Re: [PATCH 00/03] Unmapped: Separate unmapped and mapped pages

Nick Piggin
2006-03-10T04:55:37Z

Magnus Damm wrote:
> Unmapped patches - Use two LRU:s per zone.
>
> These patches break out the per-zone LRU into two separate LRU:s - one for
> mapped pages and one for unmapped pages. The patches also introduce guarantee
> support, which allows the user to set how many percent of all pages per node
> that should be kept in memory for mapped or unmapped pages. This guarantee
> makes it possible to adjust the VM behaviour depending on the workload.
>
> Reasons behind the LRU separation:
>
> - Avoid unnecessary page scanning.
> The current VM implementation rotates mapped pages on the active list
> until the number of mapped pages are high enough to start unmap and page out.
> By using two LRU:s we can avoid this scanning and shrink/rotate unmapped
> pages only, not touching mapped pages until the threshold is reached.
>
> - Make it possible to adjust the VM behaviour.
> In some cases the user might want to guarantee that a certain amount of
> pages should be kept in memory, overriding the standard behaviour. Separating
> pages into mapped and unmapped LRU:s allows guarantee with low overhead.
>
> I've performed many tests on a Dual PIII machine while varying the amount of
> RAM available. Kernel compiles on a 64MB configuration gets a small speedup,
> but the impact on other configurations and workloads seems to be unaffected.
>
> Apply on top of 2.6.16-rc5.
>
> Comments?
>

I did something similar a while back which I called split active lists.
I think it is a good idea in general and I did see fairly large speedups
with heavy swapping kbuilds, but nobody else seemed to want it :P

So you split the inactive list as well - that's going to be a bit of
change in behaviour and I'm not sure whether you gain anything.

I don't think PageMapped is a very good name for the flag.

I test mapped lazily. Much better way to go IMO.

I had further patches that got rid of reclaim_mapped completely while
I was there. It is based on crazy metrics that basically completely
change meaning if there are changes in the memory configuration of
the system, or small changes in reclaim algorithms.

--
SUSE Labs, Novell Inc.
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