[Tarantool-patches] [PATCH v3 3/3] app/fiber: wait till a full event loop iteration ends

[Serge Petrenko <sergepetrenko@tarantool.org>]

fiber.top() fills in statistics every event loop iteration,
so if it was just enabled, fiber.top() returns zero in fiber cpu
usage statistics because total time consumed by the main thread was
not yet accounted for.
Same stands for viewing top() results for a freshly created fiber:
its metrics will be zero since it hasn't lived a full ev loop iteration
yet.
Fix this by delaying the test till top() results are meaningful and add
minor refactoring.

Follow-up #2694
---
 test/app/fiber.result   | 38 +++++++++++++++++++++++++++++---------
 test/app/fiber.test.lua | 36 ++++++++++++++++++++++++++++--------
 2 files changed, 57 insertions(+), 17 deletions(-)

diff --git a/test/app/fiber.result b/test/app/fiber.result
index 4a094939f..b767bbb59 100644
--- a/test/app/fiber.result
+++ b/test/app/fiber.result
@@ -1469,6 +1469,12 @@ sum = 0
 fiber.top_enable()
 ---
 ...
+-- Wait till a full event loop iteration passes, so that
+-- top() contains meaningful results. On the ev loop iteration
+-- following fiber.top_enable() results will be zero.
+while fiber.top().cpu["1/sched"].instant == 0 do fiber.yield() end
+---
+...
 a = fiber.top()
 ---
 ...
@@ -1508,33 +1514,47 @@ sum_inst
 ---
 - 100
 ...
--- not exact due to accumulated integer division errors
-sum_avg > 99 and sum_avg < 101 or sum_avg
+-- when a fiber dies, its impact on the thread moving average
+-- persists for a couple of ev loop iterations, but it is no
+-- longer listed in fiber.top(). So sum_avg may way smaller than
+-- 100%. See gh-4625 for details and reenable the test below as
+-- soon as it is implemented.
+-- In rare cases when a fiber dies on the same event loop
+-- iteration as you issue fiber.top(), sum_inst will also be
+-- smaller than 100%, but it is so rare I don't even want to
+-- disable the test above.
+sum_avg <= 100 or sum_avg
 ---
 - true
 ...
+-- not exact due to accumulated integer division errors
+--sum_avg > 99 and sum_avg <= 100 or sum_avg
 tbl = nil
 ---
 ...
 f = fiber.new(function()\
-    for i = 1,1000 do end\
-    fiber.yield()\
-    tbl = fiber.top().cpu[fiber.self().id()..'/'..fiber.self().name()]\
+    local fiber_key = fiber.self().id()..'/'..fiber.self().name()\
+    tbl = fiber.top().cpu[fiber_key]\
+    while tbl.time == 0 do\
+        for i = 1,1000 do end\
+        fiber.yield()\
+        tbl = fiber.top().cpu[fiber_key]\
+    end\
 end)
 ---
 ...
-while f:status() ~= 'dead' do fiber.sleep(0.01) end
+while f:status() ~= 'dead' do fiber.yield() end
 ---
 ...
-tbl["average"] > 0
+tbl.average > 0
 ---
 - true
 ...
-tbl["instant"] > 0
+tbl.instant > 0
 ---
 - true
 ...
-tbl["time"] > 0
+tbl.time > 0
 ---
 - true
 ...
diff --git a/test/app/fiber.test.lua b/test/app/fiber.test.lua
index 38b85d554..7f7350acc 100644
--- a/test/app/fiber.test.lua
+++ b/test/app/fiber.test.lua
@@ -634,6 +634,12 @@ sum = 0
 -- gh-2694 fiber.top()
 fiber.top_enable()
 
+
+-- Wait till a full event loop iteration passes, so that
+-- top() contains meaningful results. On the ev loop iteration
+-- following fiber.top_enable() results will be zero.
+while fiber.top().cpu["1/sched"].instant == 0 do fiber.yield() end
+
 a = fiber.top()
 type(a)
 -- scheduler is present in fiber.top()
@@ -653,18 +659,32 @@ for k, v in pairs(a) do\
 end
 
 sum_inst
+-- when a fiber dies, its impact on the thread moving average
+-- persists for a couple of ev loop iterations, but it is no
+-- longer listed in fiber.top(). So sum_avg may way smaller than
+-- 100%. See gh-4625 for details and reenable the test below as
+-- soon as it is implemented.
+-- In rare cases when a fiber dies on the same event loop
+-- iteration as you issue fiber.top(), sum_inst will also be
+-- smaller than 100%, but it is so rare I don't even want to
+-- disable the test above.
+sum_avg <= 100 or sum_avg
 -- not exact due to accumulated integer division errors
-sum_avg > 99 and sum_avg < 101 or sum_avg
+--sum_avg > 99 and sum_avg <= 100 or sum_avg
 tbl = nil
 f = fiber.new(function()\
-    for i = 1,1000 do end\
-    fiber.yield()\
-    tbl = fiber.top().cpu[fiber.self().id()..'/'..fiber.self().name()]\
+    local fiber_key = fiber.self().id()..'/'..fiber.self().name()\
+    tbl = fiber.top().cpu[fiber_key]\
+    while tbl.time == 0 do\
+        for i = 1,1000 do end\
+        fiber.yield()\
+        tbl = fiber.top().cpu[fiber_key]\
+    end\
 end)
-while f:status() ~= 'dead' do fiber.sleep(0.01) end
-tbl["average"] > 0
-tbl["instant"] > 0
-tbl["time"] > 0
+while f:status() ~= 'dead' do fiber.yield() end
+tbl.average > 0
+tbl.instant > 0
+tbl.time > 0
 
 fiber.top_disable()
 fiber.top()
-- 
2.21.0 (Apple Git-122)