Re: [dpdk-dev] [RFC 0/6] New sync modes for ring

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 24 Feb 2020 23:29:57 +0530
Jerin Jacob <jerinjacobk@gmail.com> wrote:

> On Mon, Feb 24, 2020 at 10:29 PM Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> >
> > On Mon, 24 Feb 2020 11:35:09 +0000
> > Konstantin Ananyev <konstantin.ananyev@intel.com> wrote:
> >  
> > > Upfront note - that RFC is not a complete patch.
> > > It introduces an ABI breakage, plus it doesn't update ring_elem
> > > code properly, etc.
> > > I plan to deal with all these things in later versions.
> > > Right now I seek an initial feedback about proposed ideas.
> > > Would also ask people to repeat performance tests (see below)
> > > on their platforms to confirm the impact.
> > >
> > > More and more customers use(/try to use) DPDK based apps within
> > > overcommitted systems (multiple acttive threads over same pysical cores):
> > > VM, container deployments, etc.
> > > One quite common problem they hit: Lock-Holder-Preemption with rte_ring.
> > > LHP is quite a common problem for spin-based sync primitives
> > > (spin-locks, etc.) on overcommitted systems.
> > > The situation gets much worse when some sort of
> > > fair-locking technique is used (ticket-lock, etc.).
> > > As now not only lock-owner but also lock-waiters scheduling
> > > order matters a lot.
> > > This is a well-known problem for kernel within VMs:
> > > http://www-archive.xenproject.org/files/xensummitboston08/LHP.pdf
> > > https://www.cs.hs-rm.de/~kaiser/events/wamos2017/Slides/selcuk.pdf
> > > The problem with rte_ring is that while head accusion is sort of
> > > un-fair locking, waiting on tail is very similar to ticket lock schema -
> > > tail has to be updated in particular order.
> > > That makes current rte_ring implementation to perform
> > > really pure on some overcommited scenarios.  
> >
> > Rather than reform rte_ring to fit this scenario, it would make
> > more sense to me to introduce another primitive. The current lockless
> > ring performs very well for the isolated thread model that DPDK
> > was built around. This looks like a case of customers violating
> > the usage model of the DPDK and then being surprised at the fallout.  
> 
> I agree with Stephen here.
> 
> I think, adding more runtime check in the enqueue() and dequeue() will
> have a bad effect on the low-end cores too.
> But I agree with the problem statement that in the virtualization use
> case, It may be possible to have N virtual cores runs on a physical
> core.
> 
> IMO, The best solution would be keeping the ring API same and have a
> different flavor in "compile-time". Something like
> liburcu did for accommodating different flavors.
> 
> i.e urcu-qsbr.h and urcu-bp.h will identical definition of API. The
> application can simply include ONE header file in a C file based on
> the flavor.
> If need both at runtime. Need to have function pointer or so in the
> application and define the function in different c file by including
> the approaite flavor in C file.

This would also be a good time to consider the tradeoffs of the
heavy use of inlining that is done in rte_ring vs the impact that
has on API/ABI stability.