On Thu, 2005-08-04 at 22:27 -0500, Paul wrote: > On Thu, 2005-08-04 at 21:43 -0400, Dave Jones wrote: > > On Fri, Aug 05, 2005 at 09:22:55AM +0800, Ian Kent wrote: > > > I also find it hard to understand why it is such a problem having a larger > > > stack. As you point out, as software evolves it ultimately becomes more > > > complex. If the developers design needs it and the software is reliable > > > and efficient (aka performs well) then why not. > > > > > > A quick caclulation. > > > > > > 2000*4k is about 8M in say 1G at least. > > > > > > Not a large percentage overhead I think. > > > > Now try finding 2000 _contiguous_ pairs of pages after the machine > > has been up for a while, under load. Memory fragmentation makes > > this a really nasty problem, and the VM eats its own head after > > repeatedly scanning every page in the system. > > I thought I heard that there was some work being done in the upstream > kernel to have a process "defrag" memory in the background. This would > help alleviate this problem on systems with long up-times. actually that work is different; it is intended to defrag *userspace* pages; not kernel pages. And the existing vm already can reclaim those (by freeing them; the defrag work is there to avoid the actual free just to move them). The problem really is more complex than that, and the kernel VM got a lot of robustness back by having 4Kb stacks. (Now on x86-64 and other 64 bit machines this is FAR less of a problem; actually it's almost exclusively a x86 problem. x86 has a 1Gb lowmem zone where all kernel stacks and other kernel datastructures have to go, and the rest of memory goes into a highmem zone. This split is like quadrupling the VM pain; without this split, multi-page stacks are still not pretty but an order of magnitude less of a problem)
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