Re: [PATCH] VM: Implements the swap-out page-clustering technique

[Andi Kleen <andi@firstfloor.org>]

"Hamid R. Jahanjou" <hamid.jahanjou@gmail.com> writes:

> From: Hamid R. Jahanjou
>
> Implements the idea of swap-out page clustering from *BSD for
> Linux. Each time a candidate page is to be swapped out,
> virtually-nearby pages are scanned to find eligible pages to be
> swapped out too as a cluster. This technique increases the likelihood of
> bringing in related data on a page fault and decreases swap space
> fragmentation in the long run. Currently, Linux searches only
> physically-nearby pages which is not optimal since, over time, physically-
> adjacent pages may become unrelated.
>
> The code can be statically tuned. No benchmarks. I'm not sure whether
> the added complexity is acceptable.

Just some general comments:

First I think virtual swap clustering is a great idea in theory and
long overdue.  Hopefully the numbers will agree.

In general the code would be much nicer if you didn't pass around 
all that much in a structure (which is just a fancy way to have
a function with lots of arguments) Perhaps try to restructure
it a bit to make this smaller? Ideally clustering_info should disappear
or at least get much smaller.

Then continue_search seems to be only set to one value so it 
can be eliminated? Perhaps there is more like this.

I didn't quite understand the "adjust the value of our search by
the allocation order". The allocation order should be normally 0.
I think having a tunable for the cluster sizes would be a good idea.
At some point they might be even device dependent (e.g. on a flash
device you would like to have them roughly erase block sized) 

-Andi


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