On Fri, 2019-01-18 at 09:23 -0600, Gage Eads wrote: > For some users, the rte ring's "non-preemptive" constraint is not > acceptable; > for example, if the application uses a mixture of pinned high- > priority threads > and multiplexed low-priority threads that share a mempool. > > This patchset introduces a non-blocking ring, on top of which a > mempool can run. > Crucially, the non-blocking algorithm relies on a 128-bit compare- > and-swap, so > it is currently limited to x86_64 machines. This is also an > experimental API, > so RING_F_NB users must build with the ALLOW_EXPERIMENTAL_API flag. > > The ring uses more compare-and-swap atomic operations than the > regular rte ring: > With no contention, an enqueue of n pointers uses (1 + 2n) CAS > operations and a > dequeue of n pointers uses 2. This algorithm has worse average-case > performance > than the regular rte ring (particularly a highly-contended ring with > large bulk > accesses), however: > - For applications with preemptible pthreads, the regular rte ring's > worst-case > performance (i.e. one thread being preempted in the update_tail() > critical > section) is much worse than the non-blocking ring's. > - Software caching can mitigate the average case performance for > ring-based > algorithms. For example, a non-blocking ring based mempool (a > likely use case > for this ring) with per-thread caching. > > The non-blocking ring is enabled via a new flag, RING_F_NB. For ease- > of-use, > existing ring enqueue/dequeue functions work with both "regular" and > non-blocking rings. > > This patchset also adds non-blocking versions of ring_autotest and > ring_perf_autotest, and a non-blocking ring based mempool. > > This patchset makes one API change; a deprecation notice will be > posted in a > separate commit. > > This patchset depends on the non-blocking stack patchset[1]. > > [1] http://mails.dpdk.org/archives/dev/2019-January/123653.html > > v3: > - Avoid the ABI break by putting 64-bit head and tail values in the > same > cacheline as struct rte_ring's prod and cons members. > - Don't attempt to compile rte_atomic128_cmpset without > ALLOW_EXPERIMENTAL_API, as this would break a large number of > libraries. > - Add a helpful warning to __rte_ring_do_nb_enqueue_mp() in case > someone tries > to use RING_F_NB without the ALLOW_EXPERIMENTAL_API flag. > - Update the ring mempool to use experimental APIs > - Clarify that RINB_F_NB is only limited to x86_64 currently; > ARMv8.1-A builds > can eventually support it with the CASP instruction. ARMv8.0 should be able to implement a 128-bit atomic compare exchange operation using LDXP/STXP.
From an ARM perspective, I want all atomic operations to take memory ordering arguments (e.g. acquire, release). Not all usages of e.g. atomic compare exchange require sequential consistency (which I think what x86 cmpxchg instruction provides). DPDK functions should not be modelled after x86 behaviour. Lock-free 128-bit atomics implementations for ARM/AArch64 and x86-64 are available here: https://github.com/ARM-software/progress64/blob/master/src/lockfree.h > > v2: > - Merge separate docs commit into patch #5 > - Convert uintptr_t to size_t > - Add a compile-time check for the size of size_t > - Fix a space-after-typecast issue > - Fix an unnecessary-parentheses checkpatch warning > - Bump librte_ring's library version > > Gage Eads (5): > ring: add 64-bit headtail structure > ring: add a non-blocking implementation > test_ring: add non-blocking ring autotest > test_ring_perf: add non-blocking ring perf test > mempool/ring: add non-blocking ring handlers > > doc/guides/prog_guide/env_abstraction_layer.rst | 2 +- > drivers/mempool/ring/Makefile | 1 + > drivers/mempool/ring/meson.build | 2 + > drivers/mempool/ring/rte_mempool_ring.c | 58 ++- > lib/librte_eventdev/rte_event_ring.h | 2 +- > lib/librte_ring/Makefile | 3 +- > lib/librte_ring/rte_ring.c | 72 ++- > lib/librte_ring/rte_ring.h | 574 > ++++++++++++++++++++++-- > lib/librte_ring/rte_ring_generic_64.h | 152 +++++++ > lib/librte_ring/rte_ring_version.map | 7 + > test/test/test_ring.c | 57 ++- > test/test/test_ring_perf.c | 19 +- > 12 files changed, 874 insertions(+), 75 deletions(-) > create mode 100644 lib/librte_ring/rte_ring_generic_64.h > IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.
