Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 24 Nov 2003 20:58:24 -0800 | From | "Martin J. Bligh" <> | Subject | Re: [RFC] Make balance_dirty_pages zone aware (1/2) |
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>> 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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