Add a blob-specific cache priority (facebook#10309)

Summary:
RocksDB's `Cache` abstraction currently supports two priority levels for items: high (used for frequently accessed/highly valuable SST metablocks like index/filter blocks) and low (used for SST data blocks). Blobs are typically lower-value targets for caching than data blocks, since 1) with BlobDB, data blocks containing blob references conceptually form an index structure which has to be consulted before we can read the blob value, and 2) cached blobs represent only a single key-value, while cached data blocks generally contain multiple KVs. Since we would like to make it possible to use the same backing cache for the block cache and the blob cache, it would make sense to add a new, lower-than-low cache priority level (bottom level) for blobs so data blocks are prioritized over them.

This task is a part of facebook#10156

Pull Request resolved: facebook#10309

Reviewed By: ltamasi

Differential Revision: D38211655

Pulled By: gangliao

fbshipit-source-id: 65ef33337db4d85277cc6f9782d67c421ad71dd5

HISTORY.md

@@ -1,11 +1,12 @@
 # Rocksdb Change Log
 ## Unreleased
 ### New Features
- * Added `prepopulate_blob_cache` to ColumnFamilyOptions. If enabled, prepopulate warm/hot blobs which are already in memory into blob cache at the time of flush. On a flush, the blob that is in memory (in memtables) get flushed to the device. If using Direct IO, additional IO is incurred to read this blob back into memory again, which is avoided by enabling this option. This further helps if the workload exhibits high temporal locality, where most of the reads go to recently written data. This also helps in case of the remote file system since it involves network traffic and higher latencies.
+* Added `prepopulate_blob_cache` to ColumnFamilyOptions. If enabled, prepopulate warm/hot blobs which are already in memory into blob cache at the time of flush. On a flush, the blob that is in memory (in memtables) get flushed to the device. If using Direct IO, additional IO is incurred to read this blob back into memory again, which is avoided by enabling this option. This further helps if the workload exhibits high temporal locality, where most of the reads go to recently written data. This also helps in case of the remote file system since it involves network traffic and higher latencies.
 * Support using secondary cache with the blob cache. When creating a blob cache, the user can set a secondary blob cache by configuring `secondary_cache` in LRUCacheOptions.
 * Charge memory usage of blob cache when the backing cache of the blob cache and the block cache are different. If an operation reserving memory for blob cache exceeds the avaible space left in the block cache at some point (i.e, causing a cache full under `LRUCacheOptions::strict_capacity_limit` = true), creation will fail with `Status::MemoryLimit()`. To opt in this feature, enable charging `CacheEntryRole::kBlobCache` in `BlockBasedTableOptions::cache_usage_options`.
 * Improve subcompaction range partition so that it is likely to be more even. More evenly distribution of subcompaction will improve compaction throughput for some workloads. All input files' index blocks to sample some anchor key points from which we pick positions to partition the input range. This would introduce some CPU overhead in compaction preparation phase, if subcompaction is enabled, but it should be a small fraction of the CPU usage of the whole compaction process. This also brings a behavier change: subcompaction number is much more likely to maxed out than before.
 * Add CompactionPri::kRoundRobin, a compaction picking mode that cycles through all the files with a compact cursor in a round-robin manner. This feature is available since 7.5.
+* Added a blob-specific cache priority level - bottom level. Blobs are typically lower-value targets for caching than data blocks, since 1) with BlobDB, data blocks containing blob references conceptually form an index structure which has to be consulted before we can read the blob value, and 2) cached blobs represent only a single key-value, while cached data blocks generally contain multiple KVs. The user can specify the new option `low_pri_pool_ratio` in `LRUCacheOptions` to configure the ratio of capacity reserved for low priority cache entries (and therefore the remaining ratio is the space reserved for the bottom level), or configuring the new argument `low_pri_pool_ratio` in `NewLRUCache()` to achieve the same effect.
 
 ### Public API changes
 * Removed Customizable support for RateLimiter and removed its CreateFromString() and Type() functions.

cache/cache.cc

@@ -33,6 +33,10 @@ static std::unordered_map<std::string, OptionTypeInfo>
          {offsetof(struct LRUCacheOptions, high_pri_pool_ratio),
           OptionType::kDouble, OptionVerificationType::kNormal,
           OptionTypeFlags::kMutable}},
+        {"low_pri_pool_ratio",
+         {offsetof(struct LRUCacheOptions, low_pri_pool_ratio),
+          OptionType::kDouble, OptionVerificationType::kNormal,
+          OptionTypeFlags::kMutable}},
 };
 
