WBM: Fix stall deadlock with multiple cfs

HISTORY.md

@@ -10,6 +10,7 @@
 
 ### Bug Fixes
 * LOG Consistency:Display the pinning policy options same as block cache options / metadata cache options (#804).
+* WBM: fix allow_stall deadlock with multiple cfs.
 
 ### Miscellaneous
 * WriteController logging: Remove redundant reports when WC is not shared between dbs

db/db_impl/db_impl_write.cc

@@ -1747,9 +1747,13 @@ Status DBImpl::HandleWriteBufferManagerFlush(WriteContext* write_context) {
   if (immutable_db_options_.atomic_flush) {
     SelectColumnFamiliesForAtomicFlush(&cfds);
   } else {
-    ColumnFamilyData* cfd_picked = nullptr;
-    SequenceNumber seq_num_for_cf_picked = kMaxSequenceNumber;
-
+    // As part of https://github.com/speedb-io/speedb/pull/859, theres a need to
+    // schedule more flushes since the cfd picked for flush was the oldest one
+    // and not necessarily enough to resolve the stall issue.
+    // For this reason, schedule enough flushes so that the memory usage is at
+    // least below the flush trigger (kMutableLimit * buffer_size)
+    int64_t total_mem_to_free =
+        write_buffer_manager()->memory_above_flush_trigger();
     for (auto cfd : *versions_->GetColumnFamilySet()) {
       if (cfd->IsDropped()) {
         continue;
@@ -1759,16 +1763,15 @@ Status DBImpl::HandleWriteBufferManagerFlush(WriteContext* write_context) {
         // and no immutable memtables for which flush has yet to finish. If
         // we triggered flush on CFs already trying to flush, we would risk
         // creating too many immutable memtables leading to write stalls.
-        uint64_t seq = cfd->mem()->GetCreationSeq();
-        if (cfd_picked == nullptr || seq < seq_num_for_cf_picked) {
-          cfd_picked = cfd;
-          seq_num_for_cf_picked = seq;
+        auto mem_used = cfd->mem()->ApproximateMemoryUsageFast();
+        cfds.push_back(cfd);
+        total_mem_to_free -= mem_used;
+        if (total_mem_to_free <= 0) {
+          break;
         }
       }
     }
-    if (cfd_picked != nullptr) {
-      cfds.push_back(cfd_picked);
-    }
+
     MaybeFlushStatsCF(&cfds);
   }
   if (!cfds.empty()) {

include/rocksdb/write_buffer_manager.h

@@ -67,6 +67,8 @@ class WriteBufferManager final {
   static constexpr uint64_t kNoDelayedWriteFactor = 0U;
   static constexpr uint64_t kMaxDelayedWriteFactor = 100U;
   static constexpr uint64_t kStopDelayedWriteFactor = kMaxDelayedWriteFactor;
+  static constexpr double kMutableLimit = 0.875;
+
   enum class UsageState { kNone, kDelay, kStop };
 
  public:
@@ -152,6 +154,10 @@ class WriteBufferManager final {
     return ((inactive >= total) ? 0 : (total - inactive));
   }
 
+  int64_t memory_above_flush_trigger() {
+    return mutable_memtable_memory_usage() - buffer_size() * kMutableLimit;
+  }
+
   // Returns the total inactive memory used by memtables.
   size_t immmutable_memtable_memory_usage() const {
     return memory_inactive_.load(std::memory_order_relaxed);
@@ -180,7 +186,7 @@ class WriteBufferManager final {
     [[maybe_unused]] auto was_enabled = enabled();
 
     buffer_size_.store(new_size, std::memory_order_relaxed);
-    mutable_limit_.store(new_size * 7 / 8, std::memory_order_relaxed);
+    mutable_limit_.store(new_size * kMutableLimit, std::memory_order_relaxed);
 
     assert(was_enabled == enabled());
 

memtable/write_buffer_manager.cc

@@ -54,7 +54,7 @@ WriteBufferManager::WriteBufferManager(
     const FlushInitiationOptions& flush_initiation_options,
     uint16_t start_delay_percent)
     : buffer_size_(_buffer_size),
-      mutable_limit_(buffer_size_ * 7 / 8),
+      mutable_limit_(buffer_size_ * kMutableLimit),
       memory_used_(0),
       memory_inactive_(0),
       memory_being_freed_(0U),