Optimize FixReblogsInFeeds migration (#5538)

We have changed how we store reblogs in the redis for bigint IDs. This process is done by 1) scan all entries in users feed, and 2) re-store reblogs by 3 write commands.

However, this operation is really slow for large instances. e.g. 1hrs on friends.nico (w/ 50k users). So I have tried below tweaks.

* It checked non-reblogs by `entry[0] == entry[1]`, but this condition won't work because `entry[0]` is String while `entry[1]` is Float. Changing `entry[0].to_i == entry[1]` seems work.
  -> about 4-20x faster (feed with less reblogs will be faster)
* Write operations can be batched by pipeline
  -> about 6x faster
* Wrap operation by Lua script and execute by EVALSHA command. This really reduces packets between Ruby and Redis.
  -> about 3x faster

I've taken Lua script way, though doing other optimizations may be enough.

db/migrate/20170920032311_fix_reblogs_in_feeds.rb

@@ -3,48 +3,62 @@ class FixReblogsInFeeds < ActiveRecord::Migration[5.1]
     redis = Redis.current
     fm = FeedManager.instance
 
-    # find_each is batched on the database side.
-    User.includes(:account).find_each do |user|
-      account = user.account
+    # Old scheme:
+    # Each user's feed zset had a series of score:value entries,
+    # where "regular" statuses had the same score and value (their
+    # ID). Reblogs had a score of the reblogging status' ID, and a
+    # value of the reblogged status' ID.
 
-      # Old scheme:
-      # Each user's feed zset had a series of score:value entries,
-      # where "regular" statuses had the same score and value (their
-      # ID). Reblogs had a score of the reblogging status' ID, and a
-      # value of the reblogged status' ID.
-
-      # New scheme:
-      # The feed contains only entries with the same score and value.
-      # Reblogs result in the reblogging status being added to the
-      # feed, with an entry in a reblog tracking zset (where the score
-      # is once again set to the reblogging status' ID, and the value
-      # is set to the reblogged status' ID). This is safe for Redis'
-      # float coersion because in this reblog tracking zset, we only
-      # need the rebloggging status' ID to be able to stop tracking
-      # entries after they have gotten too far down the feed, which
-      # does not require an exact value.
-
-      # So, first, we iterate over the user's feed to find any reblogs.
-      timeline_key = fm.key(:home, account.id)
-      reblog_key = fm.key(:home, account.id, 'reblogs')
-      redis.zrange(timeline_key, 0, -1, with_scores: true).each do |entry|
-        next if entry[0] == entry[1]
+    # New scheme:
+    # The feed contains only entries with the same score and value.
+    # Reblogs result in the reblogging status being added to the
+    # feed, with an entry in a reblog tracking zset (where the score
+    # is once again set to the reblogging status' ID, and the value
+    # is set to the reblogged status' ID). This is safe for Redis'
+    # float coersion because in this reblog tracking zset, we only
+    # need the rebloggging status' ID to be able to stop tracking
+    # entries after they have gotten too far down the feed, which
+    # does not require an exact value.
+
+    # This process reads all feeds and writes 3 times for each reblogs.
+    # So we use Lua script to avoid overhead between Ruby and Redis.
+    script = <<-LUA
+      local timeline_key = KEYS[1]
+      local reblog_key = KEYS[2]
 
-        # The score and value don't match, so this is a reblog.
-        # (note that we're transitioning from IDs < 53 bits so we
-        # don't have to worry about the loss of precision)
+      -- So, first, we iterate over the user's feed to find any reblogs.
+      local items = redis.call('zrange', timeline_key, 0, -1, 'withscores')
       
-        reblogged_id, reblogging_id = entry
+      for i = 1, #items, 2 do
+        local reblogged_id = items[i]
+        local reblogging_id = items[i + 1]
+        if (reblogged_id ~= reblogging_id) then
 
-        # Remove the old entry
-        redis.zrem(timeline_key, reblogged_id)
+          -- The score and value don't match, so this is a reblog.
+          -- (note that we're transitioning from IDs < 53 bits so we
+          -- don't have to worry about the loss of precision)
 
-        # Add a new one for the reblogging status
-        redis.zadd(timeline_key, reblogging_id, reblogging_id)
+          -- Remove the old entry
+          redis.call('zrem', timeline_key, reblogged_id)
 
-        # Track the fact that this was a reblog
-        redis.zadd(reblog_key, reblogging_id, reblogged_id)
+          -- Add a new one for the reblogging status
+          redis.call('zadd', timeline_key, reblogging_id, reblogging_id)
+
+          -- Track the fact that this was a reblog
+          redis.call('zadd', reblog_key, reblogging_id, reblogged_id)
+        end
       end
+    LUA
+    script_hash = redis.script(:load, script)
+
+    # find_each is batched on the database side.
+    User.includes(:account).find_each do |user|
+      account = user.account
+
+      timeline_key = fm.key(:home, account.id)
+      reblog_key = fm.key(:home, account.id, 'reblogs')
+
+      redis.evalsha(script_hash, [timeline_key, reblog_key])
     end
   end