Restructure HPackParserTable to not use an inline table, and use a std::vector for its ring buffer instead. (#29394)

* Restructure HPackParserTable to not use an inline table, and use a std::vector for its ring buffer instead.

Before this change, it uses 6.2KiB by default, 2/3 of the size of grpc_chttp2_transport. https://screenshot.googleplex.com/6qdcybty2oCsGjG

After this change, it uses 120 bytes https://screenshot.googleplex.com/4xGWKmZZz68VE4F

This class uses up substantial amounts of memory, as it is allocated on a per-stream basis. This should reduce the size of grpc_chttp2_stream substantially

It should cause a significant memory reduction, as the HPack extension table should almost never be fully populated in terms of number of entries, especially since common headers already exist in a static table and entries are highly likely to take more memory than the absolute minimum.

* formatting

* delete unused method

* move MememtoRingBuffer

* reformat

src/core/ext/transport/chttp2/transport/hpack_parser_table.cc

@@ -20,8 +20,9 @@
 
 #include "src/core/ext/transport/chttp2/transport/hpack_parser_table.h"
 
-#include <assert.h>
-#include <string.h>
+#include <cassert>
+#include <cstddef>
+#include <cstring>
 
 #include "absl/strings/str_format.h"
 
@@ -38,29 +39,55 @@ extern grpc_core::TraceFlag grpc_http_trace;
 
 namespace grpc_core {
 
-HPackTable::HPackTable() : static_metadata_(GetStaticMementos()) {}
+void HPackTable::MementoRingBuffer::Put(Memento m) {
+  GPR_ASSERT(num_entries_ < max_entries_);
+  if (entries_.size() < max_entries_) {
+    ++num_entries_;
+    return entries_.push_back(std::move(m));
+  }
+  size_t index = (first_entry_ + num_entries_) % max_entries_;
+  entries_[index] = std::move(m);
+  ++num_entries_;
+}
 
-HPackTable::~HPackTable() = default;
+auto HPackTable::MementoRingBuffer::PopOne() -> Memento {
+  GPR_ASSERT(num_entries_ > 0);
+  size_t index = first_entry_ % max_entries_;
+  ++first_entry_;
+  --num_entries_;
+  return std::move(entries_[index]);
+}
 
-/* Evict one element from the table */
-void HPackTable::EvictOne() {
-  auto first_entry = std::move(entries_[first_entry_]);
-  GPR_ASSERT(first_entry.transport_size() <= mem_used_);
-  mem_used_ -= first_entry.transport_size();
-  first_entry_ = ((first_entry_ + 1) % entries_.size());
-  num_entries_--;
+auto HPackTable::MementoRingBuffer::Lookup(uint32_t index) const
+    -> const Memento* {
+  if (index >= num_entries_) return nullptr;
+  uint32_t offset = (num_entries_ - 1u - index + first_entry_) % max_entries_;
+  return &entries_[offset];
 }
 
-void HPackTable::Rebuild(uint32_t new_cap) {
-  EntriesVec entries;
-  entries.resize(new_cap);
+void HPackTable::MementoRingBuffer::Rebuild(uint32_t max_entries) {
+  if (max_entries == max_entries_) return;
+  std::vector<Memento> entries;
+  entries.reserve(num_entries_);
   for (size_t i = 0; i < num_entries_; i++) {
-    entries[i] = std::move(entries_[(first_entry_ + i) % entries_.size()]);
+    entries.push_back(
+        std::move(entries_[(first_entry_ + i) % entries_.size()]));
   }
   first_entry_ = 0;
   entries_.swap(entries);
 }
 
+HPackTable::HPackTable() : static_metadata_(GetStaticMementos()) {}
+
+HPackTable::~HPackTable() = default;
+
+/* Evict one element from the table */
+void HPackTable::EvictOne() {
+  auto first_entry = entries_.PopOne();
+  GPR_ASSERT(first_entry.transport_size() <= mem_used_);
+  mem_used_ -= first_entry.transport_size();
+}
+
 void HPackTable::SetMaxBytes(uint32_t max_bytes) {
   if (max_bytes_ == max_bytes) {
     return;
@@ -90,18 +117,9 @@ grpc_error_handle HPackTable::SetCurrentTableSize(uint32_t bytes) {
     EvictOne();
   }
   current_table_bytes_ = bytes;
-  max_entries_ = hpack_constants::EntriesForBytes(bytes);
-  if (max_entries_ > entries_.size()) {
-    Rebuild(max_entries_);
-  } else if (max_entries_ < entries_.size() / 3) {
-    // TODO(ctiller): move to resource quota system, only shrink under memory
-    // pressure
-    uint32_t new_cap =
-        std::max(max_entries_, static_cast<uint32_t>(kInlineEntries));
-    if (new_cap != entries_.size()) {
-      Rebuild(new_cap);
-    }
-  }
+  uint32_t new_cap = std::max(hpack_constants::EntriesForBytes(bytes),
+                              hpack_constants::kInitialTableEntries);
+  entries_.Rebuild(new_cap);
   return GRPC_ERROR_NONE;
 }
 
