Volatile Uses for Persistent Memory
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Use cases like high-performance computing (HPC), AI, and IoTA can generate a huge volume of data. Learn how Intel® Optane™ DC persistent memory can be an alternative to DRAM for applications that benefit from a very large volatile memory capacity.
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Volatile Uses for Persistent Memory
- 1. Usha Upadhyayula & Piotr Balcer Intel Data Center Group
- 2. SPDK, PMDK & Vtune™ Summit Agenda • Why Use Persistent Memory as Volatile • How to Create Volatile Region over Persistent Memory ▪ libmemkind ▪ libvmemcache 2
- 3. SPDK, PMDK & Intel® VTune™ Amplifier Summit 3 ModesataGlance • Memory Mode (volatile Memory) • Data Placement • Application does not have control over data placement between DRAM and Intel Persistent Memory • Ease of adoption: No Code Changes • Performance slower than DRAM • App Direct Mode (persistent memory) • Data Placement • Application has control over data placement between DRAM and Intel Persistent Memory • Ease of Adoption: Need to Re-architect Application • Performance at Native hardware latencies APPLICATION VOLATILE MEMORY POOL O P T A N E P E R S I S T E N T M E M O R Y D R A M A S C A C H E APPLICATION OPTANE PERSISTENT MEMORY DRAM
- 4. SPDK, PMDK & Intel® VTune™ Amplifier Summit 4 Motivation • Application Controlled Data Placement • Has Different Tiers of Data • Opportunity to use Bigger and Cheaper Volatile Memory than DRAM • Persistence not Required • Reduce Development Effort
- 5. SPDK, PMDK & Vtune™ Summit 5 Whatislibmemkind DRAM Unified Memory Management High BW Memory Intel® Optane™ DC Persistent Memory Distros Allocator Familiar API stdlib like API Availability https://github.com/memkindJeMalloc 5.0
- 6. SPDK, PMDK & Intel® VTune™ Amplifier Summit 6 HowDoesitWORK • pmem kind is file-backed • Temp file created on DAX-enabled filesystem • Temp file is memory mapped into application virtual address space • Temp file deleted when application terminates • Jemalloc manages memory allocation from this address space libmemkind is a Wrapper; Jemalloc Manages Heap
- 7. SPDK, PMDK & Intel® VTune™ Amplifier Summit 7 MemkindAPI Heap Management API • void *memkind_malloc(memkind_t kind, size_t size ) • void *memkind_calloc(memkind_t kind, size_t num, size_t size ) • void *memkind_realloc(memkind_t kind, void *ptr, size_t size ) • void memkind_free(memkind_t kind, void *ptr ) • memkind_t memkind_detect_kind(void *ptr ) Persistent Memory API • int memkind_create_pmem(const char *dir , size_t max_size , memkind_t *kind • int memkind_destroy_kind(memkind_t kind )
- 8. SPDK, PMDK & Intel® VTune™ Amplifier Summit 8 int main(int argc, char *argv[]) { struct memkind *pmem_kind = NULL; memkind_create_pmem(“/mnt/pmem”, 0, &pmem_kind); char * ptr_default = (char *)memkind_malloc(MEMKIND_DEFAULT, size); //allocate in DRAM char * ptr_pmem = (char *)memkind_malloc(pmem_kind, size); //allocate in PMEM memkind_free(MEMKIND_DEFAULT, ptr_default); memkind_free(pmem_kind, ptr_pmem); memkind_destroy_kind(pmem_kind); return 0; } Example-allocationtoDRAMandPMEM
- 9. SPDK, PMDK & Intel® VTune™ Amplifier Summit 9 • Manage Fragmentation with Memory Usage Policy • void memkind_config_set_memory_usage_policy(struct memkind_config *cfg, memkind_mem_usage_policy policy ) • MEMKIND_MEM_USAGE_POLICY_DEFAULT – default memory usage policy • MEMKIND_MEM_USAGE_POLICY_CONSERVATIVE – prioritize memory usage at cost of performance ManagingFragmentation Trade off between Memory Usage vs CPU Utilization
