Re: [PATCH] mm: Implement swap prefetching tweaks

[Andrew Morton <akpm@osdl.org>]

Con Kolivas <kernel@kolivas.org> wrote:
>
> +	/*
> +	 * get_page_state is super expensive so we only perform it every
> +	 * SWAP_CLUSTER_MAX prefetched_pages.

nr_running() is similarly expensive btw.

> 	 * We also test if we're the only
> +	 * task running anywhere. We want to have as little impact on all
> +	 * resources (cpu, disk, bus etc). As this iterates over every cpu
> +	 * we measure this infrequently.
> +	 */
> +	if (!(sp_stat.prefetched_pages % SWAP_CLUSTER_MAX)) {
> +		unsigned long cpuload = nr_running();
> +
> +		if (cpuload > 1)
> +			goto out;

Sorry, this is just wrong.  If swap prefetch is useful then it's also
useful if some task happens to be sitting over in the corner calculating
pi.

What's the actual problem here?  Someone's 3d game went blippy?  Why?  How
much?  Are we missing a cond_resched()?

> +		cpuload += nr_uninterruptible();
> +		if (cpuload > 1)
> +			goto out;

Not sure about this either.  


> +		if (ns->last_free) {
> +			if (ns->current_free + SWAP_CLUSTER_MAX <
> +				ns->last_free) {
> +					ns->last_free = ns->current_free;
>  					node_clear(node,
>  						sp_stat.prefetch_nodes);
>  					continue;
>  			}
>  		} else

That has an extra tabstop.