 static std::unordered_map<std::string, OptionTypeInfo>

cache/cache_bench_tool.cc

@@ -304,7 +304,9 @@ class CacheBench {
           FLAGS_cache_size, FLAGS_value_bytes, FLAGS_num_shard_bits,
           false /*strict_capacity_limit*/, kDefaultCacheMetadataChargePolicy);
     } else if (FLAGS_cache_type == "lru_cache") {
-      LRUCacheOptions opts(FLAGS_cache_size, FLAGS_num_shard_bits, false, 0.5);
+      LRUCacheOptions opts(FLAGS_cache_size, FLAGS_num_shard_bits,
+                           false /* strict_capacity_limit */,
+                           0.5 /* high_pri_pool_ratio */);
 #ifndef ROCKSDB_LITE
       if (!FLAGS_secondary_cache_uri.empty()) {
         Status s = SecondaryCache::CreateFromString(

cache/clock_cache.cc

@@ -697,8 +697,10 @@ void ClockCache::DisownData() {
 std::shared_ptr<Cache> NewClockCache(
     size_t capacity, int num_shard_bits, bool strict_capacity_limit,
     CacheMetadataChargePolicy metadata_charge_policy) {
-  return NewLRUCache(capacity, num_shard_bits, strict_capacity_limit, 0.5,
-                     nullptr, kDefaultToAdaptiveMutex, metadata_charge_policy);
+  return NewLRUCache(capacity, num_shard_bits, strict_capacity_limit,
+                     /* high_pri_pool_ratio */ 0.5, nullptr,
+                     kDefaultToAdaptiveMutex, metadata_charge_policy,
+                     /* low_pri_pool_ratio */ 0.5);
 }
 
 std::shared_ptr<Cache> ExperimentalNewClockCache(

cache/compressed_secondary_cache.cc

@@ -24,17 +24,18 @@ void DeletionCallback(const Slice& /*key*/, void* obj) {
 
 CompressedSecondaryCache::CompressedSecondaryCache(
     size_t capacity, int num_shard_bits, bool strict_capacity_limit,
-    double high_pri_pool_ratio,
+    double high_pri_pool_ratio, double low_pri_pool_ratio,
     std::shared_ptr<MemoryAllocator> memory_allocator, bool use_adaptive_mutex,
     CacheMetadataChargePolicy metadata_charge_policy,
     CompressionType compression_type, uint32_t compress_format_version)
     : cache_options_(capacity, num_shard_bits, strict_capacity_limit,
                      high_pri_pool_ratio, memory_allocator, use_adaptive_mutex,
                      metadata_charge_policy, compression_type,
-                     compress_format_version) {
-  cache_ = NewLRUCache(capacity, num_shard_bits, strict_capacity_limit,
-                       high_pri_pool_ratio, memory_allocator,
-                       use_adaptive_mutex, metadata_charge_policy);
+                     compress_format_version, low_pri_pool_ratio) {
+  cache_ =
+      NewLRUCache(capacity, num_shard_bits, strict_capacity_limit,
+                  high_pri_pool_ratio, memory_allocator, use_adaptive_mutex,
+                  metadata_charge_policy, low_pri_pool_ratio);
 }
 
 CompressedSecondaryCache::~CompressedSecondaryCache() { cache_.reset(); }
@@ -150,11 +151,12 @@ std::shared_ptr<SecondaryCache> NewCompressedSecondaryCache(
     double high_pri_pool_ratio,
     std::shared_ptr<MemoryAllocator> memory_allocator, bool use_adaptive_mutex,
     CacheMetadataChargePolicy metadata_charge_policy,
-    CompressionType compression_type, uint32_t compress_format_version) {
+    CompressionType compression_type, uint32_t compress_format_version,
+    double low_pri_pool_ratio) {
   return std::make_shared<CompressedSecondaryCache>(
       capacity, num_shard_bits, strict_capacity_limit, high_pri_pool_ratio,
-      memory_allocator, use_adaptive_mutex, metadata_charge_policy,
-      compression_type, compress_format_version);
+      low_pri_pool_ratio, memory_allocator, use_adaptive_mutex,
+      metadata_charge_policy, compression_type, compress_format_version);
 }
 
 std::shared_ptr<SecondaryCache> NewCompressedSecondaryCache(
@@ -165,7 +167,7 @@ std::shared_ptr<SecondaryCache> NewCompressedSecondaryCache(
       opts.capacity, opts.num_shard_bits, opts.strict_capacity_limit,
       opts.high_pri_pool_ratio, opts.memory_allocator, opts.use_adaptive_mutex,
       opts.metadata_charge_policy, opts.compression_type,
-      opts.compress_format_version);
+      opts.compress_format_version, opts.low_pri_pool_ratio);
 }
 
