[RFC 0/2] add pointer compression API

[Paul Szczepanek <paul.szczepanek@arm.com>]

This patchset is proposing adding a new EAL header with utility functions
that allow compression of arrays of pointers.

When passing caches full of pointers between threads, memory containing
the pointers is copied multiple times which is especially costly between
cores. A compression method will allow us to shrink the memory size
copied.

The compression takes advantage of the fact that pointers are usually
located in a limited memory region (like a mempool). We can compress them
by converting them to offsets from a base memory address.

Offsets can be stored in fewer bytes (dictated by the memory region size
and alignment of the pointer). For example: an 8 byte aligned pointer
which is part of a 32GB memory pool can be stored in 4 bytes. The API is
very generic and does not assume mempool pointers, any pointer can be
passed in.

Compression is based on few and fast operations and especially with vector
instructions leveraged creates minimal overhead.

The API accepts and returns arrays because the overhead means it only is
worth it when done in bulk.

Test is added that shows potential performance gain from compression. In
this test an array of pointers is passed through a ring between two cores.
It shows the gain which is dependent on the bulk operation size. In this
synthetic test run on ampere altra a substantial (up to 25%) performance
gain is seen if done in bulk size larger than 32. At 32 it breaks even and
lower sizes create a small (less than 5%) slowdown due to overhead.

In a more realistic mock application running the l3 forwarding dpdk
example that works in pipeline mode this translated into a ~5% throughput
increase on an ampere altra.

Paul Szczepanek (2):
  eal: add pointer compression functions
  test: add pointer compress tests to ring perf test

 .mailmap                           |   1 +
 app/test/test_ring.h               |  59 +++++-
 app/test/test_ring_perf.c          | 324 ++++++++++++++++++-----------
 lib/eal/include/meson.build        |   1 +
 lib/eal/include/rte_ptr_compress.h | 158 ++++++++++++++
 5 files changed, 419 insertions(+), 124 deletions(-)
 create mode 100644 lib/eal/include/rte_ptr_compress.h

-- 
2.25.1