Re: [PATCH v3] vmscan: prevent background aging of anon page in no swap system

[KOSAKI Motohiro]

> Ying Han reported that backing aging of anon pages in no swap system
> causes unnecessary TLB flush.
> 
> When I sent a patch(69c8548175), I wanted this patch but Rik pointed out
> and allowed aging of anon pages to give a chance to promote from inactive
> to active LRU.
> 
> It has a two problem.
> 
> 1) non-swap system
> 
> Never make sense to age anon pages.
> 
> 2) swap configured but still doesn't swapon
> 
> It doesn't make sense to age anon pages until swap-on time.
> But it's arguable. If we have aged anon pages by swapon, VM have moved
> anon pages from active to inactive. And in the time swapon by admin,
> the VM can't reclaim hot pages so we can protect hot pages swapout.
> 
> But let's think about it. When does swap-on happen? It depends on admin.
> we can't expect it. Nonetheless, we have done aging of anon pages to
> protect hot pages swapout. It means we lost run time overhead when
> below high watermark but gain hot page swap-[in/out] overhead when VM
> decide swapout. Is it true? Let's think more detail.
> We don't promote anon pages in case of non-swap system. So even though
> VM does aging of anon pages, the pages would be in inactive LRU for a long
> time. It means many of pages in there would mark access bit again. So access
> bit hot/code separation would be pointless.
> 
> This patch prevents unnecessary anon pages demotion in not-yet-swapon and
> non-configured swap system. Even, in non-configuared swap system
> inactive_anon_is_low can be compiled out.
> 
> It could make side effect that hot anon pages could swap out
> when admin does swap on. But I think sooner or later it would be
> steady state. So it's not a big problem.
> 
> We could lose someting but gain more thing(TLB flush and unnecessary
> function call to demote anon pages).
> 
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Ying Han <yinghan@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>

Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>





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