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Date: Tue, 18 Apr 2023 08:44:35 -0700
From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: Morten =?iso-8859-1?Q?Br=F8rup?= <mb@smartsharesystems.com>
Cc: Bruce Richardson <bruce.richardson@intel.com>, olivier.matz@6wind.com,
andrew.rybchenko@oktetlabs.ru, dev@dpdk.org
Subject: Re: [PATCH] mempool: optimize get objects with constant n
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On Tue, Apr 18, 2023 at 05:30:27PM +0200, Morten Brørup wrote:
> > From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
> > Sent: Tuesday, 18 April 2023 17.15
> >
> > 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.
>
> ARM 32-bit memcpy() [1] does it this way, so I did the same.
>
> [1]: https://elixir.bootlin.com/dpdk/v23.03/source/lib/eal/arm/include/rte_memcpy_32.h#L122
i see thanks.
>
> While I agree that a macro for __builtin_constant_p() would be good, it belongs in a patch to fix portability, not in this patch.
i agree it isn't composite of this change.
would you mind introducing it as a separate patch and depending on it or
do you feel that would delay this patch too much? i wouldn't mind doing
it myself but there is a long merge time on my patches which means i end
up having to carry the adaptations locally for weeks at a time.
thanks