Re: [PATCH] drop page cache of a single file

[Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>]

Zhang, Yanmin wrote:
> Currently, by /proc/sys/vm/drop_caches, applications could drop pagecache,
> slab(dentries and inodes), or both, but applications couldn't choose to
> just drop the page cache of one file. An user of VOD (Video-On-Demand)
> needs this capability to have more detailed control on page cache release.
> 
> Below patch against 2.6.19 implements it.
> 
> Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
> 
> ---
> 
> diff -Nraup linux-2.6.19/Documentation/filesystems/proc.txt linux-2.6.19_dropcache/Documentation/filesystems/proc.txt
> --- linux-2.6.19/Documentation/filesystems/proc.txt	2006-12-08 15:32:44.000000000 +0800
> +++ linux-2.6.19_dropcache/Documentation/filesystems/proc.txt	2006-12-28 10:20:39.000000000 +0800
> @@ -1320,6 +1320,8 @@ To free dentries and inodes:
>  	echo 2 > /proc/sys/vm/drop_caches
>  To free pagecache, dentries and inodes:
>  	echo 3 > /proc/sys/vm/drop_caches
> +To free the pagecache of one file:
> +	echo "4 /path/to/filename" > /proc/sys/vm/drop_caches
> 
>  As this is a non-destructive operation and dirty objects are not freeable, the
>  user should run `sync' first.

"sync" is the most time consuming operation.  Clean pagecache pages
are immediately reclaimable... they are actually free pages.  Writing
out dirty pages consumes time.

Hence this approach may not provide the required performance benefits
since only clean pagecache pages are marked free.  fadvise approach
would provide similar behavior.

--Vaidy

[snip]