Virtual Shared Memory (VSM)

Transcription

1 Vorlesung Rechnerarchitektur 2 Seite 76 Basic Concepts Virtual Shared (VSM) Virtual shared memory (VSM) was first developed in a Ph.D. thesis ky Kai Li (1986). The idea is to implement a coherent shared memory on a network of processors without physically shared memory. The system uses virtual addresses for memory references. The VSM address space is organized in pages which can be accessed by any node in the system. A memory-mapping manager on each node views its memory as a large cache of pages for its associated processor. Pages that are marked read-only can have copies in the physical memory of other nodes. A page currently written may reside in only one memory. The page fault mechanism of the MMU is used to handle the access and an VSM manager called by the MMU trap can perform the consistency protocol and the of pages. page based access control page fault handler Home node The node holding the primary page primary page The page located at the home node, only copies of this page are distributed to other nodes copied page A Page which is a of a primary page write permission only for primary pages without copies

2 Vorlesung Rechnerarchitektur 2 Seite 77 Read access Virtual Shared (VSM) Node # i 1. read fault 4. enter new address 3. store Node # j addr 2. read primary page 1. access to variable by a read => page fault 2. find the appropriate primary page (Node #j) and send a read to this node 3. the page to the er 4. enter new address into page table and restart page access Read access Page fault handler is software-based. The MMU traps into the OS on a page miss. Instead of transferring the missing page from swap space into memory, the page fault handler sends a message to the home node ing a of the page. Upon reception of the answer, the page is stored at the appropriate location in main memory and the process can resume execution. Functions of the page fault handler finding the source of state information by using a directory; a page has a "home" determined by (a part of) the virtual address finding the appropriate copies communicating with the copies and the page to the er; setting page access rights rescheduling of the process to hide latency

3 Vorlesung Rechnerarchitektur 2 Seite 78 Write access Virtual Shared (VSM) Node # i 4. store primary page Node # j.n access pr pa 1. write 6. enter new address.m faultaddr.m access 1. access to variable by a write => page fault 5. invalidate 5. invalidate 3. new owner 3. new owner addr.n 2. page Node # k.o co of 2. find the appropriate primary page (Node #j) and send a write to this node 3. messages to all nodes with read copies to enter new single write owner.o 4. store primary page to write er 5. invalidate all copies 6. enter new address into page table and restart page access Write access The write access in a virtual shared memory (VSM) is much more complicated than a read. The single writer coherency scheme requires the invalidation of all copies handed out for read only and the move of the page to the write er.

4 Vorlesung Rechnerarchitektur 2 Seite 79 Virtual Shared (VSM) Example of a Coherency Protocol for VSM remote write read miss INVALID write miss remote write SHARED write hit OWNED remote read LOCAL global States of the coherence protocol invalid: access not possible shared: only read-access allowed, copies are present on other nodes owned: write access possible, copies are not allowed : state for multiple writer access scheme, see next page

5 Vorlesung Rechnerarchitektur 2 Seite 8 Virtual Shared (VSM) Methode for unification of partly changed copies replicate replicate difference difference update update sequential consistency updated data original data 2. subtraction 3. determine changes.3.7 subtraction partial changed updated data Update scheme for partial changed copies