Operating System Organization
the topic is overall o/s design
lots of ways to structure an o/s -- how to decide?
looking for principles and approaches
what does an o/s *have* to do?
for e.g. desktop or server use
let apps use machine resources
multiplex resources among apps
isolate / protect
allow cooperation / interaction
we'll talk about three approaches, but others exist e.g. Java VM
what's the traditional approach? (Linux, xv6)
virtualize some resources: cpu and memory
simulate a dedicated cpu and memory system for each app
why? it's a simple model for app programmers
abstract others: storage, network, IPC
layer a sharable abstraction over h/w (file system, IP/TCP)
example: virtualize the cpu
goal: simulate a dedicated cpu for each process
we want transparent CPU multiplexing
process need not think about how it interacts w/ other processes
o/s runs different processes in turn, via clock interrupt
clock means process doesn't need to do anything special to switch
also prevents hogging
how to achieve transparency?
o/s saves state, then restores
what does o/s save?
eight regs, EIP, seg regs, eflags, page table base ptr
where does o/s save it?
o/s keeps per-process table of saved states
the return from clock interrupt restores a *different* process's state
the point: process doesn't have to worry about multiplexing!
note: while this is the traditioal approach to virtualizing the CPU,
maybe not the best: see exokernel paper
example: virtualize memory
idea: simulate a complete memory system for each process
so process has complete freedom how it uses that memory
doesn't have to worry about other processes
so addresses 0..2^32 all work, but refer to private memory
convenient: all programs can start at zero
and memory looks contiguous, good for large arrays &c
safe: can't even *name* another process's memory
again: traditional but we'll soon see it's a very limiting approach
really want apps to have more control than this style of VM implies
how can we do this? after all, the processes do in fact share the RAM
how to create address spaces?
could have only one process at a time in physical memory
would spend lots of time swapping in and out to disk
made sense 40 years ago w/ small memory machines
could use x86 segments
put each process in a different range of physical memory
CS, DS, &c point to current process's base
looks good: addresses start at zero, contiguous, isolation
this is how x86 and original unix worked
need to prevent process from modifying seg regs
but allow kernel to modify them
386 has the hardware we need
h/w "privilege level" bit: on in kernel, off in apps
and ways to jump back and forth (syscalls, interrupts, return)
but: fragmentation, all mem must be resident, can't have vm > phys
could use x86 paging hardware
MMU array w/ entry for each 4k range of "virtual" address space
refers to phy address for that "page"
this is the page table
now we don't have a fragmentation problem
o/s tells h/w to switch page table when switching process
level of indirection allows o/s to play other tricks
demand paging:
process bigger than available physical memory?
"page-out" (write) pages to disk, mark PTEs invalid
if process tries to use one of those pages, MMU causes page fault
kern finds phys mem, page-in from disk, mark PTE valid
this works because apps use only a fraction of mem at a given time
need h/w valid flag, page faults, and re-startable instructions
copy-on-write:
avoid copy implied by fork() -- won't be needed if exec()
make parent and child share the physical memory pages
if either writes, do the copy then
so need per-page write-protect flag
both of above are transparent to application
still thinks it has simple dedicated memory from 0..2^32
paging h/w has turned out to be one of the most fruitful ideas in o/s
you'll be using it a lot in labs
o/s organization
step back, what does a traditional o/s look like?
traditional organization: monolithic o/s
h/w, kernel, user
kernel is a big program: process ctl, vm, fs, network
all of kernel runs w/ full hardware privilege (very convenient)
good: easy for sub-systems to cooperate (e.g. paging and file system)
bad: interactions => complex, bugs are easy, no isolation within o/s
philosophy: convenience (for app or o/s programmer)
for any problem, either hide it from app, or add a new system call
(we need philosophy because there is not much science here)
very successful approach
alterate organization: microkernel
philosophy: IPC and user-space servers
for any problem, make a new server, talk to it w/ RPC
h/w, kernel, server processes, apps
servers: VM, FS, TCP/IP, Print, Display
split up kernel sub-systems into server processes
some servers have privileged access to some h/w (e.g. FS and disks)
apps talk to them via IPC / RPC
kernel's main job: fast IPC
good: simple/efficient kernel, sub-systems isolated, enforced better modularity
bad: cross-sub-system optimization harder, lots of IPCs may be slow
in the end, lots of good individual ideas, but overall plan didn't catch on
alternate organization: exokernel
philosophy: eliminate all abstractions
for any problem, expose h/w or info to app, let app do what it wants
h/w, kernel, environments, libOS, app
an exokernel would not provide address space, virtual cpu, file system, TCP
instead, give control to app:
phys pages, addr mappings, clock interrupts, disk i/o, net i/o
let app build nice address space if it wants, or not
should give aggressive apps much more flexibility
challenges:
how to multiplex cpu/mem/&c if you expose directly to apps?
how to get security/isolation despite apps having low-level control?
how to multiplex w/o understanding: disk (file system), incoming tcp pkts
exokernel example: memory
what are the resources? (phys pages, mappings)
what does an app need to ask the kernel to do?
pa = AllocPage()
DeallocPage(pa)
TLBwr(va, pa)
and these kernel->app upcalls:
PageFault(va)
PleaseReleaseAPage()
what does o/s need to do to make multiplexing work?
ensure app only creates mappings to phys pages it owns
track what env owns what phys pages
decide which app to ask to give up a phys page when system runs out
that app gets to decide which of its pages
simple example: shared memory
two processes want to share memory, for fast interaction
note traditional "virtual address space" doesn't allow for this
process a: pa = AllocPage()
put 0x5000 -> pa in private table
PageFault(0x5000) upcall -> TLBwr(0x5000, pa)
give pa to process b (need to tell exokernel...)
process b:
put 0x6000 -> pa in private table
...
example cool thing you could do w/ exokernel-style memory
databases like to keep a cache of disk pages in memory
problem on traditional o/s:
assume an o/s with demand-paging to/from disk
if DB caches some disk data, and o/s needs a phys page,
o/s may page-out a DB page holding a cached disk block
but that's a waste of time: if DB knew, it could release phys
page w/o writing, and later read it back from DB file (not paging area)
1. exokernel needs phys mem for some other app
2. exokernel sends DB a PleaseReleaseAPage() upcall
3. DB picks a clean page, calls DeallocPage(pa)
4. OR DB picks dirty page, writes to disk, then DeallocPage(pa)
exokernel example: cpu
what does it mean to expose cpu to app?
kernel tells app when it is taking away cpu
kernel tells app when it gives cpu to app
so if app is running and timer interrupt causes end of slice
cpu jumps from app into kernel
kernel jumps back into app at "please yield" upcall
app saves state (registers, EIP, &c)
app calls Yield()
when kernel decides to resume app
kernel jumps into app at "resume" upcall
app restores those saved registers and EIP
what cool things could an app do w/ exokernel-style cpu management?
suppose time slice ends in the middle of
acquire(lock);
...
release(lock);
you don't want the app to be holding the lock the whole time!
then maybe other apps can't make forward progress
so the "please yield" upcall can first complete the critical section
fast RPC with direct cpu management
how does traditional o/s let apps communicate?
pipes (or sockets)
picture: buffer in kernel, lots of copying and system calls
RPC probably takes 8 kernel/user crossings (read()s and write()s)
how does exokernel help?
Yield() can take a target process argument
almost a direct jump to an instruction in target process
kernel allows only entries at approved locations in target
kernel leaves regs alone, so can contain arguments
(in constrast to traditional restore of target's registers)
target app uses Yield() to return
so only 4 crossings