44 lines
1.5 KiB
Markdown
44 lines
1.5 KiB
Markdown
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---
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tags: [memory, Linux]
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created: Monday, July 08, 2024
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---
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# Memory Management Unit (MMU)
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The MMU is the bridge between the physical memory devices of the machine and the
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virtual memory space. It is a chip that sits between the CPU and the RAM and
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determines the physical location of the memory requested by the kernel as
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virtual memory.
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## Pages
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We use the term **pages** to denote blocks of virtual memory and to distinguish
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them from **addresses** as physical blocks. The MMU possesses a **page table**
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which is registry logging which pages correspond to which physical blocks.
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## Shared pages
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Virtual memory allows the sharing of files and memory by multiple processes.
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Crucially the shared data doesn't have to be within the address of each process,
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instead there is a reference in the page table that each process has access to
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the shared data.
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## Page faults
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There are two kinds of error that can occur with relation to paged memory:
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- minor page faults
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- The desired page is in main memory but the MMU doesn't currently know where
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it is
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- major page faults
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- The desired page is not in main memory at all. Therefore the kernel must
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fetch it from disk
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Minor page faults are very common and are to be expected; they resolve quickly.
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On the other hand too many major page faults can slow the system down both
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because of the time-costly process of fetching data from disk and because it
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demands more kernel resources to locate the missing page, which puts other
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processes on hold.
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