Raise MIN_DL_PER_REQUEST to 32

This change should improve overhead for downloading small numbers of
descriptors and microdescriptors by improving compression
performance and lowering directory request overhead.

Closes ticket 23220.

2 files changed, 10 insertions, 2 deletions

diff --git a/changes/bug23220 b/changes/bug23220
new file mode 100644
index 0000000000..9c2efc959a
--- /dev/null
+++ b/changes/bug23220
@@ -0,0 +1,7 @@
+  o Minor features (client directory bandwidth tuning):
+
+    - When downloading (micro)descriptors, don't split the list of
+      descriptors into multiple requests unless there are at least 32
+      descriptors that we want. Previously, we split at 4, not 32, which
+      could lead to significant overhead in HTTP request size and
+      degradation in compression performance. Closes ticket 23220.
diff --git a/src/or/routerlist.c b/src/or/routerlist.c
index 49caa875fe..9894019476 100644
--- a/src/or/routerlist.c
+++ b/src/or/routerlist.c
@@ -4971,8 +4971,9 @@ max_dl_per_request(const or_options_t *options, int purpose)
 }
 
 /** Don't split our requests so finely that we are requesting fewer than
- * this number per server. */
-#define MIN_DL_PER_REQUEST 4
+ * this number per server. (Grouping more than this at once leads to
+ * diminishing returns.) */
+#define MIN_DL_PER_REQUEST 32
 /** To prevent a single screwy cache from confusing us by selective reply,
  * try to split our requests into at least this many requests. */
 #define MIN_REQUESTS 3