Class 17
CS 170
12 March 2019

On the board
------------

1. Last time
2. Finish directories
3. mmap
4. Exam review

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1. Last time

    --indexed files

    --directories
    
        --a directory *is* a file. its data is simply a table that maps
        name to inode

2. Finish directories

    --special names: "/", ".", ".."

    --given those names, we need only two operations to navigate the
    entire name space:

	--"cd name": (change context to directory "name")
	--"ls": (list all names in current directory)


    --example:

	[DRAW PICTURE FROM LAST TIME]


    --links:

	--hard link: multiple dir entries point to same inode; inode
	contains refcount

	    "ln a b": creates a synonym ("b") for file ("a")

	    --how do we avoid cycles in the graph? (answer: can't
	    hard link to directories)

	--soft link: synonym for a *name*

	    "ln -s /d/a b": 

	    --creates a new inode, not just a new directory entry

	    --new inode has "sym link" bit set

	    --contents of that new file:

		"/d/a"

3. mmap

    --recall some syscalls: 
	fd = open(pathname, mode)
	write(fd, buf, sz)
	read(fd, buf, sz)

    --we've seen fds before, but what is it?
	--indexes into a table maintained by the kernel on behalf of the
	process
	--what's in the given entry in the table?
	    --inumber!
	    --inode, probably!
	    --and per-open-file data (file position, etc.)

    --syscall:
	void* mmap(void* addr, size_t len, int prot, int flags,
		   int fd, off_t offset);


        --means, roughly, "map the specified open file (fd) into a
        region of my virtual memory (at addr, or at a kernel-selected
        place if addr is 0), and return a pointer to it"

    --after this, loads and stores to addr[x] are
    equivalent to reading and writing to the file at offset+x.

    --how's this implemented?! (answer: through virtual memory,
    with the VA being addr [or whatever the kernel selects] and
    the PA being what? answer: the physical address storing the
    given page in the kernel's buffer cache).

    --have to deal with eviction from buffer cache, so kernel will need
    a data structure that maps from:
        Phys page --> {list of (proc,va) pairs}
      
    note that the kernel needs this data structure anyway: when a
    page is evicted from RAM, the kernel needs to be able to invalidate
    the given virtual address in the page table(s) of the process(es)
    that have the page mapped.
    
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exam review

    Ground rules: same as last time
        --75 minute exam

            --at 65 minutes, you have to stay seated; do not get up and distract
            your classmates.

            --you must hand your exam to us (we are not going to collect them).
            the purpose of this is to give everyone the same amount of time.

            --at 78 minutes, we will walk out of the room and won't accept any
            exams when we leave

            --thus you must hand in your exam at time x minutes, where:
            x <= 65 OR
            75 <= x < 78

        --exams are closed book. Also, put away electronics (phones, tablets, laptops,
        etc.).

    Material: everything we've covered: readings, labs, homeworks,
    lectures 
    
        only exception, as usual, is if there was new material covered
        in review sessions (but there shouldn't have been)

lecture topics

    I/O 
        
        --architecture

        --how CPUs and devices interact

            --mechanics (explicit I/O instructions, mem-mapped I/O,
                interrupts, memory)
            --polling vs. interrupts
            --DMA vs. programmed I/O

        --device drivers

    virtual memory

        segmentation

        paging

        segmentation vs paging

        virtual memory on the x86

	    virtual address: [10bits  10bits  12bits]

	    --entry in pgdir and page table:

	        [20 bits  more bits   bottom 3 bits]

	    --protection (user/kernel | read/write |  present/not)

	what's a TLB?

        page faults

            mechanics

            costs

            uses

        page replacement policies (FIFO, LRU, CLOCK, OPT)

        thrashing

        [latter two topics more general than paging: applies to caching
        in general]


    disks

        geometry

        performance

        interface

        scheduling (skipped in lecture; see book)


    file systems

 	--basic objects: files, directories, meta-data, links, inodes

	--how does naming work? what allows system to map
	    /usr/homes/bob/index.html to a file object?

	--types of file layout

	    --extents/contiguous, linked, FAT, indexed structure
	    
	    --classic Unix is a variant of indexed structure

        --analogy between inode and page directory

	--mmap


questions from all of you ...