Computer Science 320

1. Computer Science 320 Cache Interference

2. Unexpected Performance • The testing of the partial key search SMP program produced anomalous results • In particular, the efficiency and the experimentally determined sequential fraction EDSF were all over the place, when they should have been constant • That indicates the presence of extra synchronicity, but where?

3. EDSF vs Processors

4. Efficiency vs Processors

5. Per Thread Variables and Memory // Thread local variables. byte[] trialkey; byte[] trialciphertext; AES256Cipher cipher; // Set up thread local variables. publicvoidstart(){ trialkey = new byte [32]; System.arraycopy(partialkey, 0, trialkey, 0, 32); trialciphertext = new byte [16]; cipher = new AES256Cipher(trialkey);

6. Mapping Variables to the Cache

7. The Source of Synchronicity • The variables of threads 1 and 2 share the same cache line • Thread 1 writes to its variable, which dirties the cache line • Thread 2 must wait until the cache line is cleaned up (also called false sharing)

8. Solution: Add Padding to Variables Try to guarantee that a thread’s accessible data either fills a cache line or, if the data extends into another line, it’s never accessed by my thread If any data does extend beyond a cache line, it’s actually padding that the thread never accesses

9. Solution: Add Padding to Variables // Thread local variables. byte[] trialkey; byte[] trialciphertext; AES256Cipher cipher; // Extra padding long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe, pf; // Set up thread local variables. publicvoidstart(){ trialkey = new byte [32 + 128]; System.arraycopy(partialkey, 0, trialkey, 0, 32); trialciphertext = new byte [16 + 128]; cipher = new AES256CipherSmp(trialkey);

10. Efficiency Improvements?

11. EDSF Improvements?

12. EDSF Improvements? Turn off the JIT compiler, which produces a larger ESDF