Re: [PATCH] mempool: optimize get objects with constant n

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

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 <mb@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.richardson@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
> >