Re: too big min_free_kbytes

On Wed, Jan 26, 2011 at 02:17:46PM +0000, Mel Gorman wrote:

On Mon, Jan 24, 2011 at 04:00:34PM +0100, Andrea Arcangeli wrote:

quoted

eOn Mon, Jan 24, 2011 at 11:56:46AM +0800, Shaohua Li wrote:

quoted

Hi,
With transparent huge page, min_free_kbytes is set too big.
Before:
Node 0, zone    DMA32
  pages free     1812
        min      1424
        low      1780
        high     2136
        scanned  0
        spanned  519168
        present  511496

After:
Node 0, zone    DMA32
  pages free     482708
        min      11178
        low      13972
        high     16767
        scanned  0
        spanned  519168
        present  511496
This caused different performance problems in our test. I wonder why we
set the value so big.

It's to enable Mel's anti-frag that keeps pageblocks with movable and
unmovable stuff separated, same as "hugeadm
--set-recommended-min_free_kbytes".

Now that I checked, I'm seeing quite too much free memory with only 4G
of ram... You can see the difference with a "cp /dev/sda /dev/null" in
background interleaving these two commands:

What kernel is this and is commit
[99504748: mm: kswapd: stop high-order balancing when any suitable zone
is balanced] present in the kernel you are testing?

I'm having very little luck reproducing your scenario with
2.6.38-rc2.

Scratch that, a machine with 4G does reproduce it. The machine I was
trying was 2G. Will dig more.

-- 
Mel Gorman
Linux Technology Center
IBM Dublin Software Lab

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