@@ -122,7 +140,7 @@ grpc_error_handle HPackTable::Add(Memento md) {
     // attempt to add an entry larger than the entire table causes
     // the table to be emptied of all existing entries, and results in an
     // empty table.
-    while (num_entries_) {
+    while (entries_.num_entries()) {
       EvictOne();
     }
     return GRPC_ERROR_NONE;
@@ -136,10 +154,7 @@ grpc_error_handle HPackTable::Add(Memento md) {
 
   // copy the finalized entry in
   mem_used_ += md.transport_size();
-  entries_[(first_entry_ + num_entries_) % entries_.size()] = std::move(md);
-
-  // update accounting values
-  num_entries_++;
+  entries_.Put(std::move(md));
   return GRPC_ERROR_NONE;
 }
 
@@ -225,7 +240,7 @@ GPR_ATTRIBUTE_NOINLINE HPackTable::Memento MakeMemento(size_t i) {
 
 }  // namespace
 
-const HPackTable::StaticMementos& HPackTable::GetStaticMementos() {
+auto HPackTable::GetStaticMementos() -> const StaticMementos& {
   static const StaticMementos* const static_mementos = new StaticMementos();
   return *static_mementos;
 }

src/core/ext/transport/chttp2/transport/hpack_parser_table.h

@@ -63,7 +63,7 @@ class HPackTable {
   grpc_error_handle Add(Memento md) GRPC_MUST_USE_RESULT;
 
   // Current entry count in the table.
-  uint32_t num_entries() const { return num_entries_; }
+  uint32_t num_entries() const { return entries_.num_entries(); }
 
  private:
   struct StaticMementos {
@@ -72,28 +72,46 @@ class HPackTable {
   };
   static const StaticMementos& GetStaticMementos() GPR_ATTRIBUTE_NOINLINE;
 
-  enum { kInlineEntries = hpack_constants::kInitialTableEntries };
-  using EntriesVec = absl::InlinedVector<Memento, kInlineEntries>;
+  class MementoRingBuffer {
+   public:
+    // Rebuild this buffer with a new max_entries_ size.
+    void Rebuild(uint32_t max_entries);
+
+    // Put a new memento.
+    // REQUIRES: num_entries < max_entries
+    void Put(Memento m);
+
+    // Pop the oldest memento.
+    // REQUIRES: num_entries > 0
+    Memento PopOne();
+
+    // Lookup the entry at index, or return nullptr if none exists.
+    const Memento* Lookup(uint32_t index) const;
+
+    uint32_t max_entries() const { return max_entries_; }
+    uint32_t num_entries() const { return num_entries_; }
+
+   private:
+    // The index of the first entry in the buffer. May be greater than
+    // max_entries_, in which case a wraparound has occurred.
+    uint32_t first_entry_ = 0;
+    // How many entries are in the table.
+    uint32_t num_entries_ = 0;
+    // Maximum number of entries we could possibly fit in the table, given
+    // defined overheads.
+    uint32_t max_entries_ = hpack_constants::kInitialTableEntries;
+
+    std::vector<Memento> entries_;
+  };
 
   const Memento* LookupDynamic(uint32_t index) const {
     // Not static - find the value in the list of valid entries
     const uint32_t tbl_index = index - (hpack_constants::kLastStaticEntry + 1);
-    if (tbl_index < num_entries_) {
-      uint32_t offset =
-          (num_entries_ - 1u - tbl_index + first_entry_) % entries_.size();
-      return &entries_[offset];
-    }
-    // Invalid entry: return error
-    return nullptr;
+    return entries_.Lookup(tbl_index);
   }
 
   void EvictOne();
-  void Rebuild(uint32_t new_cap);
 
-  // The first used entry in ents.
-  uint32_t first_entry_ = 0;
-  // How many entries are in the table.
-  uint32_t num_entries_ = 0;
   // The amount of memory used by the table, according to the hpack algorithm
   uint32_t mem_used_ = 0;
   // The max memory allowed to be used by the table, according to the hpack
@@ -101,11 +119,8 @@ class HPackTable {
   uint32_t max_bytes_ = hpack_constants::kInitialTableSize;
   // The currently agreed size of the table, according to the hpack algorithm.
   uint32_t current_table_bytes_ = hpack_constants::kInitialTableSize;
-  // Maximum number of entries we could possibly fit in the table, given defined
-  // overheads.
-  uint32_t max_entries_ = hpack_constants::kInitialTableEntries;
   // HPack table entries
-  EntriesVec entries_{hpack_constants::kInitialTableEntries};
+  MementoRingBuffer entries_;
   // Mementos for static data
   const StaticMementos& static_metadata_;
 };