[dpdk-dev] [PATCH 0/3] Cuckoo hash lookup 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 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.

Second 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.

Third (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.

This patchset depends on the following patchset:
"Hash library fixes" (http://dpdk.org/ml/archives/dev/2016-August/045780.html)

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

 lib/librte_hash/rte_cuckoo_hash.c     | 427 +++++++++++++---------------------
 lib/librte_hash/rte_cuckoo_hash.h     |  24 +-
 lib/librte_hash/rte_cuckoo_hash_x86.h |  20 +-
 3 files changed, 179 insertions(+), 292 deletions(-)

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2.7.4