Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Sat, Jul 21, 2007 at 11:00:08PM +0200, Peter Zijlstra wrote:
> Scale the default max readahead size with the system memory size.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  block/ll_rw_blk.c  |    2 +-
>  include/linux/fs.h |    1 +
>  mm/readahead.c     |   32 ++++++++++++++++++++++++++++++++
>  3 files changed, 34 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/block/ll_rw_blk.c
> ===================================================================
> --- linux-2.6.orig/block/ll_rw_blk.c
> +++ linux-2.6/block/ll_rw_blk.c
> @@ -208,7 +208,7 @@ void blk_queue_make_request(request_queu
>  	blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
>  	blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
>  	q->make_request_fn = mfn;
> -	q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
> +	bdi_ra_init(&q->backing_dev_info);

fs/fuse/inode.c has another line to be converted.

>  	q->backing_dev_info.state = 0;
>  	q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
>  	blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
> Index: linux-2.6/include/linux/fs.h
> ===================================================================
> --- linux-2.6.orig/include/linux/fs.h
> +++ linux-2.6/include/linux/fs.h
> @@ -1696,6 +1696,7 @@ extern long do_splice_direct(struct file
>  
>  extern void
>  file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping);
> +extern void bdi_ra_init(struct backing_dev_info *bdi);
>  extern loff_t no_llseek(struct file *file, loff_t offset, int origin);
>  extern loff_t generic_file_llseek(struct file *file, loff_t offset, int origin);
>  extern loff_t remote_llseek(struct file *file, loff_t offset, int origin);
> Index: linux-2.6/mm/readahead.c
> ===================================================================
> --- linux-2.6.orig/mm/readahead.c
> +++ linux-2.6/mm/readahead.c
> @@ -42,6 +42,38 @@ file_ra_state_init(struct file_ra_state 
>  }
>  EXPORT_SYMBOL_GPL(file_ra_state_init);
>  
> +static unsigned long ra_pages;
> +
> +static __init int readahead_init(void)
> +{
> +	/*
> +	 * Scale the max readahead window with system memory
> +	 *
> +	 *   64M:   128K
> +	 *  128M:   180K
> +	 *  256M:   256K
> +	 *  512M:   360K
> +	 *    1G:   512K
> +	 *    2G:   724K
> +	 *    4G:  1024K
> +	 *    8G:  1448K
> +	 *   16G:  2048K
> +	 */
> +	ra_pages = int_sqrt(totalram_pages/16);
> +	if (ra_pages > (2 << (20 - PAGE_SHIFT)))
> +		ra_pages = 2 << (20 - PAGE_SHIFT);

We can elaborate on the numbers ;)

How about the following rules?
- limit it under 1MB: we have to consider latencies
- make them alignment-friendly, i.e. 128K, 256K, 512K, 1M.

My original plan is to simply do the following:

- #define VM_MAX_READAHEAD        128     /* kbytes */
+ #define VM_MAX_READAHEAD        512     /* kbytes */

I'd like to post some numbers to back-up the discussion:

  readahead   readahead
       size        miss
       128K         38%
       512K         45%
      1024K         49%

The numbers are measured on a fresh booted KDE desktop.

The majority misses come from the larger mmap read-arounds.
Sequential readahead hits are pretty high and not quite affected by
the readahead size, thanks to its size ramp-up process.

> +
> +	return 0;
> +}
> +
> +subsys_initcall(readahead_init);

Remove the global ra_pages and fold readahead_init() into bdi_ra_init()?
bdi_ra_init will only be called several times I guess.

> +
> +void bdi_ra_init(struct backing_dev_info *bdi)
> +{
> +	bdi->ra_pages = ra_pages;
> +}
> +EXPORT_SYMBOL(bdi_ra_init);