Re: swapping when there's a free memory

[Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>]

On Tue, 27 Apr 2010, KAMEZAWA Hiroyuki wrote:

> On Mon, 26 Apr 2010 15:33:33 -0700
> Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > On Sun, 25 Apr 2010 09:13:49 +0200
> > Pavel Machek <pavel@ucw.cz> wrote:
> > 
> > > Hi!
> > > 
> > > > I captured this output of vmstat. The machine was freeing cache and 
> > > > swapping out pages even when there was a plenty of free memory.
> > > > 
> > > > The machine is sparc64 with 1GB RAM with 2.6.34-rc4. This abnormal 
> > > > swapping happened during running spadfsck --- a fsck program for a custom 
> > > > filesystem that caches most reads in its internal cache --- so it reads 
> > > > buffers and allocates memory at the same time.
> > > > 
> > > > Note that sparc64 doesn't have any low/high memory zones, so it couldn't 
> > > > be explained by filling one zone and needing to allocate pages in it.
> > > 
> > > Fragmented memory + high-order allocation?
> > 
> > Yeah, could be.  I wonder which slab/slub/slob implementation you're
> > using, and what page sizes it uses for dentries, inodes, etc.  Can you
> > have a poke in /prob/slabinfo?

It uses one page-per-slab for dentries and two for inodes. But there was 
certainly no dentry or inode-based load --- the machine runs without X 
with minimum daemons, there is no major background work. There was just a 
process reading 128-kbyte blocks from a raw device and caching them in its 
userspace that triggered this. Can it be that kernel uses high-order 
allocations for reading from a buffer cache?

> And please /proc/buddyinfo and /proc/zoneinfo when the system is swappy.

It happens rarely, I don't know if I catch it at the right time. The 
report I sent, was what I found in a scrollback of vmstat. I didn't catch 
it in real time.

> Thanks,
> -Kame

Mikulas

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