immutable: when downloading an immutable file, use primary shares if they are available

Primary shares require no erasure decoding so the more primary shares you have, the less CPU is used.

diff --git a/src/allmydata/immutable/download.py b/src/allmydata/immutable/download.py
index da1d40c40481a27e56df64f0cae1ea3b1e23a9c6..fcf5c42223a0ca9daf28c638dd6fb2142526bebc 100644 (file)
--- a/src/allmydata/immutable/download.py
+++ b/src/allmydata/immutable/download.py
@@ -875,8 +875,13 @@ class FileDownloader(log.PrefixingLogMixin):
             potential_shnums = list(available_shnums - handled_shnums)
             if not potential_shnums:
                 raise NotEnoughSharesError
-            # choose a random share
-            shnum = random.choice(potential_shnums)
+            # For the next share, choose a primary share if available, else a randomly chosen
+            # secondary share.
+            potential_shnums.sort()
+            if potential_shnums[0] < self._uri.needed_shares:
+                shnum = potential_shnums[0]
+            else:
+                shnum = random.choice(potential_shnums)
             # and a random bucket that will provide it
             validated_bucket = random.choice(list(self._share_vbuckets[shnum]))
             self.active_buckets[shnum] = validated_bucket