From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 24 Nov 2003 20:58:24 -0800 From: "Martin J. Bligh" Subject: Re: [RFC] Make balance_dirty_pages zone aware (1/2) Message-ID: <1070800000.1069736303@[10.10.2.4]> In-Reply-To: <20031124170506.4024bb30.akpm@osdl.org> References: <3FBEB27D.5010007@us.ibm.com><20031123143627.1754a3f0.akpm@osdl.org><1034580000.1069688202@[10.10.2.4]><20031124100043.5416ed4c.akpm@osdl.org><39670000.1069719009@flay> <20031124170506.4024bb30.akpm@osdl.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton Cc: colpatch@us.ibm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: >> 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: aart@kvack.org