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

[Martin J. Bligh]

>> 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'm not really worried about lowmem vs highem - that was almost an 
afterthought. I'm more worried about the NUMA bit - it's easy to fill
one node's memory completely with dirty pages by just a writer running 
on that node.
 
> 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??

No, just an observation whilst looking at balance_dirty_pages, that it's
not working as intended on NUMA. It's just easy to goad Matt into a frenzy,
I guess ;-) ;-)

"dd if=/dev/zero of=foo" would trigger it, I'd think. Watching the IO
rate, it should go wierd after ram is full (on a 3 or more node system, 
so there's < 40% of RAM for each node). Yeah, I know you're going to give
me crap for not actually trying it  ... and rightly so ... but it just
seemed so obvious ... ;-) 

>> > 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?

Don't think so - see comment above re NUMA.

M.

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