linux-kernel - Re: [RFC PATCH 1/2] vmscan don't isolate too many pages in a zone

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Message-Id: <20090709121647.2395.A69D9226@jp.fujitsu.com>
Date:	Thu,  9 Jul 2009 16:01:26 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [RFC PATCH 1/2] vmscan don't isolate too many pages in a zone

Hi

> I tried the semaphore based concurrent direct reclaim throttling, and
> get these numbers. The run time is normal 30s, but can sometimes go up
> by many folds. It seems that there are more hidden problems..

Hmm....
I think I and you have different priority list. May I explain why Rik
and decide to use half of LRU pages?

the system have 4GB (=1M pages) memory. my patch allow 1M/2/32=16384
threads. I agree this is very large and inefficient. However IOW 
this is very conservative.
I believe it don't makes too strong restriction problem.

In the other hand, your patch's concurrent restriction is small constant
value (=32).
it can be more efficient and it also can makes regression. IOW it is more
aggressive approach.

e.g.
if the system have >100 CPU, my patch can get enough much reclaimer but
your patch makes tons idle cpus.

And, To recall original issue tearch us this is rarely and a bit insane
workload issue.
Then, I priotize to

1. prevent unnecessary OOM
2. no regression to typical workload
3. msgctl11 performance

IOW, I don't think msgctl11 performance is so important.
May I ask why do you think msgctl11 performance is so important?

>
> --- linux.orig/mm/vmscan.c
> +++ linux/mm/vmscan.c
> @@ -1042,6 +1042,7 @@ static unsigned long shrink_inactive_lis
>  	unsigned long nr_reclaimed = 0;
>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>  	int lumpy_reclaim = 0;
> +	static struct semaphore direct_reclaim_sem = __SEMAPHORE_INITIALIZER(direct_reclaim_sem, 32);
>  
>  	/*
>  	 * If we need a large contiguous chunk of memory, or have
> @@ -1057,6 +1058,9 @@ static unsigned long shrink_inactive_lis
>  
>  	pagevec_init(&pvec, 1);
>  
> +	if (!current_is_kswapd())
> +		down(&direct_reclaim_sem);
> +
>  	lru_add_drain();
>  	spin_lock_irq(&zone->lru_lock);
>  	do {
> @@ -1173,6 +1177,10 @@ static unsigned long shrink_inactive_lis
>  done:
>  	local_irq_enable();
>  	pagevec_release(&pvec);
> +
> +	if (!current_is_kswapd())
> +		up(&direct_reclaim_sem);
> +
>  	return nr_reclaimed;
>  }

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