pingcap/tidb

Avoid double-copy buffer when responding MySQL packets

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#32,097 opened on Feb 3, 2022

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help wantedtype/enhancementtype/performance

Description

Enhancement

When we want to return the packages to the client, our logic is:

  1. Prepare the packet and serialize it to a buffer allocated by the area.Allocator
  2. Write this data through packetIO.WritePacket, in the WritePacket, the data will be cached in a buffer writer
  3. Flush the buffer writer to the connection

Mostly, e.g, for write ok https://github.com/pingcap/tidb/blob/0c19f927419f6b043a345e4a2af5336272830229/server/conn.go#L1442, we can see that we don't need to double copy the buffer, we can serialize the data and write it directly to the connection.

Contributor guide

Tech stack
go
Domain
backendperformance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Go programmingbuffer management
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The issue proposes to avoid double copying the MySQL packet buffer in TiDB's server. Currently, the packet is serialized into a buffer, then copied into a buffered writer before flushing to the connection. The key file is server/conn.go, especially around line 1442 where the write ok packet is handled. The contributor should examine the WritePacket function and the area allocator to understand the buffer lifecycle. The goal is to write the serialized data directly to the connection without the intermediate copy. No linked pull requests were found.