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安正超
106 lines
5.1 KiB
Rust
106 lines
5.1 KiB
Rust
// Copyright 2024 RustFS Team
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//! Zero-copy I/O configuration constants.
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//!
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//! This module defines environment variables and default values for zero-copy
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//! read operations, which use memory mapping (mmap) to avoid data copying.
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// =============================================================================
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// Zero-Copy Configuration
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// =============================================================================
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/// Environment variable for zero-copy read enable.
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///
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/// When enabled, uses mmap (Unix) or optimized reads for zero-copy data access.
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/// This reduces memory copies from 3-4 to 1, lowering CPU usage by 20-30%
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/// and improving P95 latency by 15-25%.
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///
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/// - Purpose: Enable or disable zero-copy read operations
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/// - Acceptable values: `"true"` / `"false"` (case-insensitive) or a boolean typed config
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/// - Semantics: When enabled, uses mmap on Unix systems for memory-mapped file reads;
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/// falls back to regular I/O on non-Unix platforms or when mmap fails
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/// - Example: `export RUSTFS_OBJECT_ZERO_COPY_ENABLE=true`
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/// - Note: Zero-copy is safe for all workloads and provides significant performance
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/// benefits with minimal risk. Disable only if mmap-related issues are encountered.
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pub const ENV_OBJECT_ZERO_COPY_ENABLE: &str = "RUSTFS_OBJECT_ZERO_COPY_ENABLE";
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/// Default: zero-copy reads are enabled.
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///
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/// Zero-copy uses memory mapping (mmap) on Unix systems to avoid data copying
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/// between kernel and user space. This provides:
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/// - Reduced memory copies: from 3-4 copies to 1 copy
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/// - Lower CPU usage: 20-30% reduction expected
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/// - Improved latency P95: 15-25% reduction expected
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/// - Increased throughput: 10-20% improvement expected
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///
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/// On non-Unix platforms or when mmap fails, the system automatically falls back
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/// to regular I/O without errors.
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pub const DEFAULT_OBJECT_ZERO_COPY_ENABLE: bool = true;
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// =============================================================================
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// Direct I/O Configuration
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// =============================================================================
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/// Environment variable for Direct I/O enable (Linux only).
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///
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/// When enabled, uses O_DIRECT flag to bypass OS page cache for large files.
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/// This is only beneficial for specific workloads (databases, large sequential reads).
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///
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/// - Purpose: Enable or disable Direct I/O for large file operations
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/// - Acceptable values: `"true"` / `"false"` (case-insensitive) or a boolean typed config
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/// - Semantics: When enabled, files larger than the threshold will use O_DIRECT flag;
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/// this bypasses the OS page cache and transfers data directly between disk and application
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/// - Example: `export RUSTFS_OBJECT_DIRECT_IO_ENABLE=true`
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/// - Note: Direct I/O is disabled by default because it's only beneficial for specific
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/// use cases. For most workloads, the OS page cache provides better performance.
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pub const ENV_OBJECT_DIRECT_IO_ENABLE: &str = "RUSTFS_OBJECT_DIRECT_IO_ENABLE";
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/// Default: Direct I/O is disabled.
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///
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/// Direct I/O is disabled by default because it's only beneficial for specific use cases:
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/// - Large file transfers (>128MB)
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/// - Databases with their own cache
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/// - Applications requiring predictable I/O latency
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///
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/// For most workloads, the OS page cache provides better performance through:
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/// - Read-ahead caching
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/// - Write buffering
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/// - Multi-use caching (same data cached for multiple operations)
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pub const DEFAULT_OBJECT_DIRECT_IO_ENABLE: bool = false;
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/// Environment variable for Direct I/O minimum file size threshold.
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///
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/// Files smaller than this size will use regular I/O even if Direct I/O is enabled.
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/// This avoids the overhead of Direct I/O for small files where the OS page cache
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/// is more effective.
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///
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/// - Purpose: Set the minimum file size for Direct I/O operations
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/// - Unit: Bytes
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/// - Valid values: any positive integer (default: 134,217,728 bytes = 128 MB)
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/// - Semantics: Only files larger than this threshold will use Direct I/O when enabled;
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/// smaller files use regular buffered I/O
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/// - Example: `export RUSTFS_OBJECT_DIRECT_IO_THRESHOLD=268435456`
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/// - Note: The default threshold of 128MB balances the overhead of Direct I/O setup
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/// against the benefits of bypassing the page cache for large files.
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pub const ENV_OBJECT_DIRECT_IO_THRESHOLD: &str = "RUSTFS_OBJECT_DIRECT_IO_THRESHOLD";
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/// Default Direct I/O threshold: 128 MB.
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///
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/// Only files larger than 128MB will use Direct I/O when enabled.
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/// Smaller files benefit from OS page cache.
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///
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/// Formula: 128 * 1024 * 1024 = 134,217,728 bytes
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pub const DEFAULT_OBJECT_DIRECT_IO_THRESHOLD: usize = 128 * 1024 * 1024;
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