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Memory mapped files

#1
04-04-2024, 07:00 AM
You map files right into your address space when handling big data loads. I see this trick cuts down on extra copies between disk and buffers. But page faults pop up as the system pulls sections into ram. Perhaps your code treats the file like plain memory now. And access happens through simple pointers without calling read functions every time. You gain speed because the kernel manages the swaps behind the scenes. I notice shared mappings let multiple processes see changes instantly. Or faults get resolved only on demand which saves initial load time. Now this ties into how virtual memory handles larger than physical storage setups. You avoid the overhead of user space copies that slow things down otherwise.
I recall mapping a file starts with an open handle then a request to attach it. But alignment matters since pages come in fixed sizes from the hardware. Perhaps errors arise if the file shrinks while mapped which you must handle carefully. And dirty pages write back when the system decides or on unmap calls. You benefit from lazy loading that skips unused parts entirely. I think this approach shines for databases where random access patterns rule. Or in graphics where large textures load without extra steps. Now the architecture side shows how TLB entries speed up translations after first faults. You see reduced context switches compared to traditional IO loops.
But consistency issues creep in with concurrent writes from other processes. I watch for cache coherency problems on multi core chips during updates. Perhaps flushing becomes necessary at key points to keep data safe. And the OS might lock pages to prevent swapping during critical sections. You explore how this interacts with NUMA nodes for better locality. I notice performance jumps when files stay resident after initial access. Or fragmentation in the address space limits how many you can attach at once. Now think about error recovery when disk space runs out mid write. You handle these by checking return codes on map attempts.
Perhaps this method scales well for servers dealing with logs or indexes. I see reduced CPU cycles since no data shuttling occurs in user mode. But synchronization primitives help when threads modify the same regions. And unmapping releases the space without explicit closes sometimes. You test with varying file sizes to spot bottlenecks in the paging system. I recall how executable files often use similar mappings for code sections. Or swap files themselves get treated this way internally by the kernel. Now the tradeoffs include higher memory usage if everything stays pinned. You balance this against the gains in throughput for sequential scans.
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ron74
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Memory mapped files - by ron74 - 04-04-2024, 07:00 AM

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Memory mapped files

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