Tim Armstrong has uploaded a new patch set (#4). Change subject: IMPALA-4923: reduce memory transfer for selective scans ......................................................................
IMPALA-4923: reduce memory transfer for selective scans Most of the code changes are to restructure things so that the scratch batch's tuple buffer is stored in a separate MemPool from auxiliary memory such as decompression buffers. This part of the change does not change the behaviour of the scanner in itself, but allows us to recycle the tuple buffer without holding onto unused auxiliary memory. The optimisation is implemented in TryCompact(): if enough rows were filtered out during the copy from the scratch batch to the output batch, the fixed-length portions of the surviving rows (if any) are copied to a new, smaller, buffer, and the original, larger, buffer is reused for the next scratch batch. Previously the large buffer was always attached to the output batch, so a large buffer was transferred between threads for every scratch batch processed. In combination with the decompression buffer change in IMPALA-5304, this means that in many cases selective scans don't produce nearly as many empty or near-empty batches and do not attach nearly as much memory to each batch. Performance: Even on an 8 core machine I see some speedup on selective scans. Profiling with "perf top" also showed that time in TCMalloc was reduced - it went from several % of CPU time to a minimal amount. Running TPC-H on the same machine showed a ~5% overall improvement and no regressions. E.g. Q6 got 20-25% faster. I hope to do some additional cluster benchmarking on systems with more cores to verify that the severe performance problems there are fixed, but in the meantime it seems like we have enough evidence that it will at least improve things. Testing: Add a couple of selective scans that exercise the new code paths. Change-Id: I3773dc63c498e295a2c1386a15c5e69205e747ea --- M be/src/exec/hdfs-parquet-scanner.cc M be/src/exec/parquet-column-readers.cc M be/src/exec/parquet-scratch-tuple-batch.h M be/src/runtime/row-batch.cc M be/src/runtime/row-batch.h M testdata/workloads/functional-query/queries/QueryTest/parquet.test 6 files changed, 208 insertions(+), 56 deletions(-) git pull ssh://gerrit.cloudera.org:29418/Impala-ASF refs/changes/49/6949/4 -- To view, visit http://gerrit.cloudera.org:8080/6949 To unsubscribe, visit http://gerrit.cloudera.org:8080/settings Gerrit-MessageType: newpatchset Gerrit-Change-Id: I3773dc63c498e295a2c1386a15c5e69205e747ea Gerrit-PatchSet: 4 Gerrit-Project: Impala-ASF Gerrit-Branch: master Gerrit-Owner: Tim Armstrong <tarmstr...@cloudera.com> Gerrit-Reviewer: Alex Behm <alex.b...@cloudera.com> Gerrit-Reviewer: Tim Armstrong <tarmstr...@cloudera.com>
