pingcap/tidb

Eliminate unnecessary `PessimisticRollback` when write conflict occurs

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#17,597 opened on Jun 2, 2020

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challenge-programhelp wantedsig/transactiontype/enhancement

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Description

Description

When TiDB fails to acquire pessimistic locks due to write conflict, it sends PessimisticRollback to release all locks the DML needs to lock because it's possible some keys needn't be locked after retry. However, the probability is small and PessimisticRollback brings overhead:

  • TiKV process more requests and write more locks.
  • A transaction releases all its locks and if it needs to lock them again, maybe some locks are held by other ones.

Score

  • 4000

SIG Slack Channel

You can join #sig-transaction on slack in your spare time to discuss and get help with mentors or others.

Mentor(s)

  • @youjiali1995

Contact the mentors: #tidb-challenge-program channel in TiDB Community Slack Workspace

Recommended Skills

  • Golang
  • TiDB
  • Pessimistic transaction

Learning Materials

Contributor guide

Research direction
Understand the pessimistic lock acquisition flow in TiDB and TiKV, identify when PessimisticRollback is sent unnecessarily, and modify the code to skip sending rollback when write conflict occurs and locks can be retained for retry.
Tech stack
go
Domain
backenddatabase
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GoGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30