HashLinkList to log distribution of number of entries aross buckets

Summary: Add two parameters of hash linked list to log distribution of number of entries across all buckets, and a sample row when there are too many entries in one single bucket.

Test Plan: Turn it on in plain_table_db_test and see the logs.

Reviewers: haobo, ljin

Reviewed By: ljin

Subscribers: leveldb, nkg-, dhruba, yhchiang

Differential Revision: https://reviews.facebook.net/D19095

HISTORY.md

@@ -4,6 +4,7 @@
 
 ### Public API changes
 * Replaced ColumnFamilyOptions::table_properties_collectors with ColumnFamilyOptions::table_properties_collector_factories
+* Add two paramters to NewHashLinkListRepFactory() for logging on too many entries in a hash bucket when flushing.
 
 ### New Features
 * Hash index for block-based table will be materialized and reconstructed more efficiently. Previously hash index is constructed by scanning the whole table during every table open.

db/plain_table_db_test.cc

@@ -62,6 +62,7 @@ class PlainTableDBTest {
   Options CurrentOptions() {
     Options options;
     options.table_factory.reset(NewPlainTableFactory(16, 2, 0.8, 3));
+    options.memtable_factory.reset(NewHashLinkListRepFactory(4, 0, 3, true));
     options.prefix_extractor.reset(NewFixedPrefixTransform(8));
     options.allow_mmap_reads = true;
     return options;

include/rocksdb/memtablerep.h

@@ -243,8 +243,14 @@ extern MemTableRepFactory* NewHashSkipListRepFactory(
 //                      huge pages for it to be allocated, like:
 //                          sysctl -w vm.nr_hugepages=20
 //                      See linux doc Documentation/vm/hugetlbpage.txt
+// @bucket_entries_logging_threshold: if number of entries in one bucket
+//                                    exceeds this number, log about it.
+// @if_log_bucket_dist_when_flash: if true, log distribution of number of
+//                                 entries when flushing.
 extern MemTableRepFactory* NewHashLinkListRepFactory(
-    size_t bucket_count = 50000, size_t huge_page_tlb_size = 0);
+    size_t bucket_count = 50000, size_t huge_page_tlb_size = 0,
+    int bucket_entries_logging_threshold = 4096,
+    bool if_log_bucket_dist_when_flash = true);
 
 // This factory creates a cuckoo-hashing based mem-table representation.
 // Cuckoo-hash is a closed-hash strategy, in which all key/value pairs

util/hash_linklist_rep.cc

@@ -13,6 +13,7 @@
 #include "rocksdb/slice_transform.h"
 #include "port/port.h"
 #include "port/atomic_pointer.h"
+#include "util/histogram.h"
 #include "util/murmurhash.h"
 #include "db/memtable.h"
 #include "db/skiplist.h"
@@ -54,7 +55,9 @@ class HashLinkListRep : public MemTableRep {
  public:
   HashLinkListRep(const MemTableRep::KeyComparator& compare, Arena* arena,
                   const SliceTransform* transform, size_t bucket_size,
-                  size_t huge_page_tlb_size, Logger* logger);
+                  size_t huge_page_tlb_size, Logger* logger,
+                  int bucket_entries_logging_threshold,
+                  bool if_log_bucket_dist_when_flash);
 
   virtual KeyHandle Allocate(const size_t len, char** buf) override;
 
@@ -92,6 +95,10 @@ class HashLinkListRep : public MemTableRep {
 
   const MemTableRep::KeyComparator& compare_;
 
+  Logger* logger_;
+  int bucket_entries_logging_threshold_;
+  bool if_log_bucket_dist_when_flash_;
+
   bool BucketContains(Node* head, const Slice& key) const;
 
   Slice GetPrefix(const Slice& internal_key) const {
@@ -308,11 +315,16 @@ class HashLinkListRep : public MemTableRep {
 HashLinkListRep::HashLinkListRep(const MemTableRep::KeyComparator& compare,
                                  Arena* arena, const SliceTransform* transform,
                                  size_t bucket_size, size_t huge_page_tlb_size,
-                                 Logger* logger)
+                                 Logger* logger,
+                                 int bucket_entries_logging_threshold,
+                                 bool if_log_bucket_dist_when_flash)
     : MemTableRep(arena),
       bucket_size_(bucket_size),
       transform_(transform),
-      compare_(compare) {
+      compare_(compare),
+      logger_(logger),
+      bucket_entries_logging_threshold_(bucket_entries_logging_threshold),
+      if_log_bucket_dist_when_flash_(if_log_bucket_dist_when_flash) {
   char* mem = arena_->AllocateAligned(sizeof(port::AtomicPointer) * bucket_size,
                                       huge_page_tlb_size, logger);
 
