Computer Science and Engineering Copyright by Hesham El-Rewini Advanced Computer Architecture CSE 8383 February Session 13

Computer Science and Engineering Copyright by Hesham El-Rewini Next time Problem from last time Shared memory Systems Cash Coherence Protocol Contents

Computer Science and Engineering Copyright by Hesham El-Rewini Problem Assume that a switching component such as a transistor can switch in zero time. We propose to construct a disk- shaped computer chip with such a component. The only limitation is the time it takes to send electronic signals from one edge of the chip to the other. Make the simplifying assumption that electronic signals travel 300,000 kilometers per second. What must be the diameter of a round chip so that it can switch 10 9 times per second? What would the diameter be if the switching requirements were time per second?

Computer Science and Engineering Copyright by Hesham El-Rewini MIMD Shared Memory Systems Interconnection Networks MMMM PPPPP

Computer Science and Engineering Copyright by Hesham El-Rewini Shared Memory Single address space Communication via read & write Synchronization via locks

Computer Science and Engineering Copyright by Hesham El-Rewini Classification Multi-port Uniform memory Access (UMA) Non-uniform Memory Access (NUMA) Cache Only Memory Architecture (COMA) M P2P2 P1P1

Computer Science and Engineering Copyright by Hesham El-Rewini Uniform Memory Access (UMA) C P C P C P C P MMMM Bus

Computer Science and Engineering Copyright by Hesham El-Rewini Non Uniform Memory Access (NUMA) M P M P M P Interconnection Network M P

Computer Science and Engineering Copyright by Hesham El-Rewini Cache Only Memory Architecture (COMA) C P D C P D C P D C P D Interconnection Network

Computer Science and Engineering Copyright by Hesham El-Rewini Bus Based & switch based SM Systems Global Memory P C P C P C P C P C P C P C MMMM

Computer Science and Engineering Copyright by Hesham El-Rewini Bus-based Shared Memory Collection of wires and connectors Only one transaction at a time Bottleneck!! How can we solve the problem? Global Memory PPPPP

Computer Science and Engineering Copyright by Hesham El-Rewini Using Caches Global Memory P1 C1 P2 C2 P3 C3 Pn Cn - Cache Coherence problem - How many processors?

Computer Science and Engineering Copyright by Hesham El-Rewini Group Activity Variables Number of processors (n) Hit rate (h) Bus Bandwidth (B) Processor speed (V) Maximum number of processors n ?

Computer Science and Engineering Copyright by Hesham El-Rewini Group Activity

Computer Science and Engineering Copyright by Hesham El-Rewini Single Processor caching P x x Memory Cache Hit: data in the cache Miss: data is not in the cache Hit rate: h Miss rate: m = (1-h)

Computer Science and Engineering Copyright by Hesham El-Rewini Cache Coherence Policies Writing to Cache in 1 processor case Write Through Write Back

Computer Science and Engineering Copyright by Hesham El-Rewini Writing in the cache P x x Before Memory Cache P x’ Write through Memory Cache P x’ x Write back Memory Cache

Computer Science and Engineering Copyright by Hesham El-Rewini Cache Coherence P1 x P2P3 x Pn x x -Multiple copies of x -What if P1 updates x?

Computer Science and Engineering Copyright by Hesham El-Rewini Cache Coherence Policies Writing to Cache in n processor case Write Update - Write Through Write Invalidate - Write Back Write Update - Write Through Write Invalidate - Write Back

Computer Science and Engineering Copyright by Hesham El-Rewini Write-invalidate P1 x P2P3 x x P1 x’ P2P3 I x’ P1 x’ P2P3 I x BeforeWrite Through Write back

Computer Science and Engineering Copyright by Hesham El-Rewini Write-Update P1 x P2P3 x x P1 x’ P2P3 x’ P1 x’ P2P3 x’ x BeforeWrite Through Write back

Computer Science and Engineering Copyright by Hesham El-Rewini Snooping Protocols Snooping protocols are based on watching bus activities and carry out the appropriate coherency commands when necessary. Global memory is moved in blocks, and each block has a state associated with it, which determines what happens to the entire contents of the block. The state of a block might change as a result of the operations Read-Miss, Read-Hit, Write-Miss, and Write-Hit.

Computer Science and Engineering Copyright by Hesham El-Rewini Write Invalidate Write Through Multiple processors can read block copies from main memory safely until one processor updates its copy. At this time, all cache copies are invalidated and the memory is updated to remain consistent.

Computer Science and Engineering Copyright by Hesham El-Rewini Write Through- Write Invalidate (cont.) StateDescription Valid [VALID] The copy is consistent with global memory Invalid [INV] The copy is inconsistent

Computer Science and Engineering Copyright by Hesham El-Rewini Write Through- Write Invalidate (cont.) EventActions Read HitUse the local copy from the cache. Read Miss Fetch a copy from global memory. Set the state of this copy to Valid. Write HitPerform the write locally. Broadcast an Invalid command to all caches. Update the global memory. Write Miss Get a copy from global memory. Broadcast an invalid command to all caches. Update the global memory. Update the local copy and set its state to Valid. ReplaceSince memory is always consistent, no write back is needed when a block is replaced.

Computer Science and Engineering Copyright by Hesham El-Rewini Example 1 C P C Q M X = 5 P reads XP reads X Q reads XQ reads X Q updates XQ updates X Q reads XQ reads X Q updates XQ updates X P updates XP updates X Q reads XQ reads X

Computer Science and Engineering Copyright by Hesham El-Rewini Complete the table (Write through write invalidate) MemoryP’sCacheQ’sCache EventXXStateXState 0 Original value 5 1 P reads X (Read Miss) 55VALID