[IMPALA-10377] Improve the accuracy of resource estimation

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 3.4.0
• Fix Version/s: Impala 4.0.0
• Component/s: Frontend
• Labels: estimate memory statistics
• Flags: Patch
• Target Version: Impala 4.0.0
• Epic Color: ghx-label-1
• Assignee: liuyao
• Reporter: liuyao
• Created: 04/Dec/20 06:27
• Updated: 17/Apr/22 12:24
• Resolved: 13/Mar/21 02:59

Description

PlanNode does not consider some factors when estimating memory, this will cause a large error rate

 

AggregationNode

 

1.The memory occupied by hash table's own data structure is not considered. Hash table inserts a new value, which will add a bucket. The size of a bucket is 16 bytes.

2.When estimating the NDV of merge aggregation, if there are multiple grouping exprs, it may be divided by the number of Fragment Instances several times, and it should be divided only once.

3.When estimating the NDV of merge aggregation, and there are multiple grouping exprs, the estimated memory is much smaller than the actual use.

4.If there is no grouping exprs, the estimated memory is much larger than the actual use.

5.If the NDV of grouping exprs is very small, the estimated memory is much larger than the actual use.

 

SortNode

1.Estimate the memory usage of external sort. the estimated memory is much smaller than the actual use.

 

 

HashJoinNode

1.The memory occupied by hash table's own data structure is not considered.Hash Table will keep duplicate data, so the size of DuplicateNode should be considered.

2.Hash table will create multiple buckets in advance. The size of these buckets should be considered.

 

KuduScanNode

1.Estimate memory by scanning all columns,the estimated memory is much larger than the actual use.

Issue Links

• relates to: IMPALA-2945 Pre-aggregation cardinality estimates do not take into account data distribution
• relates to: IMPALA-9591 Automatically cap pre-aggregation memory consumption to match estimate
• relates to: IMPALA-7350 More accurate memory estimates for admission