[dpdk-dev] [PATCH v4 0/4] Cuckoo hash enhancements

[Pablo de Lara]

This patchset improves lookup performance on the current hash library
by changing the existing lookup bulk pipeline, with an improved pipeline,
based on a loop-and-jump model, instead of the current 4-stage 2-entry pipeline.
Also, x86 vectorized intrinsics are used to improve performance when comparing signatures.

First patch reorganizes the order of the hash structure.
The structure takes more than one 64-byte cache line, but not all
the fields are used in the lookup operation (the most common operation).
Therefore, all these fields have been moved to the first part of the structure,
so they all fit in one cache line, improving slightly the performance in some
scenarios.

Second patch modifies the order of the bucket structure.
Currently, the buckets store all the signatures together (current and alternative).
In order to be able to perform a vectorized signature comparison,
all current signatures have to be together, so the order of the bucket has been changed,
having separated all the current signatures from the alternative signatures.

Third patch introduces x86 vectorized intrinsics.
When performing a lookup bulk operation, all current signatures in a bucket
are compared against the signature of the key being looked up.
Now that they all are together, a vectorized comparison can be performed,
which takes less instructions to be carried out.
In case of having a machine with AVX2, number of entries per bucket are
increased from 4 to 8, as AVX2 allows comparing two 256-bit values, with 8x32-bit integers,
which are the 8 signatures on the bucket.

Fourth (and last) patch modifies the current pipeline of the lookup bulk function.
The new pipeline is based on a loop-and-jump model. The two key improvements are:

- Better prefetching: in this case, first 4 keys to be looked up are prefetched,
  and after that, the rest of the keys are prefetched at the time the calculation
  of the signatures are being performed. This gives more time for the CPU to
  prefetch the data requesting before actually need it, which result in less
  cache misses and therefore, higher throughput.

- Lower performance penalty when using fallback: the lookup bulk algorithm
  assumes that most times there will not be a collision in a bucket, but it might
  happen that two or more signatures are equal, which means that more than one
  key comparison might be necessary. In that case, only the key of the first hit is prefetched,
  like in the current implementation. The difference now is that if this comparison
  results in a miss, the information of the other keys to be compared has been stored,
  unlike the current implementation, which needs to perform an entire simple lookup again.

Changes in v4:
- Reordered hash structure, so alt signature is at the start
  of the next cache line, and explain in the commit message
  why it has been moved
- Reordered hash structure, so name field is on top of the structure,
  leaving all the fields used in lookup in the next cache line
  (instead of the first cache line)

Changes in v3:
- Corrected the cover letter (wrong number of patches)

Changes in v2:
- Increased entries per bucket from 4 to 8 for all cases,
  so it is not architecture dependent any longer.
- Replaced compile-time signature comparison function election
  with run-time election, so best optimization available
  will be used from a single binary.
- Reordered the hash structure, so all the fields used by lookup
  are in the same cache line (first).

Byron Marohn (3):
  hash: reorganize bucket structure
  hash: add vectorized comparison
  hash: modify lookup bulk pipeline

Pablo de Lara (1):
  hash: reorder hash structure

 lib/librte_hash/rte_cuckoo_hash.c     | 455 ++++++++++++++--------------------
 lib/librte_hash/rte_cuckoo_hash.h     |  56 +++--
 lib/librte_hash/rte_cuckoo_hash_x86.h |  20 +-
 3 files changed, 228 insertions(+), 303 deletions(-)

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2.7.4