linux-kernel - Re: [PATCH] add some drop_caches documentation and info messsge

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Date:	Thu, 25 Oct 2012 16:25:06 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add some drop_caches documentation and info messsge

On Thu 25-10-12 04:57:11, Dave Hansen wrote:
[...]
> Here's the problem: Joe Kernel Developer gets a bug report, usually
> something like "the kernel is slow", or "the kernel is eating up all my
> memory".  We then start going and digging in to the problem with the
> usual tools.  We almost *ALWAYS* get dmesg, and it's reasonably common,
> but less likely, that we get things like vmstat along with such a bug
> report.
> 
> Joe Kernel Developer digs in the statistics or the dmesg and tries to
> figure out what happened.  I've run in to a couple of cases in practice
> (and I assume Michal has too) where the bug reporter was using
> drop_caches _heavily_ and did not realize the implications.  It was
> quite hard to track down exactly how the page cache and dentries/inodes
> were getting purged.

Yes, very same here. Not that I would meet issues like that often but it
happened in the past few times and it was always a lot of burnt time.

> There are rarely oopses involved in these scenarios.
> 
> The primary goal of this patch is to make debugging those scenarios
> easier so that we can quickly realize that drop_caches is the reason our
> caches went away, not some anomalous VM activity.  A secondary goal is
> to tell the user: "Hey, maybe this isn't something you want to be doing
> all the time."

-- 
Michal Hocko
SUSE Labs
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