Re: The performance and behaviour of the anti-fragmentation related patches

[Nick Piggin]

On Thu, Mar 01, 2007 at 11:44:05PM -0800, Christoph Lameter wrote:
> On Fri, 2 Mar 2007, Nick Piggin wrote:
> 
> > > Sure we will. And you believe that the the newer controllers will be able 
> > > to magically shrink the the SG lists somehow? We will offload the 
> > > coalescing of the page structs into bios in hardware or some such thing? 
> > > And the vmscans etc too?
> > 
> > As far as pagecache page management goes, is that an issue for you?
> > I don't want to know about how many billions of pages for some operation,
> > just some profiles.
> 
> 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?

> > > > I understand you have controllers (or maybe it is a block layer limit)
> > > > that doesn't work well with 4K pages, but works OK with 16K pages.
> > > Really? This is the first that I have heard about it.
> > Maybe that's the issue you're running into.
> 
> Oh, I am running into an issue on a system that does not yet exist? I am 
> extrapolating from the problems that we commonly see now. Those will get 
> worse the more memory increases.

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 ;)

> > > > This is not something that we would introduce variable sized pagecache
> > > > for, surely.
> > > I am not sure where you get the idea that this is the sole reason why we 
> > > need to be able to handle larger contiguous chunks of memory.
> > I'm not saying that. You brought up this subject of variable sized pagecache.
> 
> You keep bringing up the 4k/16k issue into this for some reason. I want 
> just the ability to handle large amounts of memory. Larger page sizes are 
> a way to accomplish that.

As I said in my other mail to you, Linux runs on systems with 6 orders
of magnitude more struct pages than when it was first created. What's
the problem?

> > Eventually, increasing x86 page size a bit might be an idea. We could even
> > do it in software if CPU manufacturers don't for us.
> 
> A bit? Are we back to the 4k/16k issue? We need to reach 2M at mininum. 
> Some way to handle continuous memory segments of 1GB and larger 
> effectively would be great.

How did you come up with that 2MB number?

Anyway, we have hugetlbfs for things like that.

> > That doesn't buy us a great deal if you think there is this huge looming
> > problem with struct page management though.
> 
> 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.
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