Virtual Caches
42
Virtual Cach es กกกก กกกกกกก 4422005
description
Virtual Caches. กรกฎ คชฤทธิ์ 4422005. Virtual Address. Virtual Address. Physical Address. MMU. Main Memory. Virtual Cache. CPU. Data. Data. Virtual Caches. Virtual Caches. Problems with Virtual Caches Ambiguities Aliases. Physical Address. 0 x0000. 1234. * *. 0 x5000. - PowerPoint PPT Presentation
Transcript of Virtual Caches
Virtual Caches
กรกฎ คชฤทธิ์�4422005
Virtual Caches
CPUVirtualCache
MainMemory
MMU
Data Data
VirtualAddress
PhysicalAddress
VirtualAddress
Virtual Caches
Problems with Virtual Caches•Ambiguities•Aliases
Ambiguities
12
34
56
78
0 x0000
0 x5000
Physical
Address**
*
Ambiguities Virtual Address
01000
Physical Address0 5000x
Maps to
Virtual Cache
010005678
*
*
Tag Data
Ambiguities Virtual Address
01000
Physical Address0 0000x
Maps to
Virtual Cache
010005678
*
*
Tag Data
Aliases
0x0000
0x1000
0x2000
0x3000
0x4000
0x5000
VirtualAddress Space 0x000
00x100
00x200
00x300
00x400
00x500
0
PhysicalAddress Space
Managing a Virtual Cache
Context Switch• Process may use the same virtual add
ress to refer to different physical addr ess before and after context switch
• Kernel must flush everything the old p rocess had cached
• -Flushing a virtual cache can be a time consuming operation
Managing a Virtual Cache
Fork• the child receive a complete copy of
the parent’s address space• Flushing during each context switch
eliminates most inconsistencies bet ween parent’s and child’s data
• But must consider about during the copy operations
Managing a Virtual Cache - -Fork (without copy on write)
Parent’s Virtual Address Space
****
****
****
Kernel Virtual Space
Mapping to Parent’s Physical Pa
ges Temporary Mapping to
Child’s Phy sical Pages
for Copy Operation
Data Co pied to C hild’s Pa
ges
Managing a Virtual Cache
Fork• - If write through caching is used, No
special flushing is needed during for k since at least one context switch m
ust occur before the child executes• - If write back caching is used, Main m
emory must be validated before tem porary mapping is removed
Managing a Virtual Cache
Fork• if fork is implemented with co
- -py on write• - if write through, no flushing is n
eeded at fork time• - if write back, main memory mu
st be validated
Managing a Virtual Cache
Exec• Discards the process’s current addr
ess space and replaces it with a ne w one for the new program the proc
ess will run• Possible to occur ambiguities• Kernel must invalidated all user data
from cache before execution begins
Managing a Virtual Cache
Exit• Address space of process is discarded
• Last step of exit is to perform context switch that flushed th
e cache
Managing a Virtual Cache
Brk and Sbrk• use to grow and shrink the pro
cess’s bss segment• growing bss causes no cache problems
• shrink bss, kernel must invalid ates data from cache
Managing a Virtual Cache
Shared Memory and Mapped Files• no special cache management is n
eeded since the cache is flushed a t each context switch
• if single process attaches shared memory segment at different addr
ess ?
Managing a Virtual Cache
I/O
CPUVirtualCache
MainMemory I/O
MMU
Data Data Data
Virtual Address Virtual Address Physical Address Physical Address
Managing a Virtual Cache
I/O• Some problem occurs when usingDMA
• when write to device, kernel must validate main memory with cache
data before DMA starts• when read from device, cache mus
t be flushed
Managing a Virtual Cache
- User Kernel Data Ambiguities• to prevent cached kernel data
from user process, cache must be flushed
• - Apollo DN 4000, add bit indica te user or kernel mode
Virtual Caches with Keys
กรกฎ คชฤทธิ์�4422005
Virtual Caches with KeysCurren
tProces
s Key
CPUVirtualCache
MainMemory
MMU
Data Data
VirtualAddress
PhysicalAddress
VirtualAddress
Managing a Virtual Cache with Keys
Context Switch• Normally no need to flush the cach
e as long as enough keys that uniqu e can be assigned to each processe
s• when reassigning key, all entries ta
gged with affected key must be flushed
Managing a Virtual Cache with Keys
Fork• No need to Flush• Some problem
Key1
Tag0 1x00
00
0
Data10
11
97
0
Key2
Tag0 1x00
00
0
Data10
11
97
0
Managing a Virtual Cache with Keys Fork
• Some problemKey1
Tag0 1x00
00
0
Data10
11
97
0
- - -
Key1
Tag0 1x00
00
0
Data10
11
97
0
2 0 1x00
00
0
10
11
97
0
Managing a Virtual Cache with Keys
Exec• A unique key must be chosen fo
r new address space• if reassigning key, all entries ta
gged with affected key must beflushed
Managing a Virtual Cache with Keys
Exit• not necessary to invalidate cac
he at exit time• cache is flushed during contextswitch
Managing a Virtual Cache with Keys
Brk and Sbrk• Same as pure virtual cache
Managing a Virtual Cache with Keys
Shared Memory and MappedFiles• Can not share data within cache
ModM
Tag0 1x00
00
0
Data10
11
97
0
Key1
Mod-
Tag0 1x00
00
0
Data10
11
97
0
Key2
Managing a Virtual Cache with Keys
I/O• Same as pure virtual cache
Managing a Virtual Cache with Keys
- User Kernel Data Ambiguities• Kernel process have special key• User process can not access ker
nel data• Kernel process can not share us
er data
Virtual Cache Usage in MMUs
- Special purpose cache, Translation L ookside Buffer (TLB) or Address trans
lation Cache (ATC)• caching most recently used page mapping
• - - speed up virtual to physical address tra nslation process
• must be flush on each context switch
Virtual Caches
with Physical A ddress Tags
กรกฎ คชฤทธิ์�4422005
Virtual Caches with Physical Tags
CPU Virtual Cache
withPhysic
al Tags
MainMemory
MMU
Data Data
VirtualAddress
PhysicalAddress
Managing a Virtual Cache with Physical Tags
Context Switch• Normally no need to flush the cache
• No keys to mange as virtual cac he with keys
Managing a Virtual Cache with Physical Tags
Fork• No cache flushing is required fo
r fork• Data can be shared in cache• No aliases can occur
Managing a Virtual Cache with Physical Tags
Exec• Address space not be use by an
y process•exec deallocates pages, must b
e ensure that stale data is remo ved prior to allocating page to n
ew process
Managing a Virtual Cache with Physical Tags
Exit• Can be either flushed at system
call or postponed until physical page are reuse
Managing a Virtual Cache with Physical Tags
Brk and Sbrk• Like exec and exit• Can be either flushed at system
call or postponed until physical page are reuse
Managing a Virtual Cache with Physical Tags
Shared Memory and MappedFiles• Can share data in cache• Better hit ratio for shared memory
Managing a Virtual Cache with Physical Tags
I/O• Same as pure virtual cache
Managing a Virtual Cache with Physical Tags
- User Kernel Data Ambiguities• No ambiguities between user a
nd kernel data• Kernel can reference cached us
er data directly
The End
