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perf(orchestrator): increase NBD dispatch buffer by 28 bytes to coalesce header + max payload in one read

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88/100
Issue 类型
重构
描述清晰度
描述清楚
活跃度
冷清
技术栈
go
领域
infrastructure

调研方向

从 packages/orchestrator/pkg/sandbox/nbd/dispatch.go 中的 dispatchBufferSize 开始，阅读周围的 NBD dispatch 逻辑。将 raw buffer 增加 28 字节的 header 大小，然后验证现有的 NBD 包检查；完成标准是 buffer 能够容纳 4 MB payload 及其 header，同时不改变其他行为。

由索引模型根据 Issue 内容生成。

描述

Summary

The NBD dispatch read buffer (dispatchBufferSize) is set to exactly 4 MB, which is the maximum data payload size. However, every NBD request also carries a 28-byte header. This means a maximum-size read (4 MB data + 28-byte header) requires two kernel reads instead of one — the header and data cannot be received in a single recv() call.

Location: packages/orchestrator/pkg/sandbox/nbd/dispatch.go:54

// TODO: Look into optimizing the buffer reads by increasing the buffer size by 28 bytes,
// to account for a request that is 28 bytes of header + 4MB of data (this seems to be preferred kernel buffer size).
dispatchBufferSize = 4 * 1024 * 1024

Root cause

The NBD protocol frames each request as [28-byte header][N bytes payload]. When the kernel sends a 4 MB write, the dispatch loop reads the 28-byte header, then refills the buffer with up to dispatchBufferSize bytes of payload. If dispatchBufferSize == 4 MB, a 4 MB write payload fills the entire buffer, leaving no room to pre-read the next request's header in the same syscall.

Increasing the buffer by exactly 28 bytes allows the tail of one response and the header of the next to be coalesced into a single read.

Proposed fix

// dispatchBufferSize is the raw read buffer for the NBD socket.
// 4MB covers the largest possible data payload; the extra 28 bytes
// let the next request's header arrive in the same read as the
// last byte of the previous payload, saving one syscall per request.
dispatchBufferSize = 4*1024*1024 + 28

This is a single-constant change with no behavioral risk — the buffer is used only for reading from the NBD socket.

Impact

Every NBD read or write request currently requires at least two recv() calls for max-size payloads. Reducing to one call per request lowers syscall overhead for I/O-intensive sandboxes (particularly those doing large sequential reads/writes to the virtual block device).

主要语言
Go
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