  visit-tags-table-buffer()
  find-tag-noselect("-tv_summary.adb" nil nil)

>From that I concluded the expand-file-name was the cause.

Hope this helps and all the best,
Stef

-----Original Message-----
From: Eli Zaretskii <[email-masked]>
Sent: 26 May 2022 07:07
To: VAN VLIERBERGHE Stef <[email-masked]>
Cc: DE BACKER Jurgen (EXT) <[email-masked]>; [email-masked]; WAROQUIERS Philippe <[email-masked]>
Subject: Re: bug#55636: 27.2; etags performance fix when working with very big TAGS files

> From: VAN VLIERBERGHE Stef <[email-masked]>
> CC: "[email-masked]" <[email-masked]>, WAROQUIERS Philippe
> <[email-masked]>
> Date: Wed, 25 May 2022 20:42:16 +0000
>
> For us the 10 sec is reduced to below 1 sec, loading the tags file is no longer noticed after this change.
>
> I assume the reason is a huge amount of files all accessed over NFS, and expand-file-name does a lot of system calls that translate into network packets.

Actually, expand-file-name is a purely syntactical function that is supposed not to hit the filesystem at all, at l;east on Posix systems.
So I wonder why it seems to happen in your case.  Any chances that you could show a trace of system calls for those 10 sec?

Of course, making a simple change that you suggested is a no-brainer, so we might as well do it without further ado, but I'm just curious and think maybe we will learn something useful if we dig a bit deeper into your use case.

> An alternative approach is to add some switch that allows a customization that simply never calls the expand-file-name, we generate tags files that already contain absolute paths so don't need any of this logic and disabling it would also be ok for us.

That'd be less clean, I think: if we can do something automatically, it's better to do that instead of placing the burden on the user.

Again, I'm not asking these questions because I see some problem in your proposed change.  If we arrive at the conclusion that there's no reason to investigate more, we can just install that change, as it cannot possibly hurt.

Thanks.
____

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