Re: Page cache write performance issue

[Andrew Morton <akpm@xxxxxxxx>]

Nick Piggin <piggin@xxxxxxxxxxxxxxx> wrote:
>
>  Andrew probably has better ideas.

uh, is this an ia32 highmem box?

If so, you've hit the VM sour spot.  That 128M highmem zone gets 100%
filled with dirty pages and we end up doing a ton of writeout off the page
LRU.  And we do that while `dd' is cheerfully writing to a totally
different part of the disk via balance_dirty_pages().  Seekstorm ensues. 
Although last time I looked (a long time ago) the slowdown was only 2:1 -
perhaps your disk is in writethrough mode??

Basically, *any* other config is fine.  896MB and below, 1.5GB and above.

I could well understand that a minor kswapd tweak would make this bad
situation worse.  Making the dirty ratios really small (dirty_ratio less
than the 128MB) should make it go away.

If it's not ia32 then dunno.