|From:||Linus Torvalds <torvalds-AT-osdl.org>|
|To:||Arjan van de Ven <arjan-AT-infradead.org>|
|Subject:||Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19|
|Date:||Thu, 3 Nov 2005 07:51:11 -0800 (PST)|
|Cc:||"Martin J. Bligh" <mbligh-AT-mbligh.org>, Nick Piggin <nickpiggin-AT-yahoo.com.au>, Dave Hansen <haveblue-AT-us.ibm.com>, Ingo Molnar <mingo-AT-elte.hu>, Mel Gorman <mel-AT-csn.ul.ie>, Andrew Morton <akpm-AT-osdl.org>, kravetz-AT-us.ibm.com, linux-mm <linux-mm-AT-kvack.org>, Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>, lhms <lhms-devel-AT-lists.sourceforge.net>, Arjan van de Ven <arjanv-AT-infradead.org>|
On Thu, 3 Nov 2005, Arjan van de Ven wrote: > On Thu, 2005-11-03 at 07:36 -0800, Martin J. Bligh wrote: > > >> Can we quit coming up with specialist hacks for hotplug, and try to solve > > >> the generic problem please? hotplug is NOT the only issue here. Fragmentation > > >> in general is. > > >> > > > > > > Not really it isn't. There have been a few cases (e1000 being the main > > > one, and is fixed upstream) where fragmentation in general is a problem. > > > But mostly it is not. > > > > Sigh. OK, tell me how you're going to fix kernel stacks > 4K please. > > with CONFIG_4KSTACKS :) 2-page allocations are _not_ a problem. Especially not for fork()/clone(). If you don't even have 2-page contiguous areas, you are doing something _wrong_, or you're so low on memory that there's no point in forking any more. Don't confuse "fragmentation" with "perfectly spread out page allocations". Fragmentation means that it gets _exponentially_ more unlikely that you can allocate big contiguous areas. But contiguous areas of order 1 are very very likely indeed. It's only the _big_ areas that aren't going to happen. This is why fragmentation avoidance has always been totally useless. It is - only useful for big areas - very hard for big areas (Corollary: when it's easy and possible, it's not useful). Don't do it. We've never done it, and we've been fine. Claiming that fork() is a reason to do fragmentation avoidance is invalid. Linus
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