From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dpdk.org (dpdk.org [92.243.14.124]) by inbox.dpdk.org (Postfix) with ESMTP id 76A01A0613 for ; Fri, 27 Sep 2019 11:09:09 +0200 (CEST) Received: from [92.243.14.124] (localhost [127.0.0.1]) by dpdk.org (Postfix) with ESMTP id 458A04C77; Fri, 27 Sep 2019 11:09:09 +0200 (CEST) Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by dpdk.org (Postfix) with ESMTP id 1EEB631FC for ; Fri, 27 Sep 2019 11:09:06 +0200 (CEST) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by fmsmga104.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 27 Sep 2019 02:09:06 -0700 X-IronPort-AV: E=Sophos;i="5.64,554,1559545200"; d="scan'208";a="196653454" Received: from bricha3-mobl.ger.corp.intel.com ([10.237.221.95]) by fmsmga003-auth.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 27 Sep 2019 02:09:04 -0700 Date: Fri, 27 Sep 2019 10:09:01 +0100 From: Bruce Richardson To: Mattias =?iso-8859-1?Q?R=F6nnblom?= Cc: Morten =?iso-8859-1?Q?Br=F8rup?= , olivier.matz@6wind.com, stephen@networkplumber.org, harry.van.haaren@intel.com, konstantin.ananyev@intel.com, dev@dpdk.org Message-ID: <20190927090901.GA1843@bricha3-MOBL.ger.corp.intel.com> References: <20190925120355.44821-1-mb@smartsharesystems.com> <20190925120355.44821-3-mb@smartsharesystems.com> <20190926083024.GA1821@bricha3-MOBL.ger.corp.intel.com> <699b4e3e-e253-5a6a-6488-3f9b38fd8f4c@ericsson.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <699b4e3e-e253-5a6a-6488-3f9b38fd8f4c@ericsson.com> User-Agent: Mutt/1.11.4 (2019-03-13) Subject: Re: [dpdk-dev] [PATCH v2 2/2] mbuf: add bulk free function X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Thu, Sep 26, 2019 at 10:11:06PM +0200, Mattias Rönnblom wrote: > On 2019-09-26 10:30, Bruce Richardson wrote: > > On Wed, Sep 25, 2019 at 09:02:28PM +0200, Mattias Rönnblom wrote: > > > On 2019-09-25 14:03, Morten Brørup wrote: > > > > Add function for freeing a bulk of mbufs. > > > > > > > > Signed-off-by: Morten Brørup > > > > --- > > > > lib/librte_mbuf/rte_mbuf.c | 35 +++++++++++++++++++++++++++++++++++ > > > > lib/librte_mbuf/rte_mbuf.h | 16 +++++----------- > > > > 2 files changed, 40 insertions(+), 11 deletions(-) > > > > > > > > diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c > > > > index 37718d49c..b63a0eced 100644 > > > > --- a/lib/librte_mbuf/rte_mbuf.c > > > > +++ b/lib/librte_mbuf/rte_mbuf.c > > > > @@ -245,6 +245,41 @@ int rte_mbuf_check(const struct rte_mbuf *m, int is_header, > > > > return 0; > > > > } > > > > +/** > > > > + * Maximum bulk of mbufs rte_pktmbuf_free_bulk() returns to mempool. > > > > + */ > > > > +#define RTE_PKTMBUF_FREE_BULK_SZ 64 > > > > + > > > > +/* Free a bulk of mbufs back into their original mempools. */ > > > > +void rte_pktmbuf_free_bulk(struct rte_mbuf **mbufs, unsigned int count) > > > > +{ > > > > + struct rte_mbuf *m, *free[RTE_PKTMBUF_FREE_BULK_SZ]; > > > > + unsigned int idx, nb_free = 0; > > > > + > > > > + for (idx = 0; idx < count; idx++) { > > > > + m = mbufs[idx]; > > > > + if (unlikely(m == NULL)) > > > > + continue; > > > > + > > > > + __rte_mbuf_sanity_check(m, 1); > > > > + m = rte_pktmbuf_prefree_seg(m); > > > > + if (unlikely(m == NULL)) > > > > + continue; > > > > + > > > > + if (nb_free >= RTE_PKTMBUF_FREE_BULK_SZ || > > > > + (nb_free > 0 && m->pool != free[0]->pool)) { > > > > > > Maybe an unlikely() would be in order here? > > > > > I'd caution against it, since it can penalize the cold branch a lot. If a > > branch really is predictable the HW branch predictors generally are good > > enough to handle it at runtime. So long as a path is a valid path for a > > runtime app, i.e. not something like a fatal error only ever hit once in an > > entire run, I'd tend to omit likely()/unlikely() calls unless profiling > > shows a real performance difference. > > > > Let's see if I understand you: your worry is that wrapping that expression > in an unlikely() will lead to code that is slower (than w/o the hint), if > during runtime the probability turns out to be 50/50? > While not an expert, I believe that the use of likely/unlikely can cause the unexpected part of the branch to be moved to a different part of the code and potentially be more expensive to call, meaning that the performance may be poorer even if the probability is lower than 50/50. > Wouldn't leaving out unlikely() just lead to the compiler using its fancy > heuristics in an attempt to come to a conclusion, what path is the more > likely? > > About HW branch prediction - I'm sure it's good, but still the compiler > needs to decided which code code path requires a branch, and which need not. > Even if HW branch prediction successfully predicted a branch being taken, > actually branching is going to be somewhat more expensive than to not > branch? The cost difference between a taken and untaken branch should be unnoticable so long as the branch is correctly predicted - which if does always go one way, it will be each time each time after the first. Overall, though, I suspect the presence of likely/unlikely is going to make any real difference, so I'd therefore err on the side of leaving it out in the absense of evidence that it helps in some cases. /Bruce