Re: [PATCH 0/3 v2] mm: Batch page reclamation under shink_page_list

[Tim Chen]

On Wed, 2012-09-12 at 12:27 -0700, Andrew Morton wrote:

> 
> That sounds good, although more details on the performance changes
> would be appreciated - after all, that's the entire point of the
> patchset.
> 
> And we shouldn't only test for improvements - we should also test for
> degradation.  What workloads might be harmed by this change?  I'd suggest
> 
> - a single process which opens N files and reads one page from each
>   one, then repeats.  So there are no contiguous LRU pages which share
>   the same ->mapping.  Get some page reclaim happening, measure the
>   impact.
> 
> - The batching means that we now do multiple passes over pageframes
>   where we used to do things in a single pass.  Walking all those new
>   page lists will be expensive if they are lengthy enough to cause L1
>   cache evictions.

I need to address both your concerns and Mel's concerns about the
downside of prolonging the holding page locks for the pages to be
unmmaped for patch 1 in the series.  I'll try to do some testing to see
what kind of benefit I get by only batching operations under the
i_mmap_mutex (i.e. patch 2 and 3 only) and not do batch unmap. Those
other changes don't have the downsides of prolonged page locking and we
can incorporate them with less risks.

> 
>   What would be a test for this?  A simple, single-threaded walk
>   through a file, I guess?

Thanks for your test suggestions.  I will do tests along your
suggestions when I generate the next iterations of the patch. 

I've been playing with these patches for a while and they are based on
3.4 kernel.  I'll move them to 3.6 kernel in my next iteration.

> 
> Mel's review comments were useful, thanks.

Very much appreciate comments from you, Mel and Minchan. I'll try to
incorporate them in my changes. 

Tim

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