linux-kernel - Re: Swappiness vs. mmap() and interactive response

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Date:	Tue, 28 Apr 2009 14:35:29 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Elladan <elladan@...imo.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>, Rik van Riel <riel@...hat.com>
Subject: Re: Swappiness vs. mmap() and interactive response

(cc to linux-mm and Rik)


> Hi,
> 
> So, I just set up Ubuntu Jaunty (using Linux 2.6.28) on a quad core phenom box,
> and then I did the following (with XFS over LVM):
> 
> mv /500gig/of/data/on/disk/one /disk/two
> 
> This quickly caused the system to. grind.. to... a.... complete..... halt.
> Basically every UI operation, including the mouse in Xorg, started experiencing
> multiple second lag and delays.  This made the system essentially unusable --
> for example, just flipping to the window where the "mv" command was running
> took 10 seconds on more than one occasion.  Basically a "click and get coffee"
> interface.

I have some question and request.

1. please post your /proc/meminfo
2. Do above copy make tons swap-out? IOW your disk read much faster than write?
3. cache limitation of memcgroup solve this problem?
4. Which disk have your /bin and /usr/bin?



> 
> There was no particular kernel CPU load -- the SATA DMA seemed fine.
> 
> If I actively used the GUI, then the pieces I was using would work better, but
> they'd start experiencing astonishing latency again if I just let the UI sit
> for a little while.  From this, I diagnosed that the problem was probably
> related to the VM paging out my GUI.
> 
> Next, I set the following:
> 
> echo 0 > /proc/sys/vm/swappiness
> 
> ... hoping it would prevent paging out of the UI in favor of file data that's
> only used once.  It did appear to help to a small degree, but not much.  The
> system is still effectively unusable while a file copy is going on.
> 
> From this, I diagnosed that most likely, the kernel was paging out all my
> application file mmap() data (such as my executables and shared libraries) in
> favor of total garbage VM load from the file copy.
> 
> I don't know how to verify that this is true definitively.  Are there some
> magic numbers in /proc I can look at?  However, I did run latencytop, and it
> showed massive 2000+ msec latency in the page fault handler, as well as in
> various operations such as XFS read.  
> 
> Could this be something else?  There were some long delays in latencytop from
> various apps doing fsync as well, but it seems unlikely that this would destroy
> latency in Xorg, and again, latency improved whenever I touched an app, for
> that app.
> 
> Is there any way to fix this, short of rewriting the VM myself?  For example,
> is there some way I could convince this VM that pages with active mappings are
> valuable?
> 
> Thanks.



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