Re: Reduce pool cache items when there is no contention

[David Gwynne]

> On 5 Jul 2017, at 20:11, Visa Hankala  wrote:
> 
> The current pool cache code increases the number of items that can be
> cached locally in response to lock contention. This patch adds a tweak
> that lowers the number when contention does not occur. The idea is to
> let resources be returned to the common pool when pressure on the cache
> has decreased.
> 
> This change alone does not move elements from the cache to the common
> pool. The cache has to have some activity for the moving to happen.
> 
> It is likely that `pr_cache_items' does not settle on a single value
> but oscillates within some range. In my testing, I have not seen any
> wild swings that would indicate positive feedback in the process.
> The adjustments are constant per step and happen relatively rarely.
> 
> OK?

ok

> 
> Index: kern/subr_pool.c
> ===
> RCS file: src/sys/kern/subr_pool.c,v
> retrieving revision 1.217
> diff -u -p -r1.217 subr_pool.c
> --- kern/subr_pool.c  23 Jun 2017 01:21:55 -  1.217
> +++ kern/subr_pool.c  5 Jul 2017 09:27:40 -
> @@ -1957,6 +1957,9 @@ pool_cache_gc(struct pool *pp)
>   if ((contention - pp->pr_cache_contention_prev) > 8 /* magic */) {
>   if ((ncpusfound * 8 * 2) <= pp->pr_cache_nitems)
>   pp->pr_cache_items += 8;
> + } else if ((contention - pp->pr_cache_contention_prev) == 0) {
> + if (pp->pr_cache_items > 8)
> + pp->pr_cache_items--;
>   }
>   pp->pr_cache_contention_prev = contention;
> }
> 

Reduce pool cache items when there is no contention

[Visa Hankala]

The current pool cache code increases the number of items that can be
cached locally in response to lock contention. This patch adds a tweak
that lowers the number when contention does not occur. The idea is to
let resources be returned to the common pool when pressure on the cache
has decreased.

This change alone does not move elements from the cache to the common
pool. The cache has to have some activity for the moving to happen.

It is likely that `pr_cache_items' does not settle on a single value
but oscillates within some range. In my testing, I have not seen any
wild swings that would indicate positive feedback in the process.
The adjustments are constant per step and happen relatively rarely.

OK?

Index: kern/subr_pool.c
===
RCS file: src/sys/kern/subr_pool.c,v
retrieving revision 1.217
diff -u -p -r1.217 subr_pool.c
--- kern/subr_pool.c23 Jun 2017 01:21:55 -  1.217
+++ kern/subr_pool.c5 Jul 2017 09:27:40 -
@@ -1957,6 +1957,9 @@ pool_cache_gc(struct pool *pp)
if ((contention - pp->pr_cache_contention_prev) > 8 /* magic */) {
if ((ncpusfound * 8 * 2) <= pp->pr_cache_nitems)
pp->pr_cache_items += 8;
+   } else if ((contention - pp->pr_cache_contention_prev) == 0) {
+   if (pp->pr_cache_items > 8)
+   pp->pr_cache_items--;
}
pp->pr_cache_contention_prev = contention;
 }