linux-kernel - Re: [PATCH][BUGFIX][RFC] fix soft lock up at NFS mount by making limitation of dentry

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Date:	Sat, 08 Mar 2008 17:33:06 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Kentaro Makita <k-makita@...css.fujitsu.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	dgc@....com
Subject: Re: [PATCH][BUGFIX][RFC] fix soft lock up at NFS mount by making limitation of dentry_unused

Hi makita-san

in general, I agreed with many people disallow >1min hang up.

> > No, we need a smarter free list structure. There have been several attempts
> > at this in the past. Two that I can recall off the top of my head:
> > 
> > 	- per node unused LRUs
> > 	- per superblock unusued LRUs
> 
>  I know there is such attempt already, but they are not in main-line.
>  I think this is not a smart but simple way to avoid this ploblem.

I think 2 improvement is not exclusive.
your patch is nice, but we need David's patch too. 

because 2 patch purpose is different.

per superblock lru:   improve typical performance.
limit of unused list: prevent too long hang up.

many time hang up happend at worst case even introduce per superblock lru.
and
unused list traversal doesn't improvement even introduce limit of unused list.

I hope both.


> >> Tested on Intel Itanium 2 9050 (dualcore) x12 MEM 24GB , kernel-2.6.25-rc4
> >> I found no peformance regression in my tests.
> > 
> > Try something that relies on leaving the working set on the unused
> > list, like NFS server benchmarks that have a working set of tens of
> > million of files....
> > 
>  Okay, I'll try some benchmarks and report results...

good luck.


>   	spin_unlock(&dentry->d_lock);
>  	spin_unlock(&dcache_lock);
> +	/* Prune unused dentry over threshold level */
> +	int nr_in_use = (dentry_stat.nr_dentry - dentry_stat.nr_unused);
> +	if (dentry_stat.nr_dentry > nr_in_use * (dentry_unused_ratio / 100))
> +		prune_dcache(dentry_stat.nr_unused * 5 / 100 , NULL);
>  	return;

Why don't you make sysctl adjustable interface of dentry_unused_ratio?


- kosaki


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