Re: Swappiness vs. mmap() and interactive response

[Wu Fengguang]

On Tue, Apr 28, 2009 at 09:48:39AM +0200, Peter Zijlstra wrote:
> On Tue, 2009-04-28 at 14:35 +0900, KOSAKI Motohiro wrote:
> > (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?
> > 
> 
> FWIW I fundamentally object to 3 as being a solution.
> 
> I still think the idea of read-ahead driven drop-behind is a good one,
> alas last time we brought that up people thought differently.

The semi-drop-behind is a great idea for the desktop - to put just
accessed pages to end of LRU. However I'm still afraid it vastly
changes the caching behavior and wont work well as expected in server
workloads - shall we verify this?

Back to this big-cp-hurts-responsibility issue. Background write
requests can easily pass the io scheduler's obstacles and fill up
the disk queue. Now every read request will have to wait 10+ writes
- leading to 10x slow down of major page faults.

I reach this conclusion based on recent CFQ code reviews. Will bring up
a queue depth limiting patch for more exercises..

Thanks,
Fengguang

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