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Date:	Mon, 28 Jan 2008 19:30:05 +0000
From:	Pavel Machek <pavel@....cz>
To:	Theodore Tso <tytso@....edu>, Valdis.Kletnieks@...edu,
	David Chinner <dgc@....com>,
	Valerie Henson <val@...consulting.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andreas Dilger <adilger@...sterfs.com>,
	Ric Wheeler <ric@....com>
Subject: Re: [RFC] Parallelize IO for e2fsck

Hi!

> It's been discussed before, but I suspect the main reason why it was
> never done is no one submitted a patch.  Also, the problem is actually
> a pretty complex one.  There are a couple of different stages where
> you might want to send an alert to processes:
> 
>     * Data is starting to get ejected from page/buffer cache
>     * System is starting to swap
>     * System is starting to really struggle to find memory
>     * System is starting an out-of-memory killer
> 
> AIX's SIGDANGER really did the last two, where the OOM killer would
> tend to avoid processes that had a SIGDANGER handler in favor of
> processes that were SIGDANGER unaware.
> 
> Then there is the additional complexity in Linux that you have
> multiple zones of memory, which at least on the historically more
> popular x86 was highly, highly important.  You could say that whenever
> there is sufficient memory pressure in any zone that you start
> ejecting data from caches or start to swap that you start sending the
> signals --- but on x86 systems with lowmem, that could happen quite
> frequently, and since a user process has no idea whether its resources
> are in lowmem or highmem, there's not much you can do about this.

As user pages are always in highmem, this should be easy to decide:
only send SIGDANGER when highmem is full. (Yes, there are
inodes/dentries/file descriptors in lowmem, but I doubt apps will
respond to SIGDANGER by closing files).
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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