Re: Regression from 2.6.36

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 13 avril 2011 A  14:16 -0700, Andrew Morton a A(C)crit :

> So am I correct in believing that this regression is due to the
> high-order allocations putting excess stress onto page reclaim?
> 

I suppose so.

> If so, then how large _are_ these allocations?  This perhaps can be
> determined from /proc/slabinfo.  They must be pretty huge, because slub
> likes to do excessively-large allocations and the system handles that
> reasonably well.
> 
> I suppose that a suitable fix would be
> 
> 
> From: Andrew Morton <akpm@linux-foundation.org>
> 
> Azurit reports large increases in system time after 2.6.36 when running
> Apache.  It was bisected down to a892e2d7dcdfa6c76e6 ("vfs: use kmalloc()
> to allocate fdmem if possible").
> 
> That patch caused the vfs to use kmalloc() for very large allocations and
> this is causing excessive work (and presumably excessive reclaim) within
> the page allocator.
> 
> Fix it by falling back to vmalloc() earlier - when the allocation attempt
> would have been considered "costly" by reclaim.
> 
> Reported-by: azurIt <azurit@pobox.sk>
> Cc: Changli Gao <xiaosuo@gmail.com>
> Cc: Americo Wang <xiyou.wangcong@gmail.com>
> Cc: Jiri Slaby <jslaby@suse.cz>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Mel Gorman <mel@csn.ul.ie>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  fs/file.c |   17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff -puN fs/file.c~a fs/file.c
> --- a/fs/file.c~a
> +++ a/fs/file.c
> @@ -39,14 +39,17 @@ int sysctl_nr_open_max = 1024 * 1024; /*
>   */
>  static DEFINE_PER_CPU(struct fdtable_defer, fdtable_defer_list);
>  
> -static inline void *alloc_fdmem(unsigned int size)
> +static void *alloc_fdmem(unsigned int size)
>  {
> -	void *data;
> -
> -	data = kmalloc(size, GFP_KERNEL|__GFP_NOWARN);
> -	if (data != NULL)
> -		return data;
> -
> +	/*
> +	 * Very large allocations can stress page reclaim, so fall back to
> +	 * vmalloc() if the allocation size will be considered "large" by the VM.
> +	 */
> +	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) {
> +		void *data = kmalloc(size, GFP_KERNEL|__GFP_NOWARN);
> +		if (data != NULL)
> +			return data;
> +	}
>  	return vmalloc(size);
>  }
>  
> _
> 

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

#define PAGE_ALLOC_COSTLY_ORDER 3

On x86_64, this means we try kmalloc() up to 4096 files in fdtable.

Thanks !


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