linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Date:	Sat, 26 Oct 2013 00:32:25 +0100
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Diego Calleja <diegocg@...il.com>
Cc:	"Artem S. Tashkinov" <t.artem@...os.com>, david@...g.hm,
	neilb@...e.de, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	axboe@...nel.dk, linux-mm@...ck.org
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

On Fri, Oct 25, 2013 at 09:40:13PM +0200, Diego Calleja wrote:
> El Viernes, 25 de octubre de 2013 18:26:23 Artem S. Tashkinov escribió:
> > Oct 25, 2013 05:26:45 PM, david wrote:
> > >actually, I think the problem is more the impact of the huge write later
> > >on.
> > Exactly. And not being able to use applications which show you IO
> > performance like Midnight Commander. You might prefer to use "cp -a" but I
> > cannot imagine my life without being able to see the progress of a copying
> > operation. With the current dirty cache there's no way to understand how
> > you storage media actually behaves.
> 
> 
> This is a problem I also have been suffering for a long time. It's not so much 
> how much and when the systems syncs dirty data, but how unreponsive the 
> desktop becomes when it happens (usually, with rsync + large files). Most 
> programs become completely unreponsive, specially if they have a large memory 
> consumption (ie. the browser). I need to pause rsync and wait until the 
> systems writes out all dirty data if I want to do simple things like scrolling 
> or do any action that uses I/O, otherwise I need to wait minutes.

That's a problem. And it's kind of independent of the dirty threshold
-- if you are doing large file copies in the background, it will lead
to continuous disk writes and stalls anyway -- the large dirty threshold
merely delays the write IO time.

> I have 16 GB of RAM and excluding the browser (which usually uses about half 
> of a GB) and KDE itself, there are no memory hogs, so it seem like it's 
> something that shouldn't happen. I can understand that I/O operations are 
> laggy when there is some other intensive I/O ongoing, but right now the system 
> becomes completely unreponsive. If I am unlucky and Konsole also becomes 
> unreponsive, I need to switch to a VT (which also takes time).
> 
> I haven't reported it before in part because I didn't know how to do it, "my 
> browser stalls" is not a very useful description and I didn't know what kind 
> of data I'm supposed to report.

What's the kernel you are running? And it's writing to a hard disk?
The stalls are most likely caused by either one of

1) write IO starves read IO
2) direct page reclaim blocked when
   - trying to writeout PG_dirty pages
   - trying to lock PG_writeback pages

Which may be confirmed by running

        ps -eo ppid,pid,user,stat,pcpu,comm,wchan:32
or
        echo w > /proc/sysrq-trigger    # and check dmesg

during the stalls. The latter command works more reliably.

Thanks,
Fengguang
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