On Tue, Apr 18, 2023 at 01:29:49PM +0200, Morten Brørup wrote: > > From: Bruce Richardson [mailto:bruce.richard...@intel.com] > > Sent: Tuesday, 18 April 2023 13.07 > > > > On Tue, Apr 11, 2023 at 08:48:45AM +0200, Morten Brørup wrote: > > > When getting objects from the mempool, the number of objects to get is > > > often constant at build time. > > > > > > This patch adds another code path for this case, so the compiler can > > > optimize more, e.g. unroll the copy loop when the entire request is > > > satisfied from the cache. > > > > > > On an Intel(R) Xeon(R) E5-2620 v4 CPU, and compiled with gcc 9.4.0, > > > mempool_perf_test with constant n shows an increase in rate_persec by an > > > average of 17 %, minimum 9.5 %, maximum 24 %. > > > > > > The code path where the number of objects to get is unknown at build time > > > remains essentially unchanged. > > > > > > Signed-off-by: Morten Brørup <m...@smartsharesystems.com> > > > > Change looks a good idea. Some suggestions inline below, which you may want > > to > > take on board for any future version. I'd strongly suggest adding some > > extra clarifying code comments, as I suggest below. > > With those exta code comments: > > > > Acked-by: Bruce Richardson <bruce.richard...@intel.com> > > > > > --- > > > lib/mempool/rte_mempool.h | 24 +++++++++++++++++++++--- > > > 1 file changed, 21 insertions(+), 3 deletions(-) > > > > > > diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h > > > index 9f530db24b..ade0100ec7 100644 > > > --- a/lib/mempool/rte_mempool.h > > > +++ b/lib/mempool/rte_mempool.h > > > @@ -1500,15 +1500,33 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, > > void **obj_table, > > > if (unlikely(cache == NULL)) > > > goto driver_dequeue; > > > > > > - /* Use the cache as much as we have to return hot objects first */ > > > - len = RTE_MIN(remaining, cache->len); > > > cache_objs = &cache->objs[cache->len]; > > > + > > > + if (__extension__(__builtin_constant_p(n)) && n <= cache->len) { > > > + /* > > > + * The request size is known at build time, and > > > + * the entire request can be satisfied from the cache, > > > + * so let the compiler unroll the fixed length copy loop. > > > + */ > > > + cache->len -= n; > > > + for (index = 0; index < n; index++) > > > + *obj_table++ = *--cache_objs; > > > + > > > > This loop looks a little awkward to me. Would it be clearer (and perhaps > > easier for compilers to unroll efficiently if it was rewritten as: > > > > cache->len -= n; > > cache_objs = &cache->objs[cache->len]; > > for (index = 0; index < n; index++) > > obj_table[index] = cache_objs[index]; > > The mempool cache is a stack, so the copy loop needs get the objects in > decrementing order. I.e. the source index decrements and the destination > index increments. >
BTW: Please add this as a comment in the code too, above the loop to avoid future developers (or even future me), asking this question again! /Bruce
