linux-kernel - Re: inotify limits - thousands (tens of thousands?) of watches

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Date:	Wed, 20 May 2009 22:58:26 +0900
From:	hooanon05@...oo.co.jp
To:	Marcin Krol <mrkafk@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: inotify limits - thousands (tens of thousands?) of watches 

Marcin Krol:
> I'm not a kernel programmer, but I want to develop a program that would 
> watch modifications in *all* user directories on a busy server using 
> inotify.
	:::
> 1. is it safe? that is, will it not lock the kernel up, or cause 
> excessive memory consumption?
> 
> 2. is it economic in terms of CPU time and RAM? I have no idea how to 
> even measure such a thing happening in the kernel..

The maximum number of inotify instances per user is limited to
/proc/sys/fs/inotify/max_user_instances which is 128 by default.
And also the number of watches per user is limited by
/proc/sys/fs/inotify/max_user_watches (8192 by defaut).
Theoretically you may be able to monitor 8192 directories, but it
consumes memory.

If periodical rsync is out of question for you, how about kprobe?
While I don't think it is a beautiful solution, to set a hook to
vfs_mkdir, vfs_unlink, etc and to compare the target super_block may
work for you.
See Documentation/kprobe.txt in detail.

J. R. Okajima
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