The Buffer Cache Jeff Chase Duke University. The kernel syscall trap/returnfault/return...
-
Upload
camron-scott -
Category
Documents
-
view
217 -
download
0
Transcript of The Buffer Cache Jeff Chase Duke University. The kernel syscall trap/returnfault/return...
The Buffer Cache
Jeff ChaseDuke University
The kernel
syscall trap/return fault/return
interrupt/return
system call layer: file APIfault entry: VM page faults
I/O completions timer ticks
memory management: block/page cache policy
DFS
DBufferCache DBuffer
VirtualDisk
startRequest(dbuf, r/w)
ioComplete()
read(), write()startFetch(), startPush()waitValid(), waitClean()
DBuffer dbuf = getBlock(blockID)releaseBlock(dbuf)
create, destroy, read, write a dfilelist dfiles
DeFiler interfaces: overview
Memory Allocation
How should an OS allocate its memory resources among contending demands?– Virtual address spaces: fork, exec, sbrk, page fault.
– The kernel controls how many machine memory frames back the pages of each virtual address space.
– The kernel can take memory away from a VAS at any time.
– The kernel always gets control if a VAS (or rather a thread running within a VAS) asks for more.
– The kernel controls how much machine memory to use as a cache for data blocks whose home is on slow storage.
– Policy choices: which pages or blocks to keep in memory? And which ones to evict from memory to make room for others?
registerscachesL1/L2
L3
main memory (RAM)
disk, other storage, network RAM
off-core
off-chip
off-module
small and fast
(ns)
big and slow(ms)
Memory/storage hierarchy
• In general, each layer is a cache over the layer below.
– inclusion property
• Technology trends rapid change
• The triangle is expanding vertically bigger gaps, more levels
Terms to knowcache index/directorycache line/entry, associativitycache hit/miss, hit ratiospatial locality of referencetemporal locality of referenceeviction / replacementwrite-through / writebackdirty/clean
Memory as a cache
memory(frames)
data
data
virtual address spaces
files and filesystems,databases,
other storage objects
disk and other storagenetwork RAM
page/block read/write accesses
backing storage volumes(pages and blocks)
Processes access external storage objects through file
APIs and VM abstraction. The OS kernel manages caching
of pages/blocks in main memory.
The Buffer Cache
Memory
Filecache
Proc
Ritchie and Thompson The UNIX Time-Sharing
System, 1974
Editing Ritchie/Thompson
Memory
Filecache
Proc
The system maintains a buffer cache (block cache, file cache) to reduce the number of I/O operations.
Suppose a process makes a system call to access a single byte of a file. UNIX determines the affected disk block, and finds the block if it is resident in the cache. If it is not resident, UNIX allocates a cache buffer and reads the block into the buffer from the disk.
Then, if the op is a write, it replaces the affected byte in the buffer. A buffer with modified data is marked dirty: an entry is made in a list of blocks to be written. The write call may then return. The actual write may not be completed until a later time.
If the op is a read, it picks the requested byte out of the buffer and returns it, leaving the block in the cache.
DBufferCache DBuffer
read(), write()startFetch(), startPush()waitValid(), waitClean()
DBuffer dbuf = getBlock(blockID)releaseBlock(dbuf)
The DeFiler buffer cache
Device I/O interfaceAsynchronous I/O to/from buffersblock read and writeBlocks numbered by blockIDs
File abstraction implemented in upper DFS layer.All knowledge of how files are laid out on disk is at this layer.Access underlying disk volume through buffer cache API.Obtain buffers (dbufs), write/read to/from buffers, orchestrate I/O.
Page/block cache internalsHASH(blockID)
Each frame/buffer of memory is described by a meta-object (header).
Resident pages or blocks are accessible through through a global hash table.
An ordered list of eviction candidates winds through the hash chains.
Some frames/buffers are free (no valid data). These are on a free list.
DBufferCache internals
DBufferCache DBuffer
DBuffer dbuf = getBlock(blockID)
I/O cache buffersEach is byte[blocksize]
Buffer headersDBuffer dbuf
There is a one-to-one correspondence of dbufs to buffers.
HASH(blockID) Any given block (blockID) is either resident or not. If resident, then it has exactly one copy (dbuf) in the cache. If it is resident then getBlock finds the dbuf (cache hit).This requires some kind of cache index, e.g., a hash table.
DBufferCache internals
DBufferCache DBuffer
I/O cache buffersEach is byte[blocksize]
Buffer headersDBuffer dbuf
There is a one-to-one correspondence of dbufs to buffers.
HASH(blockID) If the requested block is not resident, then getBlock allocates a dbuf for the block and places the correct block contents in its buffer (cache miss). If there are no free dbufs in the cache, then we must evict some other block from the cache and reuse its dbuf.
DBuffer dbuf = getBlock(blockID)
Page/block cache internals
HASH(blockID)
cache directory
List(s) of free buffers (bufs) or eviction candidates. These dbufs might be listed in the cache directory if they contain useful data, or not, if they are truly free.
