Re: [PATCH] readahead: add notes on readahead size

[Vivek Goyal]

On Fri, Feb 26, 2010 at 10:48:37AM +0800, Wu Fengguang wrote:
> > readahead: limit read-ahead size for small memory systems
> > 
> > When lifting the default readahead size from 128KB to 512KB,
> > make sure it won't add memory pressure to small memory systems.
> 
> btw, I wrote some comments to summarize the now complex readahead size
> rules..
> 
> ==
> readahead: add notes on readahead size
> 
> Basically, currently the default max readahead size
> - is 512k
> - is boot time configurable with "readahead="
> and is auto scaled down:
> - for small devices
> - for small memory systems (read-around size alone)
> 
> CC: Matt Mackall <mpm@xxxxxxxxxxx>
> CC: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
>  mm/readahead.c |   22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> --- linux.orig/mm/readahead.c	2010-02-26 10:11:41.000000000 +0800
> +++ linux/mm/readahead.c	2010-02-26 10:11:55.000000000 +0800
> @@ -7,6 +7,28 @@
>   *		Initial version.
>   */
>  
> +/*
> + * Notes on readahead size.
> + *
> + * The default max readahead size is VM_MAX_READAHEAD=512k,
> + * which can be changed by user with boot time parameter "readahead="
> + * or runtime interface "/sys/devices/virtual/bdi/default/read_ahead_kb".
> + * The latter normally only takes effect in future for hot added devices.
> + *
> + * The effective max readahead size for each block device can be accessed with
> + * 1) the `blockdev` command
> + * 2) /sys/block/sda/queue/read_ahead_kb
> + * 3) /sys/devices/virtual/bdi/$(env stat -c '%t:%T' /dev/sda)/read_ahead_kb
> + *
> + * They are typically initialized with the global default size, however may be
> + * auto scaled down for small devices in add_disk(). NFS, software RAID, btrfs
> + * etc. have special rules to setup their default readahead size.
> + *
> + * The mmap read-around size typically equals with readahead size, with an
> + * extra limit proportional to system memory size.  For example, a 64MB box
> + * will have a 64KB read-around size limit, 128MB mem => 128KB limit, etc.
> + */
> +

Great. I was confused among so many ways to control read ahead size. This
documentation helps a lot.

Vivek

>  #include <linux/kernel.h>
>  #include <linux/fs.h>
>  #include <linux/memcontrol.h>
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