Re: [PATCH v4 1/4] mm: introduce MADV_COLD

[Johannes Weiner]

On Thu, Jul 11, 2019 at 10:25:25AM +0900, Minchan Kim wrote:
> When a process expects no accesses to a certain memory range, it could
> give a hint to kernel that the pages can be reclaimed when memory pressure
> happens but data should be preserved for future use.  This could reduce
> workingset eviction so it ends up increasing performance.
> 
> This patch introduces the new MADV_COLD hint to madvise(2) syscall.
> MADV_COLD can be used by a process to mark a memory range as not expected
> to be used in the near future. The hint can help kernel in deciding which
> pages to evict early during memory pressure.
> 
> It works for every LRU pages like MADV_[DONTNEED|FREE]. IOW, It moves
> 
> 	active file page -> inactive file LRU
> 	active anon page -> inacdtive anon LRU
> 
> Unlike MADV_FREE, it doesn't move active anonymous pages to inactive
> file LRU's head because MADV_COLD is a little bit different symantic.
> MADV_FREE means it's okay to discard when the memory pressure because
> the content of the page is *garbage* so freeing such pages is almost zero
> overhead since we don't need to swap out and access afterward causes just
> minor fault. Thus, it would make sense to put those freeable pages in
> inactive file LRU to compete other used-once pages. It makes sense for
> implmentaion point of view, too because it's not swapbacked memory any
> longer until it would be re-dirtied. Even, it could give a bonus to make
> them be reclaimed on swapless system. However, MADV_COLD doesn't mean
> garbage so reclaiming them requires swap-out/in in the end so it's bigger
> cost. Since we have designed VM LRU aging based on cost-model, anonymous
> cold pages would be better to position inactive anon's LRU list, not file
> LRU. Furthermore, it would help to avoid unnecessary scanning if system
> doesn't have a swap device. Let's start simpler way without adding
> complexity at this moment. However, keep in mind, too that it's a caveat
> that workloads with a lot of pages cache are likely to ignore MADV_COLD
> on anonymous memory because we rarely age anonymous LRU lists.
> 
> * man-page material
> 
> MADV_COLD (since Linux x.x)
> 
> Pages in the specified regions will be treated as less-recently-accessed
> compared to pages in the system with similar access frequencies.
> In contrast to MADV_FREE, the contents of the region are preserved
> regardless of subsequent writes to pages.
> 
> MADV_COLD cannot be applied to locked pages, Huge TLB pages, or VM_PFNMAP
> pages.
> 
> * v2
>  * add up the warn with lots of page cache workload - mhocko
>  * add man page stuff - dave
> 
> * v1
>  * remove page_mapcount filter - hannes, mhocko
>  * remove idle page handling - joelaf
> 
> * RFCv2
>  * add more description - mhocko
> 
> * RFCv1
>  * renaming from MADV_COOL to MADV_COLD - hannes
> 
> * internal review
>  * use clear_page_youn in deactivate_page - joelaf
>  * Revise the description - surenb
>  * Renaming from MADV_WARM to MADV_COOL - surenb
> 
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>