Decoupling Translation Lookaside Buffer Coherence from Cache Coherence

International audienceMany multicore and manycore architectures support hardware cache coherence. However, most of them rely on software techniques to maintain Translation Lookaside Buffer (TLB) coherence, namely the TLB shootdown routine, which is a costly procedure, known to be hardly scalable. The TSAR architecture is a manycore architecture including hardware TLB coherence, but in which the TLB coherence mechanism is tightly coupled to the cache coherence protocol, resulting in useless TLB invalidations. We propose to improve this existing TLB coherence scheme by adding a hardware module which allows separating data from metadata for cache lines containing address translation. This allows to eliminate the need to invalidate TLB entries when a line containing a translation is evicted from the L1 cache. Our solution does not modify the cache coherence protocol, does not increase the critical path in the L1 cache, and even results in little memory savings. Performance results show that our solution allows to eliminate from 90% to 95% of TLB scans operations, and from 50% to 80% of TLB flushes. This in turn results in an overall performance improvement of 5% to 20% of execution times on a 16-core architecture