PA: Parameterize minimum prepared contexts per bidder thread

This will help reduce wasted contexts until we implement a better
heuristic for the number of contexts.

Bug: 378135925
Change-Id: I0a4e410582117a1063acebfb52b768a565191f1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6163726
Reviewed-by: Russ Hamilton <behamilton@google.com>
Commit-Queue: Abigail Katcoff <abigailkatcoff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1404918}

content/services/auction_worklet/bidder_worklet.cc

@@ -291,9 +291,10 @@ size_t CalculateNumberOfContextsToPreparePerThread(
     size_t threads) {
   return fmin(
       features::kFledgeMaxBidderContextsPerThreadInAdvance.Get(),
-      fmax(1, (max_expected_required_contexts *
-               features::kFledgeBidderContextsMultiplier.Get()) /
-                  (threads * features::kFledgeBidderContextsDivisor.Get())));
+      fmax(features::kFledgeMinBidderContextsPerThreadInAdvance.Get(),
+           (max_expected_required_contexts *
+            features::kFledgeBidderContextsMultiplier.Get()) /
+               (threads * features::kFledgeBidderContextsDivisor.Get())));
 }
 
 // Check if trusted bidding signals, if any, are same-origin or cross-origin.

content/services/auction_worklet/bidder_worklet_unittest.cc

@@ -9373,7 +9373,8 @@ TEST_P(BidderWorkletMultiThreadingTest, CreatesCorrectNumberOfPremadeContexts) {
       features::kFledgePrepareBidderContextsInAdvance,
       {{"BidderContextsDivisor", "2"},
        {"BidderContextsMultiplier", "1"},
-       {"MaxBidderContextsPerThread", "4"}});
+       {"MaxBidderContextsPerThread", "4"},
+       {"MinBidderContextsPerThread", "1"}});
   interest_group_trusted_bidding_signals_url_ = GURL("https://signals.test/");
   const GURL kFullSignalsUrl(
       "https://signals.test/?hostname=top.window.test&interestGroupNames=Fred");

content/services/auction_worklet/public/cpp/auction_worklet_features.cc

@@ -46,6 +46,11 @@ BASE_FEATURE_PARAM(int,
                    &kFledgePrepareBidderContextsInAdvance,
                    "MaxBidderContextsPerThread",
                    10);
+BASE_FEATURE_PARAM(int,
+                   kFledgeMinBidderContextsPerThreadInAdvance,
+                   &kFledgePrepareBidderContextsInAdvance,
+                   "MinBidderContextsPerThread",
+                   1);
 BASE_FEATURE_PARAM(int,
                    kFledgeBidderContextsDivisor,
                    &kFledgePrepareBidderContextsInAdvance,

content/services/auction_worklet/public/cpp/auction_worklet_features.h

@@ -40,6 +40,9 @@ CONTENT_EXPORT BASE_DECLARE_FEATURE(kFledgePrepareBidderContextsInAdvance);
 CONTENT_EXPORT BASE_DECLARE_FEATURE_PARAM(
     int,
     kFledgeMaxBidderContextsPerThreadInAdvance);
+CONTENT_EXPORT BASE_DECLARE_FEATURE_PARAM(
+    int,
+    kFledgeMinBidderContextsPerThreadInAdvance);
 CONTENT_EXPORT BASE_DECLARE_FEATURE_PARAM(int, kFledgeBidderContextsDivisor);
 CONTENT_EXPORT BASE_DECLARE_FEATURE_PARAM(int, kFledgeBidderContextsMultiplier);