Change HeaderTable::names_map values to a small_vector

Summary:
std::list is generally a bad choice, and this one is particularly bad for performance. Each iteration in getIndex essentially incurs a cache miss, which under load can be going all the way to DRAM.

Change the map to always a (flat) F14ValueMap, and optimize further by having the values inline most of the time. 7 is chosen as an empirically good value, while not being outrageously large.

Reviewed By: kvtsoy

Differential Revision: D58037451

fbshipit-source-id: 7a546aa00cab65c95a2ee1c25a79a45caa71c774

proxygen/lib/http/codec/HTTP1xCodec.h

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <list>
 #include <proxygen/lib/http/HTTPMessage.h>
 #include <proxygen/lib/http/codec/HTTPCodec.h>
 #include <proxygen/lib/http/codec/TransportDirection.h>

proxygen/lib/http/codec/compress/HeaderTable.cpp

@@ -122,7 +122,7 @@ uint32_t HeaderTable::removeLast() {
     DCHECK(names_it != names_.end());
     auto& ilist = names_it->second;
     DCHECK_EQ(ilist.front(), t);
-    ilist.pop_front();
+    ilist.erase(ilist.begin());
 
     // remove the name if there are no indices associated with it
     if (ilist.empty()) {

proxygen/lib/http/codec/compress/HeaderTable.h

@@ -8,11 +8,11 @@
 
 #pragma once
 
-#include <list>
 #include <string>
 #include <vector>
 
 #include <folly/container/F14Map.h>
+#include <folly/small_vector.h>
 #include <proxygen/lib/http/codec/compress/HPACKHeader.h>
 
 namespace proxygen {
@@ -24,8 +24,10 @@ namespace proxygen {
 
 class HeaderTable {
  public:
-  // TODO: std::vector might be faster than std::list in the use case?
-  using names_map = folly::F14FastMap<HPACKHeaderName, std::list<uint32_t>>;
+  // 7 is chosen as an empirically good value that's not too big.
+  // small_vector spills to the heap beyond that.
+  using names_map =
+      folly::F14ValueMap<HPACKHeaderName, folly::small_vector<uint32_t, 7>>;
 
   explicit HeaderTable(uint32_t capacityVal) {
     init(capacityVal);

proxygen/lib/http/codec/compress/test/HPACKContextTests.cpp

@@ -83,11 +83,11 @@ TEST_F(HPACKContextTests, StaticTableHeaderNamesAreCommon) {
                                               "max-forwards",
                                               "if-range",
                                               "refresh"};
-  for (std::pair<HPACKHeaderName, std::list<uint32_t>> entry : table.names()) {
-    EXPECT_TRUE(entry.first.isCommonHeader() ||
-                uncommonStaticEntries.find(entry.first.get()) !=
+  for (const auto& [header, _] : table.names()) {
+    EXPECT_TRUE(header.isCommonHeader() ||
+                uncommonStaticEntries.find(header.get()) !=
                     uncommonStaticEntries.end())
-        << entry.first.get();
+        << header.get();
   }
 }