From: "Mattias Rönnblom" <mattias.ronnblom@ericsson.com>
To: Bruce Richardson <bruce.richardson@intel.com>
Cc: "Morten Brørup" <mb@smartsharesystems.com>,
olivier.matz@6wind.com, stephen@networkplumber.org,
harry.van.haaren@intel.com, konstantin.ananyev@intel.com,
dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH v2 2/2] mbuf: add bulk free function
Date: Thu, 26 Sep 2019 22:11:06 +0200 [thread overview]
Message-ID: <699b4e3e-e253-5a6a-6488-3f9b38fd8f4c@ericsson.com> (raw)
In-Reply-To: <20190926083024.GA1821@bricha3-MOBL.ger.corp.intel.com>
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 <mb@smartsharesystems.com>
>>> ---
>>> 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?
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?
next prev parent reply other threads:[~2019-09-26 20:11 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-25 12:03 [dpdk-dev] [PATCH v2 0/2] " Morten Brørup
2019-09-25 12:03 ` [dpdk-dev] [PATCH v2 1/2] " Morten Brørup
2019-09-25 12:03 ` [dpdk-dev] [PATCH v2 2/2] " Morten Brørup
[not found] ` <20190925120542.A51B41BE84@dpdk.org>
2019-09-25 12:17 ` [dpdk-dev] |WARNING| pw59738 " Morten Brørup
2019-09-25 23:37 ` Stephen Hemminger
2019-09-27 6:42 ` Morten Brørup
2019-09-25 19:02 ` [dpdk-dev] " Mattias Rönnblom
2019-09-26 8:30 ` Bruce Richardson
2019-09-26 20:11 ` Mattias Rönnblom [this message]
2019-09-27 9:09 ` Bruce Richardson
2019-09-26 9:26 ` Andrew Rybchenko
2019-09-26 15:35 ` Morten Brørup
2019-09-26 10:23 ` Ananyev, Konstantin
2019-09-27 10:22 ` Morten Brørup
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