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

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> Subject: [dpdk-dev] [RFC 0/6] New sync modes for ring
> 
> Upfront note - that RFC is not a complete patch.
> It introduces an ABI breakage, plus it doesn't update ring_elem code properly,
As per the current rules, these changes (in the current form) will be accepted only for 20.11 release. How do we address this for immediate requirements like RCU defer APIs? 
I suggest that we move forward with my RFC (taking into consideration your feedback) to make progress on RCU APIs.

> 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.
> While it is probably not possible to completely resolve this problem in
> userspace only (without some kernel communication/intervention), removing
> fairness in tail update can mitigate it significantly.
> So this RFC proposes two new optional ring synchronization modes:
> 1) Head/Tail Sync (HTS) mode
> In that mode enqueue/dequeue operation is fully serialized:
>     only one thread at a time is allowed to perform given op.
>     As another enhancement provide ability to split enqueue/dequeue
>     operation into two phases:
>       - enqueue/dequeue start
>       - enqueue/dequeue finish
>     That allows user to inspect objects in the ring without removing
>     them from it (aka MT safe peek).
IMO, this will not address the problem described above. For ex: when a producer updates the head and gets scheduled out, other producers have to spin. The problem is probably worse as with non-HTS case moving of the head and copying of the ring elements can happen in parallel between the producers (similarly for consumers).
IMO, HTS should not be a configurable flag. In RCU requirement, a MP enqueue and HTS dequeue are required.

> 2) Relaxed Tail Sync (RTS)
> The main difference from original MP/MC algorithm is that tail value is
> increased not by every thread that finished enqueue/dequeue, but only by the
> last one.
> That allows threads to avoid spinning on ring tail value, leaving actual tail value
> change to the last thread in the update queue.
This can be a configurable flag on the ring.
I am not sure how this solves the problem you have stated above completely. Updating the count from all intermediate threads is still required to update the value of the head. But yes, it reduces the severity of the problem by not enforcing the order in which the tail is updated.
I also think it introduces the problem on the other side of the ring because the tail is not updated soon enough (the other side has to wait longer for the elements to become available). It also introduces another configuration parameter (HTD_MAX_DEF) which they have to deal with.
Users have to still implement the current hypervisor related solutions.
IMO, we should run the benchmark for this on an over committed setup to understand the benefits.

> 
> Test results on IA (see below) show significant improvements for average
> enqueue/dequeue op times on overcommitted systems.
> For 'classic' DPDK deployments (one thread per core) original MP/MC
> algorithm still shows best numbers, though for 64-bit target RTS numbers are
> not that far away.
> Numbers were produced by ring_stress_*autotest (first patch in these series).
> 
> X86_64 @ Intel(R) Xeon(R) Platinum 8160 CPU @ 2.10GHz
> DEQ+ENQ average cycles/obj
> 
>                                                 MP/MC      HTS     RTS
> 1thread@1core(--lcores=6-7)                     8.00       8.15    8.99
> 2thread@2core(--lcores=6-8)                     19.14      19.61   20.35
> 4thread@4core(--lcores=6-10)                    29.43      29.79   31.82
> 8thread@8core(--lcores=6-14)                    110.59     192.81  119.50
> 16thread@16core(--lcores=6-22)                  461.03     813.12  495.59
> 32thread/@32core(--lcores='6-22,55-70')         982.90     1972.38 1160.51
> 
> 2thread@1core(--lcores='6,(10-11)@7'            20140.50   23.58   25.14
> 4thread@2core(--lcores='6,(10-11)@7,(20-21)@8'  153680.60  76.88   80.05
> 8thread@2core(--lcores='6,(10-13)@7,(20-23)@8'  280314.32  294.72  318.79
> 16thread@2core(--lcores='6,(10-17)@7,(20-27)@8' 643176.59  1144.02
> 1175.14 32thread@2core(--lcores='6,(10-25)@7,(30-45)@8' 4264238.80
> 4627.48 4892.68
> 
> 8thread@2core(--lcores='6,(10-17)@(7,8))'       321085.98  298.59  307.47
> 16thread@4core(--lcores='6,(20-35)@(7-10))'     1900705.61 575.35  678.29
> 32thread@4core(--lcores='6,(20-51)@(7-10))'     5510445.85 2164.36 2714.12
> 
> i686 @ Intel(R) Xeon(R) Platinum 8160 CPU @ 2.10GHz
> DEQ+ENQ average cycles/obj
> 
>                                                 MP/MC      HTS     RTS
> 1thread@1core(--lcores=6-7)                     7.85       12.13   11.31
> 2thread@2core(--lcores=6-8)                     17.89      24.52   21.86
> 8thread@8core(--lcores=6-14)                    32.58      354.20  54.58
> 32thread/@32core(--lcores='6-22,55-70')         813.77     6072.41 2169.91
> 
> 2thread@1core(--lcores='6,(10-11)@7'            16095.00   36.06   34.74
> 8thread@2core(--lcores='6,(10-13)@7,(20-23)@8'  1140354.54 346.61  361.57
> 16thread@2core(--lcores='6,(10-17)@7,(20-27)@8' 1920417.86 1314.90
> 1416.65
> 
> 8thread@2core(--lcores='6,(10-17)@(7,8))'       594358.61  332.70  357.74
> 32thread@4core(--lcores='6,(20-51)@(7-10))'     5319896.86 2836.44 3028.87
> 
> Konstantin Ananyev (6):
>   test/ring: add contention stress test
>   ring: rework ring layout to allow new sync schemes
>   ring: introduce RTS ring mode
>   test/ring: add contention stress test for RTS ring
>   ring: introduce HTS ring mode
>   test/ring: add contention stress test for HTS ring
> 
>  app/test/Makefile                      |   3 +
>  app/test/meson.build                   |   3 +
>  app/test/test_pdump.c                  |   6 +-
>  app/test/test_ring_hts_stress.c        |  28 ++
>  app/test/test_ring_rts_stress.c        |  28 ++
>  app/test/test_ring_stress.c            |  27 ++
>  app/test/test_ring_stress.h            | 477 +++++++++++++++++++
>  lib/librte_pdump/rte_pdump.c           |   2 +-
>  lib/librte_port/rte_port_ring.c        |  12 +-
>  lib/librte_ring/Makefile               |   4 +-
>  lib/librte_ring/meson.build            |   4 +-
>  lib/librte_ring/rte_ring.c             |  84 +++-
>  lib/librte_ring/rte_ring.h             | 619 +++++++++++++++++++++++--
>  lib/librte_ring/rte_ring_elem.h        |   8 +-
>  lib/librte_ring/rte_ring_hts_generic.h | 228 +++++++++
> lib/librte_ring/rte_ring_rts_generic.h | 240 ++++++++++
>  16 files changed, 1721 insertions(+), 52 deletions(-)  create mode 100644
> app/test/test_ring_hts_stress.c  create mode 100644
> app/test/test_ring_rts_stress.c  create mode 100644
> app/test/test_ring_stress.c  create mode 100644 app/test/test_ring_stress.h
> create mode 100644 lib/librte_ring/rte_ring_hts_generic.h
>  create mode 100644 lib/librte_ring/rte_ring_rts_generic.h
> 
> --
> 2.17.1