Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays

From: Thomas Monjalon <thomas@monjalon.net>
To: Jerin Jacob <jerinjacobk@gmail.com>
Cc: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
	"Richardson, Bruce" <bruce.richardson@intel.com>,
	"Morten Brørup" <mb@smartsharesystems.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	"olivier.matz@6wind.com" <olivier.matz@6wind.com>,
	"andrew.rybchenko@oktetlabs.ru" <andrew.rybchenko@oktetlabs.ru>,
	"honnappa.nagarahalli@arm.com" <honnappa.nagarahalli@arm.com>,
	"Yigit, Ferruh" <ferruh.yigit@intel.com>,
	"jerinj@marvell.com" <jerinj@marvell.com>,
	"gakhil@marvell.com" <gakhil@marvell.com>
Subject: Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays
Date: Tue, 15 Jun 2021 11:18:11 +0200	[thread overview]
Message-ID: <1725976.ZNbi1HEfbS@thomas> (raw)
In-Reply-To: <CALBAE1OVu2xoXK7TsETVA+DoOFdmNCF+rRcOjQBfN-SdshrdZw@mail.gmail.com>

15/06/2021 10:00, Jerin Jacob:
> On Tue, Jun 15, 2021 at 12:22 PM Thomas Monjalon <thomas@monjalon.net> wrote:
> > 14/06/2021 17:48, Jerin Jacob:
> > > On Mon, Jun 14, 2021 at 8:29 PM Ananyev, Konstantin
> > > <konstantin.ananyev@intel.com> wrote:
> > > > > 14/06/2021 15:15, Bruce Richardson:
> > > > > > While I dislike the hard-coded limits in DPDK, I'm also not convinced that
> > > > > > we should switch away from the flat arrays or that we need fully dynamic
> > > > > > arrays that grow/shrink at runtime for ethdevs. I would suggest a half-way
> > > > > > house here, where we keep the ethdevs as an array, but one allocated/sized
> > > > > > at runtime rather than statically. This would allow us to have a
> > > > > > compile-time default value, but, for use cases that need it, allow use of a
> > > > > > flag e.g.  "max-ethdevs" to change the size of the parameter given to the
> > > > > > malloc call for the array.  This max limit could then be provided to apps
> > > > > > too if they want to match any array sizes. [Alternatively those apps could
> > > > > > check the provided size and error out if the size has been increased beyond
> > > > > > what the app is designed to use?]. There would be no extra dereferences per
> > > > > > rx/tx burst call in this scenario so performance should be the same as
> > > > > > before (potentially better if array is in hugepage memory, I suppose).
> > > > >
> > > > > I think we need some benchmarks to decide what is the best tradeoff.
> > > > > I spent time on this implementation, but sorry I won't have time for benchmarks.
> > > > > Volunteers?
> > > >
> > > > I had only a quick look at your approach so far.
> > > > But from what I can read, in MT environment your suggestion will require
> > > > extra synchronization for each read-write access to such parray element (lock, rcu, ...).
> > > > I think what Bruce suggests will be much ligther, easier to implement and less error prone.
> > > > At least for rte_ethdevs[] and friends.
> > >
> > > +1
> >
> > Please could you have a deeper look and tell me why we need more locks?
> 
> We don't need more locks (It is fat mutex) now in the implementation.
> 
> If it needs to use in fastpath, we need more state of art
> synchronization like RCU.
> 
> Also, you can take look at VPP dynamic array implementation which is
> used in fastpath.
> 
> https://docs.fd.io/vpp/21.10/db/d65/vec_8h.html
> 
> So the question is the use case for this API. Is it for slowpath item
> like ethdev[] memory
> or fastpath items like holding an array of mbuf etc.

As I replied to Morten, it is for read in fast path
and alloc/free in slow path.
I should highlight this in the commit log if there is a v2.
That's why there is a mutex in alloc/free and nothing in read access.

> > The element pointers doesn't change.
> > Only the array pointer change at resize,
> > but the old one is still usable until the next resize.
> > I think we don't need more.