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From: Thomas Monjalon <thomas@monjalon.net>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>,
 Wathsala Wathawana Vithanage <wathsala.vithanage@arm.com>,
 Tyler Retzlaff <roretzla@linux.microsoft.com>,
 Ruifeng Wang <Ruifeng.Wang@arm.com>, "dev@dpdk.org" <dev@dpdk.org>,
 nd <nd@arm.com>, Dhruv Tripathi <Dhruv.Tripathi@arm.com>,
 Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
 Jack Bond-Preston <jack.bond-preston@foss.arm.com>,
 Nick Connolly <Nick.Connolly@arm.com>, Vinod Krishna <Vinod.Krishna@arm.com>, 
 "david.marchand@redhat.com" <david.marchand@redhat.com>
Subject: Re: [PATCH v2 2/2] eal: add Arm WFET in power management intrinsics
Date: Thu, 04 Jul 2024 20:59:29 +0200
Message-ID: <3741901.S9oNm88NmQ@thomas>
In-Reply-To: <20240704075515.75a9a0cd@hermes.local>
References: <20240604044401.3577707-2-wathsala.vithanage@arm.com>
 <4208140.dumfJixkPq@thomas> <20240704075515.75a9a0cd@hermes.local>
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04/07/2024 16:55, Stephen Hemminger:
> On Thu, 04 Jul 2024 16:14:42 +0200
> Thomas Monjalon <thomas@monjalon.net> wrote:
>=20
> > > > Let=E2=80=99s ask Pavan why this flag is used in cn10k driver.
> > > >=20
> > > > From our perspective, WFE is available on all the supported arm pla=
tforms in
> > > > DPDK.
> > > > Therefore, RTE_ARM_USE_WFE should be treated as a flag to choose be=
tween
> > > > WFE
> > > > and non-WFE code paths due to performance reasons rather than as a =
flag
> > > > that indicates
> > > > the availability of the instruction on the target CPU.
> > > >  =20
> > >=20
> > > We are using this flag to allow application to choose between WFE and=
 non-WFE code path.
> > > The non-WFE path performs slightly better. =20
> >=20
> > What's the benefit of the WFE path then?
>=20
> WFE saves power at the expense of latency.

Yes maybe there is a misunderstanding.
Pavan can you confirm you were saying "throughput is better on non-WFE"?
but "power consumption is lower on WFE path"?

> Maybe some form of hybrid approach would work best and could
> be always used.
>=20
> For example, many implementations of mutex do a short spin poll
> then fall back to a waiting primitive (like futex).