- 10. SPDK, PMDK & Intel® VTune™ Amplifier Summit 10 Fragmentation 0.5 2.5 4.5 6.5 8.5 10.5 12.5 14.5 1 4… 9… 1… 1… 2… 2… 3… 3… 4… 4… 5… 5… 6… 6… 7… 7… 8… 8… 9… 9… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 1… Fragmentationpercentage Number of allocations Fragmentation MEMKIND_MEM_USAGE_POLICY_DEFAULT MEMKIND_MEM_USAGE_POLICY_CONSERVATIVE
- 11. SPDK, PMDK & Intel® VTune™ Amplifier Summit 11 Summary • libmemkind Provides • Unified memory management across different “kinds of memory” • Familiar API to manage different kinds of memory • Tutorial and Code Samples • https://software.intel.com/en-us/articles/use-memkind-to-manage-volatile- memory-on-intel-optane-persistent-memory • https://github.com/memkind/memkind/tree/master/examples https://github.com/memkind
- 12. libvmemcachePiotr Balcer <piotr.balcer@intel.com> Intel® Data Center Group
- 13. SPDK, PMDK & Vtune™ Summit Problemstatement Local LRU cache Support for large capacities available with persistent memory (many terabytes per server) Lightweight, efficient and embeddable In-memory Scalable 13
- 14. SPDK, PMDK & Vtune™ Summit Existingsolutions In-memory databases tend to rely on malloc() in some form for allocating memory for entries ▪ Which means allocating anonymous memory Persistent Memory is exposed by the operating system through normal file-system operations ▪ Which means allocating byte-addressable PMEM needs to use file memory mapping (fsdax). We could modify the allocator of an existing in-memory database and be done with it, right? ☺ 14
- 15. SPDK, PMDK & Vtune™ Summit 15 Fragmentation Manual dynamic memory management a’la dlmalloc/jemalloc/tcmalloc/palloc causes fragmentation Applications with substantial expected runtime durations need a way to combat this problem ▪ Compacting GC (Java, .NET) ▪ Defragmentation (Redis, Apache Ignite) ▪ Slab allocation (memcached) Especially so if there’s substantial expected variety in allocated sizes
- 16. SPDK, PMDK & Vtune™ Summit 16 Extentallocation If fragmentation is unavoidable, and defragmentation/compacting is CPU and memory bandwidth intensive, let’s embrace it! Usually only done in relatively large blocks in file-systems. But on PMEM, we are no longer restricted by large transfer units (sectors, pages etc)
- 17. SPDK, PMDK & Vtune™ Summit 17 Scalablereplacementpolicy Performance of libvmemcache was bottlenecked by naïve implementation of LRU based on a doubly-linked list. With 100st of threads, most of the time of any request was spent waiting on a list lock… Locking per-node doesn’t solve the problem…
- 18. SPDK, PMDK & Vtune™ Summit 18 BufferedLRU Our solution was quite simple. We’ve added a non-blocking ring-buffer which buffers the list-move operations This way, the list only needs to get locked during eviction or when the ring-buffer is full.
- 19. SPDK, PMDK & Vtune™ Summit Lightweight,embeddable,in-memorycaching https://github.com/pmem/vmemcache VMEMcache *cache = vmemcache_new("/tmp", VMEMCACHE_MIN_POOL, VMEMCACHE_MIN_EXTENT, VMEMCACHE_REPLACEMENT_LRU); const char *key = "foo"; vmemcache_put(cache, key, strlen(key), "bar", sizeof("bar")); char buf[128]; ssize_t len = vmemcache_get(cache, key, strlen(key), buf, sizeof(buf), 0, NULL); vmemcache_delete(cache); libvmemcache has normal get/put APIs, optional replacement policy, and configurable extent size. Works with terabyte-sized in-memory workloads without a sweat, with very high space utilization. Also works on regular DRAM. 19