On Tue, Apr 18, 2023 at 12:06:42PM +0100, Bruce Richardson wrote:
> 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) {
don't take direct dependency on compiler builtins. define a macro so we
don't have to play shotgun surgery later.
also what is the purpose of using __extension__ here? are you annotating
the use of __builtin_constant_p() or is there more? because if that's
the only reason i see no need to use __extension__ when already using a
compiler specific builtin like this, that it is not standard is implied
and enforced by a compile break.
> > + /*
> > + * 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];
>
> alternatively those last two lines can be replaced by a memcpy, which the
> compiler should nicely optimize itself, for constant size copy:
>
> memcpy(obj_table, cache_objs, sizeof(obj_table[0]) * n);
>
> > + RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> > + RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
> > +
> > + return 0;
> > + }
> > +
> > + /* Use the cache as much as we have to return hot objects first */
> > + len = __extension__(__builtin_constant_p(n)) ? cache->len :
> > + RTE_MIN(remaining, cache->len);
>
> This line confused me a lot, until I applied the patch and stared at it a
> lot :-). Why would the length value depend upon whether or not n is a
> compile-time constant?
> I think it would really help here to add in a comment stating that when n
> is a compile-time constant, at this point it much be "> cache->len" since
> the previous block was untaken, therefore we don't need to call RTE_MIN.
>
> > cache->len -= len;
> > remaining -= len;
> > for (index = 0; index < len; index++)
> > *obj_table++ = *--cache_objs;
> >
> > - if (remaining == 0) {
> > + if (!__extension__(__builtin_constant_p(n)) && remaining == 0) {
> > /* The entire request is satisfied from the cache. */
> >
> > RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
>
> Once I'd worked out the logic for the above conditional check, then this
> conditional adjustment was clear. I just wonder if a further comment might
> help here.
>
> I am also wondering if having larger blocks for the constant and
> non-constant cases may help. It would lead to some duplication but may
> clarify the code.
>
> > --
> > 2.17.1
> >
