[Tarantool-patches] [PATCH vshard 09/11] ref: introduce vshard.storage.ref module

[Vladislav Shpilevoy]

Hi! Thanks for the review!

>> +local function ref_session_new(sid)
>> +    -- Session object does store its internal hot attributes in a table. Because
>> +    -- it would mean access to any session attribute would cost at least one
>> +    -- table indexing operation. Instead, all internal fields are stored as
>> +    -- upvalues referenced by the methods defined as closures.
>> +    --
>> +    -- This means session creation may not very suitable for jitting, but it is
>> +    -- very rare and attempts to optimize the most common case.
>> +    --
>> +    -- Still the public functions take 'self' object to make it look normally.
>> +    -- They even use it a bit.
>> +
>> +    -- Ref map to get ref object by its ID.
>> +    local ref_map = {}
>> +    -- Ref heap sorted by their deadlines.
>> +    local ref_heap = lheap.new(heap_min_deadline_cmp)
>> +    -- Total number of refs of the session. Is used to drop the session without
>> +    -- fullscan of the ref map. Heap size can't be used because not all refs are
>> +    -- stored here. See more on that below.
>> +    local count = 0
> 
> Maybe it's better to rename it to "global_count". Sometimes it's quite confusing to see `M.count +=` near `count += `.
> 
> Also you have "global_map" and "global_heap" so no reasons to call it just "count".

I have global_map and global_heap variables because I also have normal map and
heap, local to the session. To distinguish between them I added 'global_'
prefix to the global ones.

The count here is not global. It is local to the session. But I see the point.
I renamed it to `ref_count` to be consistent with `ref_map` and `ref_heap`.

====================
diff --git a/vshard/storage/ref.lua b/vshard/storage/ref.lua
index 7589cb9..27f7804 100644
--- a/vshard/storage/ref.lua
+++ b/vshard/storage/ref.lua
@@ -84,7 +84,7 @@ local function ref_session_new(sid)
     -- Total number of refs of the session. Is used to drop the session without
     -- fullscan of the ref map. Heap size can't be used because not all refs are
     -- stored here. See more on that below.
-    local count = 0
+    local ref_count = 0
     -- Cache global session storages as upvalues to save on M indexing.
     local global_heap = M.session_heap
     local global_map = M.session_map
@@ -94,9 +94,9 @@ local function ref_session_new(sid)
         assert(new_count >= 0)
         M.count = new_count
 
-        new_count = count - del_count
+        new_count = ref_count - del_count
         assert(new_count >= 0)
-        count = new_count
+        ref_count = new_count
     end
 
     local function ref_session_update_deadline(self)
@@ -224,7 +224,7 @@ local function ref_session_new(sid)
             self.deadline = deadline
             global_heap:update(self)
         end
-        count = count + 1
+        ref_count = ref_count + 1
         M.count = M.count + 1
         return true
     end
@@ -260,7 +260,7 @@ local function ref_session_new(sid)
     local function ref_session_kill(self)
         global_map[sid] = nil
         global_heap:remove(self)
-        ref_session_discount(self, count)
+        ref_session_discount(self, ref_count)
     end
 
     -- Don't use __index. It is useless since all sessions use closures as
====================

>> +
>> +    --
>> +    -- GC expired refs until they end or the limit on the number of iterations
>> +    -- is exhausted. The limit is supposed to prevent too long GC which would
>> +    -- occupy TX thread unfairly.
>> +    --
>> +    -- Returns false if nothing to GC, or number of iterations left from the
>> +    -- limit. The caller is supposed to yield when 0 is returned, and retry GC
>> +    -- until it returns false.
>> +    -- The function itself does not yield, because it is used from a more
>> +    -- generic function GCing all sessions. It would not ever yield if all
>> +    -- sessions would have less than limit refs, even if total ref count would
>> +    -- be much bigger.
>> +    --
>> +    -- Besides, the session might be killed during general GC. There must not be
>> +    -- any yields in session methods so as not to introduce a support of dead
>> +    -- sessions.
>> +    --
>> +    local function ref_session_gc(self, limit, now)
>> +        if self.deadline >= now then
>> +            return false
>> +        end
> 
> Here you mix "booleans" and "numbers" as return values. Maybe it's better to return "nil" here?

No problem:

====================
diff --git a/vshard/storage/ref.lua b/vshard/storage/ref.lua
index 27f7804..d31e3ed 100644
--- a/vshard/storage/ref.lua
+++ b/vshard/storage/ref.lua
@@ -164,9 +164,9 @@ local function ref_session_new(sid)
     -- is exhausted. The limit is supposed to prevent too long GC which would
     -- occupy TX thread unfairly.
     --
-    -- Returns false if nothing to GC, or number of iterations left from the
+    -- Returns nil if nothing to GC, or number of iterations left from the
     -- limit. The caller is supposed to yield when 0 is returned, and retry GC
-    -- until it returns false.
+    -- until it returns nil.
     -- The function itself does not yield, because it is used from a more
     -- generic function GCing all sessions. It would not ever yield if all
     -- sessions would have less than limit refs, even if total ref count would
@@ -178,7 +178,7 @@ local function ref_session_new(sid)
     --
     local function ref_session_gc(self, limit, now)
         if self.deadline >= now then
-            return false
+            return nil
         end
         local top = ref_heap:top()
         local del = 1
====================

>> +
>> +    -- Don't use __index. It is useless since all sessions use closures as
>> +    -- methods. Also it is probably slower because on each method call would
>> +    -- need to get the metatable, get __index, find the method here. While now
>> +    -- it is only an index operation on the session object.
> 
> Side note: for heap you still use "__index" even heap uses closures as methods.

Indeed, I should have thought of this. I updated the part1 branch, and rebased the
part2 branch. See the part1 email thread for the diff.