Remove enableFastNegativeLookup option from cachelib

Summary: fast negative lookup is alway turned on in production. Remove the option to turn it off.

Reviewed By: therealgymmy

Differential Revision: D51477963

fbshipit-source-id: 2159e6f4e51d2cf6c4f4688dfafe0af701133f61

cachelib/allocator/nvmcache/NvmCache-inl.h

@@ -150,8 +150,8 @@ typename NvmCache<C>::WriteHandle NvmCache<C>::find(HashedKey hk) {
     // For concurrent put, if it is already enqueued, its put context already
     // exists. If it is not enqueued yet (in-flight) the above invalidateToken
     // will prevent the put from being enqueued.
-    if (config_.enableFastNegativeLookups && it == fillMap.end() &&
-        !putContexts_[shard].hasContexts() && !navyCache_->couldExist(hk)) {
+    if (it == fillMap.end() && !putContexts_[shard].hasContexts() &&
+        !navyCache_->couldExist(hk)) {
       stats().numNvmGetMiss.inc();
       stats().numNvmGetMissFast.inc();
       return WriteHandle{};
@@ -234,8 +234,8 @@ bool NvmCache<C>::couldExistFast(HashedKey hk) {
   // For concurrent put, if it is already enqueued, its put context already
   // exists. If it is not enqueued yet (in-flight) the above invalidateToken
   // will prevent the put from being enqueued.
-  if (config_.enableFastNegativeLookups && it == fillMap.end() &&
-      !putContexts_[shard].hasContexts() && !navyCache_->couldExist(hk)) {
+  if (it == fillMap.end() && !putContexts_[shard].hasContexts() &&
+      !navyCache_->couldExist(hk)) {
     return false;
   }
 
@@ -813,8 +813,7 @@ void NvmCache<C>::remove(HashedKey hk, DeleteTombStoneGuard tombstone) {
   // (in-flight puts) before we check for couldExist.  Any put contexts
   // created after couldExist api returns does not matter, since the put
   // token is invalidated before all of this begins.
-  if (config_.enableFastNegativeLookups && !putContexts_[shard].hasContexts() &&
-      !navyCache_->couldExist(hk)) {
+  if (!putContexts_[shard].hasContexts() && !navyCache_->couldExist(hk)) {
     stats().numNvmSkippedDeletes.inc();
     return;
   }

cachelib/allocator/nvmcache/NvmCache.h

@@ -111,10 +111,6 @@ class NvmCache {
     // If true, only store the orignal size the user requested.
     bool truncateItemToOriginalAllocSizeInNvm{false};
 
-    // when enabled, nvmcache will attempt to resolve misses without incurring
-    // thread hops by using synchronous methods.
-    bool enableFastNegativeLookups{true};
-
     // serialize the config for debugging purposes
     std::map<std::string, std::string> serialize() const;
 

cachelib/allocator/nvmcache/tests/NvmCacheTests.cpp

@@ -137,7 +137,6 @@ TEST_F(NvmCacheTest, BasicGet) {
 
 TEST_F(NvmCacheTest, CouldExistFast) {
   // Enable fast negative lookup
-  this->allocConfig_.nvmConfig->enableFastNegativeLookups = true;
   this->makeCache();
 
   auto& nvm = this->cache();

website/docs/Cache_Library_User_Guides/CacheLib_configs.md

@@ -38,7 +38,6 @@ Configs to initialize NVM cache lives in `CacheAllocatorConfig::nvmConfig` and a
 * `setDropNvmCacheOnShmNew`: This flag is used to determine whether the NVM cache would start truncated.
 * `enableNvmCacheEncryption`: This sets CacheAllocatorConfig::nvmConfig::deviceEncryptor.
 * `enableNvmCacheTruncateAllocSize`: This sets CacheAllocatorConfig::nvmConfig::truncateItemToOriginalAllocSizeInNvm.
-* `enableFastNegativeLookups`: This enables/disables the feature to resolve NVM cache misses quickly in the sync path of `find(...)` call without kicking the lookup for NVM cache. This is essential to reduce the overall end-to-end latency, but comes with the penalty of small overhead added in the hit cases for additional accesses to index tables or bloom filters. The default is enabled.
 * `setNvmCacheAdmissionPolicy`/`enableRejectFirstAPForNvm`: Sets the NvmAdmissionPolicy. Notice that the field lives with CacheAllocatorConfig.
 * `setNvmAdmissionMinTTL`: Sets the NVM admission min TTL. Similarly this lives directly with CacheAllocatorConfig.
 * `enableNvmCache`: Sets `CacheAllocatorConfig::nvmConfig` directly. This function should be called first if you intend to turn on NVM cache. And the other functions above would correctly modify the nvmConfig.