[IMPALA-5988] Improve MemPool::TryAllocate() efficiency for small strings

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: Impala 2.11.0
• Component/s: Backend
• Labels: perf
• Target Version: Product Backlog
• Epic Color: ghx-label-5
• Assignee: Tim Armstrong
• Reporter: Tim Armstrong
• Created: 26/Sep/17 23:16
• Updated: 05/Oct/17 05:26
• Resolved: 05/Oct/17 05:26

Description

There are some places in the code where we allocate many small strings from MemPool::TryAllocate(). The code is not particularly well optimised for this case. Specifically:

• We always realign to the default alignment even when alignment isn't required.
• The fast path through the code could be streamlined.

I noticed this while working on IMPALA-5307 where I'm adding extra calls to allocate strings per-row.