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From: Morten Brørup <mb@smartsharesystems.com>
To: "Honnappa Nagarahalli" <Honnappa.Nagarahalli@arm.com>,
	<thomas@monjalon.net>, "Jerin Jacob" <jerinjacobk@gmail.com>,
	"Konstantin Ananyev" <konstantin.ananyev@intel.com>,
	<jerinj@marvell.com>
Cc: <dev@dpdk.org>, "Olivier Matz" <olivier.matz@6wind.com>,
	"David Christensen" <drc@linux.vnet.ibm.com>,
	"Stephen Hemminger" <stephen@networkplumber.org>,
	"nd" <nd@arm.com>, "nd" <nd@arm.com>
Subject: Re: [dpdk-dev] [RFC] ring: make ring implementation non-inlined
Date: Wed, 1 Jul 2020 09:27:02 +0200
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35C610D9@smartserver.smartshare.dk> (raw)
In-Reply-To: <DB6PR0802MB2216BCFE9C1E7644DEA1DEB0986F0@DB6PR0802MB2216.eurprd08.prod.outlook.com>

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Honnappa
> Nagarahalli
> Sent: Wednesday, July 1, 2020 1:16 AM
> 
> <snip>
> 
> > Subject: Re: [dpdk-dev] [RFC] ring: make ring implementation non-
> inlined
> >
> > 26/03/2020 09:04, Morten Brørup:
> > > From: Jerin Jacob
> > > > On Fri, Mar 20, 2020 Konstantin Ananyev wrote:
> > > > >
> > > > > As was discussed here:
> > > > > http://mails.dpdk.org/archives/dev/2020-February/158586.html
> > > > > this RFC aimed to hide ring internals into .c and make all ring
> > > > > functions non-inlined. In theory that might help to maintain
> ABI
> > > > > stability in future.
> > > > > This is just a POC to measure the impact of proposed idea,
> proper
> > > > > implementation would definetly need some extra effort.
> > > > > On IA box (SKX) ring_perf_autotest shows ~20-30 cycles extra
> for
> > > > > enqueue+dequeue pair. On some more realistic code, I suspect
> > > > > the impact it might be a bit higher.
> > > > > For MP/MC bulk transfers degradation seems quite small, though
> for
> > > > > SP/SC and/or small transfers it is more then noticable (see
> exact
> > > > > numbers below).
> > > > > From my perspective we'd probably keep it inlined for now to
> avoid
> > > > > any non-anticipated perfomance degradations.
> > > > > Though intersted to see perf results and opinions from other
> > > > > interested parties.
> > > >
> > > > +1
> >
> > Konstantin, thank you for doing some measures
> >
> >
> > > > My reasoning is a bit different, DPDK is using in embedded boxes
> too
> > > > where performance has more weight than ABI stuff.
> > >
> > > As a network appliance vendor I can confirm that we certainly care
> > > more about performance than ABI stability.
> > > ABI stability is irrelevant for us;
> > > and API instability is a non-recurring engineering cost each time
> we
> > > choose to switch to a new DPDK version, which we only do if we
> cannot
> > > avoid it, e.g. due to new drivers, security fixes or new features
> that
> > > we want to use.
> > >
> > > For us, the trend pointed in the wrong direction when DPDK switched
> > > the preference towards runtime configurability and deprecated
> compile
> > > time configurability. I do understand the reasoning behind it, and
> the
> > > impact is minimal, so we accept it.
> >
> > The code can be optimized by removing some instructions with #ifdef.
> > But the complexity of managing #ifdef enabling/disabling, depending
> on the
> > platform and the use case, would be huge.
> > We try to have a reasonable code "always enabled" which performs well
> in all
> > cases. This is a design choice which makes DPDK a library, not a pool
> of code
> > to cherry-pick.
> >
> > > However, if DPDK starts sacrificing performance of the core
> libraries
> > > for the benefits of the GNU/Linux distributors, network appliance
> > > vendors may put more effort into sticking with old DPDK versions
> > > instead of updating.
> >
> > The initial choice regarding ABI compatibility was "do not care".
> > Recently, the decision was done to care about ABI compatibility as
> priority
> > number 2. The priority number 1 remains the performance.
> > That's a reason for allowing some ABI breakages in some specific
> releases
> > announced in advance.
> >
> > > > I think we need to focus first on slow path APIs ABI stuff.
> >
> > Yes we should not degrade fast path performance for the sake of
> avoiding
> > uncertain future ABI issues.
> >
> > Morten, Jerin, thank you for the feedback.
> I think we have a consensus here not to make any changes to inline
> functions for now.
> Should we mark this as 'Deferred or Rejected'?

Rejected.

There is no need for this modification now, and no actual use cases for it in the road map. In other words: This modification has no use cases; it is purely academic. Many other suggestions have been rejected for the reason that they have no current use cases.

As Thomas mentioned, DPDK has transitioned towards being a library, rather than a pool of code to cherry-pick from. I have learned to live with this.

Being a library doesn't mean that functions cannot be exposed as inline code in the library header files. DPDK is mainly a high performance library with a tradition of exposing many of its internals in its API, and we should keep it this way. We certainly don't want an opaque API hiding all of its internals, passing around void pointers.

However, it was still an interesting experiment to investigate the performance cost.


  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-20 16:41 Konstantin Ananyev
2020-03-20 17:54 ` Stephen Hemminger
2020-03-21  1:03   ` Ananyev, Konstantin
2020-03-25 21:09 ` Jerin Jacob
2020-03-26  0:28   ` Ananyev, Konstantin
2020-03-26  8:04   ` Morten Brørup
2020-03-31 23:25     ` Thomas Monjalon
2020-06-30 23:15       ` Honnappa Nagarahalli
2020-07-01  7:27         ` Morten Brørup [this message]
2020-07-01 12:21           ` Ananyev, Konstantin
2020-07-01 14:11             ` Honnappa Nagarahalli
2020-07-01 14:31               ` David Marchand

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