Re: [RFC 2/3] vmscan hook

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 17 Jan 2012 17:13:57 +0900
Minchan Kim <minchan@kernel.org> wrote:

> This patch insert memory pressure notify point into vmscan.c
> Most problem in system slowness is swap-in. swap-in is a synchronous
> opeartion so that it affects heavily system response.
> 
> This patch alert it when reclaimer start to reclaim inactive anon list.
> It seems rather earlier but not bad than too late.
> 
> Other alert point is when there is few cache pages
> In this implementation, if it is (cache < free pages),
> memory pressure notify happens. It has to need more testing and tuning
> or other hueristic. Any suggesion are welcome.
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>

In my 1st impression, isn't this too simple ?


> ---
>  mm/vmscan.c |   28 ++++++++++++++++++++++++++++
>  1 files changed, 28 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2880396..cfa2e2d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -43,6 +43,7 @@
>  #include <linux/sysctl.h>
>  #include <linux/oom.h>
>  #include <linux/prefetch.h>
> +#include <linux/low_mem_notify.h>
>  
>  #include <asm/tlbflush.h>
>  #include <asm/div64.h>
> @@ -2082,16 +2083,43 @@ static void shrink_mem_cgroup_zone(int priority, struct mem_cgroup_zone *mz,
>  {
>  	unsigned long nr[NR_LRU_LISTS];
>  	unsigned long nr_to_scan;
> +
>  	enum lru_list lru;
>  	unsigned long nr_reclaimed, nr_scanned;
>  	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
>  	struct blk_plug plug;
> +#ifdef CONFIG_LOW_MEM_NOTIFY
> +	bool low_mem = false;
> +	unsigned long free, file;
> +#endif
>  
>  restart:
>  	nr_reclaimed = 0;
>  	nr_scanned = sc->nr_scanned;
>  	get_scan_count(mz, sc, nr, priority);
> +#ifdef CONFIG_LOW_MEM_NOTIFY
> +	/* We want to avoid swapout */
> +	if (nr[LRU_INACTIVE_ANON])
> +		low_mem = true;

IIUC, nr[LRU_INACTIVE_ANON] can be easily > 0.
And get_scan_count() now check per-memcg-lru. So, this only works when
memcg is not used.


> +	/*
> +	 * We want to avoid dropping page cache excessively
> +	 * in no swap system
> +	 */
> +	if (nr_swap_pages <= 0) {
> +		free = zone_page_state(mz->zone, NR_FREE_PAGES);
> +		file = zone_page_state(mz->zone, NR_ACTIVE_FILE) +
> +			zone_page_state(mz->zone, NR_INACTIVE_FILE);
> +		/*
> +		 * If we have very few page cache pages,
> +		 * notify to user
> +		 */
> +		if (file < free)
> +			low_mem = true;
> +	}

I can't understand why you think you can check lowmem condition by "file < free".
And I don't think using per-zone data is good.
(I'm not sure how many zones embeded guys using..)

Another idea:
1. can't we use some technique like cleancache to detect the condition ?
2. can't we measure page-in/page-out distance by recording something ?
3. NR_ANON + NR_FILE_MAPPED can't mean the amount of core memory if we can
   ignore the data file cache ?
4. how about checking kswapd's busy status ?



Thanks,
-Kame

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