[PATCH] mempool: micro-optimize put function

[Morten Brørup]

> From: Andrew Rybchenko [mailto:andrew.rybchenko at oktetlabs.ru]
> Sent: Wednesday, 16 November 2022 12.05
> 
> On 11/16/22 13:18, Morten Brørup wrote:
> > Micro-optimization:
> > Reduced the most likely code path in the generic put function by
> moving an
> > unlikely check out of the most likely code path and further down.
> >
> > Also updated the comments in the function.
> >
> > Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
> > ---
> >   lib/mempool/rte_mempool.h | 35 ++++++++++++++++++-----------------
> >   1 file changed, 18 insertions(+), 17 deletions(-)
> >
> > diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> > index 9f530db24b..aba90dbb5b 100644
> > --- a/lib/mempool/rte_mempool.h
> > +++ b/lib/mempool/rte_mempool.h
> > @@ -1364,32 +1364,33 @@ rte_mempool_do_generic_put(struct rte_mempool
> *mp, void * const *obj_table,
> >   {
> >   	void **cache_objs;
> >
> > -	/* No cache provided */
> > +	/* No cache provided? */
> >   	if (unlikely(cache == NULL))
> >   		goto driver_enqueue;
> >
> > -	/* increment stat now, adding in mempool always success */
> > +	/* Increment stats now, adding in mempool always succeeds. */
> >   	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_bulk, 1);
> >   	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, n);
> >
> > -	/* The request itself is too big for the cache */
> > -	if (unlikely(n > cache->flushthresh))
> > -		goto driver_enqueue_stats_incremented;
> 
> I've kept the check here since it protects against overflow in len plus
> n below if n is really huge.

We can fix that, see below.

> 
> > -
> > -	/*
> > -	 * The cache follows the following algorithm:
> > -	 *   1. If the objects cannot be added to the cache without
> crossing
> > -	 *      the flush threshold, flush the cache to the backend.
> > -	 *   2. Add the objects to the cache.
> > -	 */
> > -
> > -	if (cache->len + n <= cache->flushthresh) {
> > +	if (likely(cache->len + n <= cache->flushthresh)) {

It is an invariant that cache->len <= cache->flushthresh, so the above comparison can be rewritten to protect against overflow:

if (likely(n <= cache->flushthresh - cache->len)) {

> > +		/*
> > +		 * The objects can be added to the cache without crossing
> the
> > +		 * flush threshold.
> > +		 */
> >   		cache_objs = &cache->objs[cache->len];
> >   		cache->len += n;
> > -	} else {
> > +	} else if (likely(n <= cache->flushthresh)) {
> > +		/*
> > +		 * The request itself fits into the cache.
> > +		 * But first, the cache must be flushed to the backend, so
> > +		 * adding the objects does not cross the flush threshold.
> > +		 */
> >   		cache_objs = &cache->objs[0];
> >   		rte_mempool_ops_enqueue_bulk(mp, cache_objs, cache->len);
> >   		cache->len = n;
> > +	} else {
> > +		/* The request itself is too big for the cache. */
> > +		goto driver_enqueue_stats_incremented;
> >   	}
> >
> >   	/* Add the objects to the cache. */
> > @@ -1399,13 +1400,13 @@ rte_mempool_do_generic_put(struct rte_mempool
> *mp, void * const *obj_table,
> >
> >   driver_enqueue:
> >
> > -	/* increment stat now, adding in mempool always success */
> > +	/* Increment stats now, adding in mempool always succeeds. */
> >   	RTE_MEMPOOL_STAT_ADD(mp, put_bulk, 1);
> >   	RTE_MEMPOOL_STAT_ADD(mp, put_objs, n);
> >
> >   driver_enqueue_stats_incremented:
> >
> > -	/* push objects to the backend */
> > +	/* Push the objects to the backend. */
> >   	rte_mempool_ops_enqueue_bulk(mp, obj_table, n);
> >   }
> >
>