20170615 [學習筆記] Linux 系統程式 (15)

一、作業系統

Each process needs minimum number of frames
Example: IBM 370 – 6 pages to handle SS MOVE instruction:
- instruction is 6 bytes, might span 2 pages
- 2 pages to handle from
- 2 pages to handle to
Maximum of course is total frames in the system
Two major allocation schemes
- fixed allocation
- priority allocation
Many variations

Equal allocation – For example, if there are 100 frames (after allocating frames for the OS) and 5 processes, give each process 20 frames
- Keep some as free frame buffer pool
Proportional allocation – Allocate according to the size of process
- Dynamic as degree of multiprogramming, process sizes change

Use a proportional allocation scheme using priorities rather than size
If process P i generates a page fault,
- select for replacement one of its frames
- select for replacement a frame from a process with lower priority number

Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another
- But then process execution time can vary greatly
- But greater throughput so more common
Local replacement – each process selects from only its own set of allocated frames
- More consistent per-process performance
- But possibly underutilized memory

So far all memory accessed equally
Many systems are NUMA – speed of access to memory varies
- Consider system boards containing CPUs and memory, interconnected over a system bus
Optimal performance comes from allocating memory “close to” the CPU on which the thread is scheduled
- And modifying the scheduler to schedule the thread on the same system board when possible
- Solved by Solaris by creating lgroups
  - Structure to track CPU / Memory low latency groups
  - Used my schedule and pager
  - When possible schedule all threads of a process and allocate all memory for that process within the lgroup

Why does demand paging work? Locality model
- Process migrates from one locality to another
- Localities may overlap
Why does thrashing occur? size of locality > total memory size
- Limit effects by using local or priority page replacement

More direct approach than WSS
Establish “acceptable” page-fault frequency rate and use local replacement policy
- If actual rate too low, process loses frame
- If actual rate too high, process gains frame

Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory
A file is initially read using demand paging
- A page-sized portion of the file is read from the file system into a physical page
- Subsequent reads/writes to/from the file are treated as ordinary memory accesses
Simplifies and speeds file access by driving file I/O through memory rather than read() and write() system calls
Also allows several processes to map the same file allowing the pages in memory to be shared
But when does written data make it to disk?
- Periodically and / or at file close() time
- For example, when the pager scans for dirty pages

To reduce the large number of page faults that occurs at process startup
Prepage all or some of the pages a process will need, before they are referenced
But if prepaged pages are unused, I/O and memory was wasted
Assume s pages are prepaged and α of the pages is used
- Is cost of s * α save pages faults > or < than the cost of prepaging s * (1-α) unnecessary pages?
- α near zero » prepaging loses

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