To replace a dbufRemove from free/eviction list.Remove from cache directory.Change dbuf blockID and status.Enter in directory w/ new blockID.Re-register on eviction list.Beware of concurrent accesses.
Dbuffer (dbuf) states
A DBuffer dbuf returned by getBlock is always associated with exactly one block in the disk volume. But it might or might not be “in sync” with the underlying disk contents.
DBuffer
read(…)write(...)startFetch(), startPush()waitValid(), waitClean()
DFS
A dbuf is valid iff it has the “correct” copy of the data. A dbuf is dirty iff it is valid and has an update (a write) that has not yet been written to disk. A valid dbuf is clean if it is not dirty.
Your DeFiler should return only valid data to a client. That may require you to zero the dbuf or fetch data from the disk. Your DeFiler should ensure that all dirty data is eventually pushed to disk.
DBuffer
startFetch(), startPush()waitValid(), waitClean()
Asynchronous I/O on dbufs
startRequest(dbuf, r/w)
ioComplete()
VirtualDisk
device threads
Device I/O interfaceAsync I/O on dbufs
Start I/O on a dbuf by posting it to a producer/consumer queue for service by a device thread.
Thread upcalls dbuf ioComplete when I/O operation is done.
Client threads may wait on the dbuf for asynchronous I/O to complete.
startFetch(), startPush()
waitValid(), waitClean()
DFS
DBufferCache DBuffer
VirtualDisk
startRequest(dbuf, r/w); ioComplete()
dbuf = getBlock(blockID)releaseBlock(dbuf)
More dbuf states
Do not evict a dbuf that is in active use (busy)!
A dbuf is pinned if I/O is in progress, i.e., a disk request has started but not yet completed.
A dbuf is held if DFS obtained a reference to the dbuf from getBlock but has not yet released the dbuf.
read(), write()startFetch(), startPush()waitValid(), waitClean()
DBuffer dbuf = getBlock(blockID)releaseBlock(dbuf)sync()
create, destroy, read, write a dfilelist dfiles
File system layer (DFS)
DBufferCache DBuffer
“inode”
Allocate blocks to files and file metadata.Allocate DFileIDs to files.
Track which blockIDs and DFileIDs are free and which are in use.
Maintain a block map “inode” for each file, as metadata stored on disk.
A Filesystem On Disk
111000100010110110111101
100110100011000100010101
001011100001100101000100
sector 0
allocationbitmap file
0
rain: 32
hail: 48
0
wind: 18
snow: 62
once upon a time/n in a l
and far far away, lived th
sector 1
directoryfile
Data
A Filesystem On Disk
111000100010110110111101
100110100011000100010101
001011100001100101000100
sector 0
allocationbitmap file
0
rain: 32
hail: 48
0
wind: 18
snow: 62
once upon a time/n in a l
and far far away, lived th
sector 1
directoryfile
Metadata
read(), write()startFetch(), startPush()waitValid(), waitClean()
DBuffer dbuf = getBlock(blockID)releaseBlock(dbuf)sync()
create, destroy, read, write a dfilelist dfiles
Managing files
DBufferCache DBuffer
“inode”
Each file has a size: it is the first byte offset in the file that has never been written. Never return data past a file’s size.
Fetch blocks for data and metadata (or zero new ones fresh), read and write in place, and push dirty blocks back to the disk.
Serialize DFS read/write on each inode.
Representing a File On Disk
logicalblock 0
logicalblock 1
logicalblock 2
once upon a time/nin a l
and far far away,/nlived t
he wise and sagewizard.
block mapIndex by logical block number
maps to a blockID
“inode”
file attributes e.g., size
blockIDaccess blocks through the block cache with getBlock, startFetch, waitValid, read,releaseBlock.
Filesystem layout on disk
111000100010110110111101
inode 0bitmap file
0
rain: 32
hail: 48
once upon a time/n in a l
and far far away, lived th
inode 1root directory
inode
file blocks
111000100010110110111101
100110100011000100010101
001011100001100101000100
allocationbitmap file
blocks
0
wind: 18
snow: 62
inode 1root directory
fixed locations on disk
This is a toy example (Nachos).
Filesystem layout on disk
111000100010110110111101
inode 0bitmap file
0
rain: 32
hail: 48
once upon a time/n in a l
and far far away, lived th
inode 1root directory
inode
file blocks
Your DeFiler volume is small. You can keep the free block/inode maps in memory. You don’t need metadata structures on disk for that. But you have to scan the disk to rebuild the in-memory structures on initialization.
X X
XX
DeFiler has no directories.You just need to keep track of which DFileIDs are currently valid, and return a list.
DeFiler must be able to find all valid inodes on disk.
Disk layout: the easy way
once upon a time/n in a l
and far far away, lived th
inode
file blocks
Given a list of valid inodes,you can determine which inodes and blocks are free and which are in use.
DeFiler must be able to find all valid inodes on disk.