Memory Mapping (mmap)

SUMMARY

Memory mapping (mmap) is a system call that maps files directly into a process's memory address space, enabling direct file access without explicit read/write operations. In database systems, mmap provides efficient data access by leveraging the operating system's virtual memory and page cache mechanisms.

How memory mapping works

Memory mapping creates a direct correlation between a file on disk and a range of virtual memory addresses. When a program accesses these memory addresses, the operating system automatically handles loading the corresponding file pages from disk into physical memory.

This mechanism provides several advantages:

Transparent page caching by the operating system
Reduced system call overhead compared to traditional read/write operations
Ability to share memory-mapped regions across processes
Support for zero-copy operations

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Memory mapping in time-series databases

Time-series databases often use memory mapping for efficient data access patterns. For example, QuestDB employs memory mapping for its storage engine to achieve high-performance read operations.

Key benefits in time-series contexts include:

Fast random access to historical data
Efficient sequential scans for time-range queries
Reduced memory pressure through OS-managed paging
Improved cache utilization through the page cache

Performance considerations

While memory mapping offers significant advantages, there are important considerations:

Virtual memory limits: The total size of mapped files cannot exceed available virtual memory address space
Page faults: Initial access to unmapped pages incurs page faults, causing disk I/O
Memory pressure: Large mappings can impact system-wide memory availability
Flush control: Less direct control over when modified pages are written to disk

Best practices for database systems

When implementing memory mapping in database systems:

Consider file size and access patterns
Monitor system memory pressure
Implement proper error handling for mapping failures
Use appropriate mapping flags (read-only vs. read-write)

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Memory mapping and data integrity

Memory mapping introduces specific considerations for data integrity:

Crash recovery: Memory-mapped files may have pending writes in the page cache during system crashes
Consistency: Multiple processes accessing the same mapped regions need proper synchronization
Durability: Explicit sync operations may be needed to ensure data persistence

Example system call pattern for safe memory mapping:

fd = open("data.bin", O_RDWR);
data = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
// ... access data directly through pointer ...
msync(data, size, MS_SYNC); // ensure durability
munmap(data, size);

Monitoring and optimization

To maintain optimal performance with memory-mapped files:

Monitor page fault rates
Track memory pressure indicators
Observe I/O patterns
Consider pre-faulting pages for critical sections

Memory mapping remains a crucial technique for high-performance database systems, particularly when dealing with time-series data that requires both random access and sequential scan capabilities.