Re: [PATCH 3/3] readahead: introduce context readahead algorithm

[Ingo Molnar <mingo@elte.hu>]

* Wu Fengguang <fengguang.wu@intel.com> wrote:

> Introduce page cache context based readahead algorithm.
> This is to better support concurrent read streams in general.

>  /*
> + * Count contiguously cached pages from @offset-1 to @offset-@max,
> + * this count is a conservative estimation of
> + * 	- length of the sequential read sequence, or
> + * 	- thrashing threshold in memory tight systems
> + */
> +static pgoff_t count_history_pages(struct address_space *mapping,
> +				   struct file_ra_state *ra,
> +				   pgoff_t offset, unsigned long max)
> +{
> +	pgoff_t head;
> +
> +	rcu_read_lock();
> +	head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
> +	rcu_read_unlock();
> +
> +	return offset - 1 - head;
> +}

Very elegant method! I suspect this will work far better 
than adding various increasingly more complex heuristics.

Emphatically-Acked-by: Ingo Molnar <mingo@elte.hu>

	Ingo