On Fri, Sep 30, 2011 at 10:35:25AM +0300, Pekka Enberg wrote:

Hi Johannes!

On Fri, Sep 30, 2011 at 10:17 AM, Johannes Weiner [off-list ref] wrote:

quoted

But there is a flaw in that we have a zoned page allocator which does
not care about the global state but rather the state of individual
memory zones.  And right now there is nothing that prevents one zone
from filling up with dirty pages while other zones are spared, which
frequently leads to situations where kswapd, in order to restore the
watermark of free pages, does indeed have to write pages from that
zone's LRU list.  This can interfere so badly with IO from the flusher
threads that major filesystems (btrfs, xfs, ext4) mostly ignore write
requests from reclaim already, taking away the VM's only possibility
to keep such a zone balanced, aside from hoping the flushers will soon
clean pages from that zone.

The obvious question is: how did you test this? Can you share the results?

Meh, sorry about that, they were in the series introduction the last
time and I forgot to copy them over.

I did single-threaded, linear writing to an USB stick as the effect is
most pronounced with slow backing devices.

[ The write deferring on ext4 because of delalloc is so extreme that I
  could trigger it even with simple linear writers on a mediocre
  rotating disk, though.  I can not access the logfiles right now, but
  the nr_vmscan_writes went practically away here as well and runtime
  was unaffected with the patched kernel. ]

			Test results

15M DMA + 3246M DMA32 + 504M Normal = 3765M memory
40% dirty ratio, 10% background ratio
16G USB thumb drive
10 runs of dd if=/dev/zero of=disk/zeroes bs=32k count=$((10 << 15))

		seconds			nr_vmscan_write
		        (stddev)	       min|     median|        max
xfs
vanilla:	 549.747( 3.492)	     0.000|      0.000|      0.000
patched:	 550.996( 3.802)	     0.000|      0.000|      0.000

fuse-ntfs
vanilla:	1183.094(53.178)	 54349.000|  59341.000|  65163.000
patched:	 558.049(17.914)	     0.000|      0.000|     43.000

btrfs
vanilla:	 573.679(14.015)	156657.000| 460178.000| 606926.000
patched:	 563.365(11.368)	     0.000|      0.000|   1362.000

ext4
vanilla:	 561.197(15.782)	     0.000|2725438.000|4143837.000
patched:	 568.806(17.496)	     0.000|      0.000|      0.000

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