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

[Ananyev, Konstantin]

> > > > 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
> These slides seem to indicate that the problems are mitigated through the Hypervisor configuration. Do we still need to address the issues?

I am not really an expert here, but AFAIK current mitigations deal mostly with guest kernel:
linux implements PV version of spinlocks (unfair and/or based on hypercall availability),
hypervisor might make decision itself based on is guest in user/kernel mode,
plus on some special cpu instructions. 
We do spin in user-space mode.
Might be hypervisors became smarter these days, but so far,
I heard about few different customers that hit such problem.
As an example, NA DPDK summit presentation:
https://dpdkna2019.sched.com/event/WYBG/dpdk-containers-challenges-solutions-wang-yong-zte
page 16 (problem #4) describes same issue.

> 
> > > > 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.
> It is hard to imagine that there are data plane applications deployed in such environments. Wouldn't this affect the performance terribly?

It wouldn't reach same performance as isolated threads, 
but for some tasks it might be enough. 
AFAIK, one quite common scenario - few isolated threads(/processes) doing
actual IO and then spread packets over dozens(/hundreds) non-isolated
consumers. 

> 
> >
> > 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.
> >
> > #include <urcu-qsbr.h> /* QSBR RCU flavor */ #include <urcu-bp.h> /*
> > Bulletproof RCU flavor */