Linux Memory Management

Readahead：

• Linux kernel system call that loads a file’s content into page cache. This prefetches the files so when it is subsequently accessed, the content is read from main memory rather then hdd, resulting in faster response time
• Readahead 的基本原理是預先載入開機過程可能使用的檔案到記憶體中 (page cache)。如此開機程式執行時，便可節省讀入檔案的時間，進而加快開機速度

Page cache：

• 用來加速系統讀取檔案速度
• 當資料 write to / read from storage, 資料也會儲存一份到空閒的記憶體中 (unused areas of memory), act as cache
• 當該資料再度需要時就可以從 cache 裏讀出, 加速速度
• CentOS 6 中, free -m 出現的 cached 欄位就是 page cache 目前使用的大小
• 當目前的記憶體大小無法滿足程式的記憶體需求時, areas of page cache that are no longer in use will be automatically deleted

Dirty page：

• 當寫入資料時, 會先寫入 page cache, 然後當作 dirty pages
• Dirty pages means 儲存在 page cache 的資料但需要寫入 storage (hdd, NAS 等)
• 測試
  • dd if=/dev/zero of=testfile.txt bs=1M count=10
  • cat /proc/meminfo | grep Dirty, Drity 數字會增加約 10240 KB
  • 執行 sync 後數字會立即降低, 代表 page cache 資料已回寫到 hdd
  • 系統也會透過 flush 自動將 dirty pages 寫入 hdd

Flush

• Kernel process flushes dirty pages from page cache to storage (hdd, NAS 等)
• On CentOS 6, try
  • ps aux | grep flush ; [flush-253:0]
  • grep ^ /sys/class/block/*/dev
  • Compare both results
  • Extra reading

TLB-translation lookaside buffer

• Memory cache used to reduce the time taken to access a user memory location ( page table in main memory)
• Sits between CPU and main memory
• Is associative memory
• Used by MMU when virtual-physical address mapping is required
• Stores the recent translation of virtual memory to physical memory (recent access of page table entries)
• Access to main memory is slower then access to CPU cache, so below
• TLB is mostly stored on CPU cache such as L1, L2, L3 cache (?)
• More reading, reading, reading, reading, reading, reading

Paging：

• Physical memory break down to frame, pfn is physical frame number

Physical memory：

• Page frame is the smallest fixed-length contiguous block of physical memory
• Page frame is 4KB (4096bytes) in size
• Use command getconf PAGESIZE to get the page frame size
• More reading

Paging daemon：

• Background process
• Responsible to maintain a pool of free clean page frames
• Checks if at least 20% of frames are free every 250ms (/proc/sys/vm/dirty_ratio)
  • Select pages to evict using the replacement algorithm
  • Schedule disk writes for dirty pages (flush process?)

Swapper：

• When paging daemon is not keeping up with the demand for free pages on the system
• Swapper swaps out the entire process to free momery
• Typically swaps out large, sleeping processes in order to free memory quickly

Virtual memory：

• A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table
• Size of a page is equal to the size of page frame
• Page is initially read-only
• When someone wants to write to page, it traps kernel, then OS copies the page and change it to read-write (copy on write)
• Process can only access to virtual memory so each process is virtual memory isolated

Page fault：

• When a desired page is not in the memory
• Traps to kernel to exec further process

Page table：

• Table for process to map virtual address to physical address
• Each process has it’s on page table
• TLB is used to cache and speed up the mapping access time between virtual and physical address
• Page table is stored in main memory (physical memory)

Invert page table：

Multi-level paging：

• Linux uses three-level page tables
  • Global directory
  • Page middle directory
  • Page table
  • Page

MMU-momery management unit：

• The job of MMU is to translate page number to (page)frame number