@@ -416,15 +428,36 @@ MemTableRep::Iterator* HashLinkListRep::GetIterator(Arena* alloc_arena) {
   // allocate a new arena of similar size to the one currently in use
   Arena* new_arena = new Arena(arena_->BlockSize());
   auto list = new FullList(compare_, new_arena);
+  HistogramImpl keys_per_bucket_hist;
+
   for (size_t i = 0; i < bucket_size_; ++i) {
+    int count = 0;
+    bool num_entries_printed = false;
     auto bucket = GetBucket(i);
     if (bucket != nullptr) {
       Iterator itr(this, bucket);
       for (itr.SeekToHead(); itr.Valid(); itr.Next()) {
         list->Insert(itr.key());
+        if (logger_ != nullptr &&
+            ++count >= bucket_entries_logging_threshold_ &&
+            !num_entries_printed) {
+          num_entries_printed = true;
+          Info(logger_, "HashLinkedList bucket %zu has more than %d "
+               "entries. %dth key: %s",
+               i, count, count,
+               GetLengthPrefixedSlice(itr.key()).ToString(true).c_str());
+        }
       }
     }
+    if (if_log_bucket_dist_when_flash_) {
+      keys_per_bucket_hist.Add(count);
+    }
   }
+  if (if_log_bucket_dist_when_flash_ && logger_ != nullptr) {
+    Info(logger_, "hashLinkedList Entry distribution among buckets: %s",
+         keys_per_bucket_hist.ToString().c_str());
+  }
+
   if (alloc_arena == nullptr) {
     return new FullListIterator(list, new_arena);
   } else {
@@ -482,13 +515,17 @@ Node* HashLinkListRep::FindGreaterOrEqualInBucket(Node* head,
 MemTableRep* HashLinkListRepFactory::CreateMemTableRep(
     const MemTableRep::KeyComparator& compare, Arena* arena,
     const SliceTransform* transform, Logger* logger) {
-  return new HashLinkListRep(compare, arena, transform, bucket_count_,
-                             huge_page_tlb_size_, logger);
+  return new HashLinkListRep(
+      compare, arena, transform, bucket_count_, huge_page_tlb_size_, logger,
+      bucket_entries_logging_threshold_, if_log_bucket_dist_when_flash_);
 }
 
-MemTableRepFactory* NewHashLinkListRepFactory(size_t bucket_count,
-                                              size_t huge_page_tlb_size) {
-  return new HashLinkListRepFactory(bucket_count, huge_page_tlb_size);
+MemTableRepFactory* NewHashLinkListRepFactory(
+    size_t bucket_count, size_t huge_page_tlb_size,
+    int bucket_entries_logging_threshold, bool if_log_bucket_dist_when_flash) {
+  return new HashLinkListRepFactory(bucket_count, huge_page_tlb_size,
+                                    bucket_entries_logging_threshold,
+                                    if_log_bucket_dist_when_flash);
 }
 
 } // namespace rocksdb

util/hash_linklist_rep.h

@@ -16,8 +16,13 @@ namespace rocksdb {
 class HashLinkListRepFactory : public MemTableRepFactory {
  public:
   explicit HashLinkListRepFactory(size_t bucket_count,
-                                  size_t huge_page_tlb_size)
-      : bucket_count_(bucket_count), huge_page_tlb_size_(huge_page_tlb_size) {}
+                                  size_t huge_page_tlb_size,
+                                  int bucket_entries_logging_threshold,
+                                  bool if_log_bucket_dist_when_flash)
+      : bucket_count_(bucket_count),
+        huge_page_tlb_size_(huge_page_tlb_size),
+        bucket_entries_logging_threshold_(bucket_entries_logging_threshold),
+        if_log_bucket_dist_when_flash_(if_log_bucket_dist_when_flash) {}
 
   virtual ~HashLinkListRepFactory() {}
 
@@ -32,6 +37,8 @@ class HashLinkListRepFactory : public MemTableRepFactory {
  private:
   const size_t bucket_count_;
   const size_t huge_page_tlb_size_;
+  int bucket_entries_logging_threshold_;
+  bool if_log_bucket_dist_when_flash_;
 };
 
 }