Re: [RFC] Make balance_dirty_pages zone aware (1/2)

[Andrew Morton]

"Martin J. Bligh" <mbligh@xxxxxxxxxxx> wrote:
>
> > What trouble?
> 
> Well ... not so sure of this as I once was ... so be gentle with me ;-)
> But if the system has been running for a while, memory is full of pagecache,
> etc. We try to allocate from the local node, fail, and fall back to the
> other nodes, which are all full as well. Then we wake up kswapd, but all
> pages in this node are dirty, so we block for ages on writeout, making 
> mem allocate really latent and slow (which was presumably what
> balance_dirty_pages was there to solve in the first place). 

It is possible.  You'd be pretty unlucky to dirty so much lowmem when there
is such a huge amount of highmem floating about, but yes, if you tried hard
enough...

I have a feeling that some observed problem must have prompted this coding
frenzy from Matthew.  Surely some problem was observed, and this patch
fixed it up??

> > If we make the dirty threshold a proportion of the initial amount of free
> > memory in ZONE_NORMAL, as is done in 2.4 it will not be possible to fill
> > any node with dirty pages.
> 
> True. But that seems a bit extreme for a system with 64GB of RAM, and only
> 896Mb in ZONE_NORMAL ;-) Doesn't really seem like the right way to fix it.
> 

Increasing /proc/sys/vm/lower_zone_protection can be used to teach the VM
to not use lowmem for pagecache.  Does this solve the elusive problem too?

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