Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:

>>Despite what people were trying to tell me at Ottawa, this patch
>>set really does add quite a lot of complexity to the page
>>allocator, and it seems to be increasingly only of benefit to
>>dynamically allocating hugepages and memory hot unplug.
> 
> 
> Remember that Rohit is seeing ~10% variation between runs of scientific
> software, and that his patch to use higher-order pages to preload the
> percpu-pages magazines fixed that up.  I assume this means that it provided
> up to 10% speedup, which is a lot.
> 

OK, I wasn't aware of this. I wonder what other approaches we could
try to add a bit of colour to our pages? I bet something simple like
trying to hand out alternate odd/even pages per task might help.

> But the patch caused page allocator fragmentation and several reports of
> gigE Tx buffer allocation failures, so I dropped it.
> 
> We think that Mel's patches will allow us to reintroduce Rohit's
> optimisation.
> 
> 
>>If that is the case, do we really want to make such sacrifices
>>for the huge machines that want these things? What about just
>>making an extra zone for easy-to-reclaim things to live in?
>>
>>This could possibly even be resized at runtime according to
>>demand with the memory hotplug stuff (though I haven't been
>>following that).
>>
>>Don't take this as criticism of the actual implementation or its
>>effectiveness.
>>
> 
> 
> But yes, adding additional complexity is a black mark, and these patches
> add quite a bit.  (Ditto the fine-looking adaptive readahead patches, btw).
> 

They do look quite fine. They seem to get their claws pretty deep
into page reclaim, but I guess that is to be expected if we want
to increase readahead smarts much more.

However, I'm hoping bits of that can be merged at a time, and
interfaces and page reclaim stuff can be discussed and the best
option taken. No such luck with these patches AFAIKS - simply
adding another level of page groups, and another level of
heuristics to the page allocator is going to hurt. By definition.
I do wonder why zones can't be used... though I'm sure there are
good reasons.

-- 
SUSE Labs, Novell Inc.

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