 }  // namespace ROCKSDB_NAMESPACE

cache/compressed_secondary_cache.h

@@ -54,7 +54,7 @@ class CompressedSecondaryCache : public SecondaryCache {
  public:
   CompressedSecondaryCache(
       size_t capacity, int num_shard_bits, bool strict_capacity_limit,
-      double high_pri_pool_ratio,
+      double high_pri_pool_ratio, double low_pri_pool_ratio,
       std::shared_ptr<MemoryAllocator> memory_allocator = nullptr,
       bool use_adaptive_mutex = kDefaultToAdaptiveMutex,
       CacheMetadataChargePolicy metadata_charge_policy =

cache/compressed_secondary_cache_test.cc

@@ -238,9 +238,11 @@ class CompressedSecondaryCacheTest : public testing::Test {
     secondary_cache_opts.metadata_charge_policy = kDontChargeCacheMetadata;
     std::shared_ptr<SecondaryCache> secondary_cache =
         NewCompressedSecondaryCache(secondary_cache_opts);
-    LRUCacheOptions lru_cache_opts(1024, 0, false, 0.5, nullptr,
-                                   kDefaultToAdaptiveMutex,
-                                   kDontChargeCacheMetadata);
+    LRUCacheOptions lru_cache_opts(
+        1024 /* capacity */, 0 /* num_shard_bits */,
+        false /* strict_capacity_limit */, 0.5 /* high_pri_pool_ratio */,
+        nullptr /* memory_allocator */, kDefaultToAdaptiveMutex,
+        kDontChargeCacheMetadata, 0.5 /* low_pri_pool_ratio */);
     lru_cache_opts.secondary_cache = secondary_cache;
     std::shared_ptr<Cache> cache = NewLRUCache(lru_cache_opts);
     std::shared_ptr<Statistics> stats = CreateDBStatistics();
@@ -322,8 +324,11 @@ class CompressedSecondaryCacheTest : public testing::Test {
     std::shared_ptr<SecondaryCache> secondary_cache =
         NewCompressedSecondaryCache(secondary_cache_opts);
 
-    LRUCacheOptions opts(1024, 0, false, 0.5, nullptr, kDefaultToAdaptiveMutex,
-                         kDontChargeCacheMetadata);
+    LRUCacheOptions opts(
+        1024 /* capacity */, 0 /* num_shard_bits */,
+        false /* strict_capacity_limit */, 0.5 /* high_pri_pool_ratio */,
+        nullptr /* memory_allocator */, kDefaultToAdaptiveMutex,
+        kDontChargeCacheMetadata, 0.5 /* low_pri_pool_ratio */);
     opts.secondary_cache = secondary_cache;
     std::shared_ptr<Cache> cache = NewLRUCache(opts);
 
@@ -368,8 +373,11 @@ class CompressedSecondaryCacheTest : public testing::Test {
     std::shared_ptr<SecondaryCache> secondary_cache =
         NewCompressedSecondaryCache(secondary_cache_opts);
 
-    LRUCacheOptions opts(1024, 0, false, 0.5, nullptr, kDefaultToAdaptiveMutex,
-                         kDontChargeCacheMetadata);
+    LRUCacheOptions opts(
+        1024 /* capacity */, 0 /* num_shard_bits */,
+        false /* strict_capacity_limit */, 0.5 /* high_pri_pool_ratio */,
+        nullptr /* memory_allocator */, kDefaultToAdaptiveMutex,
+        kDontChargeCacheMetadata, 0.5 /* low_pri_pool_ratio */);
     opts.secondary_cache = secondary_cache;
     std::shared_ptr<Cache> cache = NewLRUCache(opts);
 
@@ -424,8 +432,11 @@ class CompressedSecondaryCacheTest : public testing::Test {
     std::shared_ptr<SecondaryCache> secondary_cache =
         NewCompressedSecondaryCache(secondary_cache_opts);
 
-    LRUCacheOptions opts(1024, 0, false, 0.5, nullptr, kDefaultToAdaptiveMutex,
-                         kDontChargeCacheMetadata);
+    LRUCacheOptions opts(
+        1024 /* capacity */, 0 /* num_shard_bits */,
+        false /* strict_capacity_limit */, 0.5 /* high_pri_pool_ratio */,
+        nullptr /* memory_allocator */, kDefaultToAdaptiveMutex,
+        kDontChargeCacheMetadata, 0.5 /* low_pri_pool_ratio */);
     opts.secondary_cache = secondary_cache;
     std::shared_ptr<Cache> cache = NewLRUCache(opts);
 
@@ -480,8 +491,11 @@ class CompressedSecondaryCacheTest : public testing::Test {
     std::shared_ptr<SecondaryCache> secondary_cache =
         NewCompressedSecondaryCache(secondary_cache_opts);
 
-    LRUCacheOptions opts(1024, 0, /*_strict_capacity_limit=*/true, 0.5, nullptr,
-                         kDefaultToAdaptiveMutex, kDontChargeCacheMetadata);
+    LRUCacheOptions opts(
+        1024 /* capacity */, 0 /* num_shard_bits */,
+        true /* strict_capacity_limit */, 0.5 /* high_pri_pool_ratio */,
+        nullptr /* memory_allocator */, kDefaultToAdaptiveMutex,
+        kDontChargeCacheMetadata, 0.5 /* low_pri_pool_ratio */);
     opts.secondary_cache = secondary_cache;
     std::shared_ptr<Cache> cache = NewLRUCache(opts);