Re: The performance and behaviour of the anti-fragmentation relatedpatches

[Christoph Lameter]

On Fri, 2 Mar 2007, Nick Piggin wrote:

> > If there are billions of pages in the system and we are allocating and 
> > deallocating then pages need to be aged. If there are just few pages 
> > freeable then we run into issues.
> 
> page writeout and vmscan don't work too badly. What are the issues?

Slow downs up to livelocks with large memory configurations.

> So what problems that you commonly see now? Some of us here don't
> have 4TB of memory, so you actually have to tell us ;)

Oh just run a 32GB SMP system with sparsely freeable pages and lots of 
allocs and frees and you will see it too. F.e try Linus tree and mlock 
a large portion of the memory and then see the fun starting. See also 
Rik's list of pathological cases on this.

> How did you come up with that 2MB number?

Huge page size. The only basic choice on x86_64

> Anyway, we have hugetlbfs for things like that.

Good to know that direct io works.

> > I am not the first one.... See Rik's posts regarding the reasons for his 
> > new page replacement algorithms.
> 
> Different issue, isn't it? Rik wants to be smarter in figuring out which
> pages to throw away. More work per page == worse for you.

Rik is trying to solve the same issue in a different way. He is trying to 
manage gazillion entries better instead of reducing the entries to be 
managed. That can only work in a limited way. Drastic reductions in the 
entries to be manages have good effects in multiple ways. Reduce 
management overhead, improve I/O throughput, etc etc.

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