Re: [dpdk-dev] [EXT] [PATCH v3 6/6] spinlock: ticket based to improve fairness

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 3 Jan 2019 18:35:31 +0000
Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com> wrote:

> > > > > >
> > > > > > On Thu, 2018-12-27 at 12:13 +0800, Gavin Hu wrote:  
> > > > > > > -----------------------------------------------------------
> > > > > > > ----
> > > > > > > ----
> > > > > > > ---
> > > > > > > From: Joyce Kong <joyce.kong@arm.com>
> > > > > > >
> > > > > > > The old implementation is unfair, some threads may take locks
> > > > > > > aggressively  
> > > > > >
> > > > > > I think, one issue here is x86 and ppc follows traditional
> > > > > > spinlock and
> > > > > > arm64 will be following ticket lock for spinlock implementation.
> > > > > > This would change application behaviour on arm64 compared to
> > > > > > x86
> > > > > > and
> > > > > > ppc.
> > > > > >
> > > > > > How about having a separate API for ticket lock? That would
> > > > > > give, # application choice to use the locking strategy #
> > > > > > application behaviour will be same across all arch.  
> > > > >
> > > > > Ok, will do in v4 to have a new named rte_ticket_spinlock API.  
> > > >
> > > > I would prefer rte_ticketlock_[lock/unlock/trylock/is_locked] name
> > > > instead of rte_ticket_spinlock_lock etc to reduce the length of the
> > > > API.  
> > >
> > > NAK to adding new API for this.
> > >
> > > I want the best possible locks for all applications and all
> > > architectures.
> > > These should be called spinlock so there is no requirement for
> > > application to change to get better performance. Why not just
> > > implement the best algorithm across the board. Yes, this means
> > > collaboration or working on the other guys architecture.  
> IMO, the ticket lock should be a separate API. Spin lock (as implemented today) and ticket lock have different characteristics in terms of behavior as well as resources required. For ex: spin lock might be sufficient for a low contention use case and it requires less number of cache lines. Ticket lock needs more number of cache lines and might be good for use cases with more contention. The decision to use spin lock vs ticket lock should be left to the application.

The problem is that changing applications is hard. Real world applications are non-trivial
and large. That is while doing things global or at the library level are easier.
Do you want to go back and evaluate every lock in VPP, NFF-go, OVS, Tungsten Fabric, ....

Either a config or runtime option seems best to me.