Lua Performance

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Overview

This page is copied from the CA wiki. The widget used in the performance tests is available from the CA SVN.

Other Considerations

It is a well known axiom in computing that

"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil"
- Donald Knuth

Lua coders should keep that in mind, and especially when visiting this page. Readability and maintainability are in most cases just as important, and optimizing code for every last ounce of performance can severely impact those qualities. On the other hand, some of the optimizations suggested have little bearing on readability and should generally always be applied, e.g. localization of API functions, or actually make for neater code e.g. the use of or rather than a nil-check. Generally speaking, optimize only once you are sure that there is or will be a performance bottleneck.

Performance Tests

TEST 1: Localize

Code:

  local min = math.min


Results:

Non-local: 0.719 (158%)
Localized: 0.453 (100%)

Conclusion:

Yes, we should localize all standard Lua and Spring API functions.


TEST 2: Localized Class-Methods (with only 3 accesses!)

Code 1:

  for i=1,1000000 do
    local x = class.test()
    local y = class.test()
    local z = class.test()
  end


Code 2:

  for i=1,1000000 do
    local test = class.test
    local x = test()
    local y = test()
    local z = test()
  end


Results:

Normal way: 1.203 (102%)
Localized: 1.172 (100%)

Conclusion:

No, it isn't faster to localize a class method IN the function call.


TEST 3: Unpack A Table

Code 1:

  for i=1,1000000 do
    local x = min( a[1],a[2],a[3],a[4] )
  end


Code 2:

  local unpack = unpack
  for i=1,1000000 do
    local x = min( unpack(a) )
  end


Code 3:

  local function unpack4(a)
    return a[1],a[2],a[3],a[4]
  end
  for i=1,1000000 do
    local x = min( unpack4(a) )
  end


Results:

with [ ]: 0.485 (100%)
unpack(): 1.093 (225%)
custom unpack4: 0.641 (131%)

Conclusion:

Don't use unpack() in time critical code!


TEST 4: Determine Maximum And Set It ('>' vs. max)

Code 1:

  local max = math.max
  for i=1,1000000 do
     x = max(random(cnt),x)
  end


Code 2:

  for i=1,1000000 do
    local r = random(cnt)
    if (r>x) then x = r end
  end


Results:

math.max: 0.437 (156%)
'if > then': 0.282 (100%)

Conclusion:

Don't use math.[max|min]() in time critical code!


TEST 5: Nil Checks ('if' vs. 'or')

Code 1:

  for i=1,1000000 do
    local y,x
    if (random()>0.5) then y=1 end 
    if (y==nil) then x=1 else x=y end
  end


Code 2:

  for i=1,1000000 do
    local y
    if (random()>0.5) then y=1 end 
    local x=y or 1
  end


Results:

nil-check: 0.297 (106%)
a=x or y: 0.281 (100%)

Conclusion:

The or-operator is faster than a nil-check. Use it!


TEST 6: 'x^2' vs. 'x*x'

Code 1:

  for i=1,1000000 do
     local y = x^2
  end


Code 2:

  for i=1,1000000 do
     local y = x*x
  end


Results:

x^2: 1.422 (110%)
x*x: 1.297 (100%)

Conclusion:

The second syntax is marginally faster


TEST 7: Modulus Operators (math.mod vs. %)

Code 1:

  local fmod = math.fmod
  for i=1,1000000 do
    if (fmod(i,30)<1) then
      local x = 1
    end
  end


Code 2:

  for i=1,1000000 do
    if ((i%30)<1) then
      local x = 1
    end
  end


Results:

math.mod: 0.281 (355%)
%: 0.079 (100%)

Conclusion:

Don't use math.fmod() for positive numbers (for negative ones % and fmod() have different results!)


TEST 8: Functions As Param For Other Functions

Code 1:

  local func1 = function(a,b,func) 
    return func(a+b) 
  end

  for i=1,1000000 do
    local x = func1(1,2,function(a) return a*2 end)
  end


Code 2:

  local func1 = function(a,b,func) 
    return func(a+b) 
  end
  local func2 = function(a) 
    return a*2 
  end

  for i=1,1000000 do
    local x = func1(1,2,func2)
  end


Results:

defined in function param: 3.890 (1144%)
defined as local: 0.344 (100%)

Conclusion:

REALLY, LOCALIZE YOUR FUNCTIONS ALWAYS BEFORE SENDING THEM INTO ANOTHER FUNCTION!!! i.e if you use gl.BeginEnd(), gl.CreateList(), ...!!!


TEST 9: for-loops

Code 1:

  for i=1,1000000 do
    for j,v in pairs(a) do
      x=v
    end
  end


Code 2:

  for i=1,1000000 do
    for j,v in ipairs(a) do
      x=v
    end
  end


Code 3:

  for i=1,1000000 do
    for i=1,100 do
      x=a[i]
    end
  end


Code 4:

  for i=1,1000000 do
    for i=1,#a do
      x=a[i]
    end
  end


Code 5:

  for i=1,1000000 do
    local length = #a
    for i=1,length do
      x=a[i]
    end
  end


Results:

pairs: 3.078 (217%)
ipairs: 3.344 (236%)
for i=1,x do: 1.422 (100%)
for i=1,#atable do 1.422 (100%)
for i=1,atable_length do: 1.562 (110%)

Conclusion:

Don't use pairs() or ipairs() in critical code! Try to save the table-size somewhere and use for i=1,x do!


TEST 10: Array Access (with [ ]) vs. Object Access (with .method)

Code 1:

  for i=1,1000000 do
    x = a["foo"]
  end


Code 2:

  for i=1,1000000 do
    x = a.foo
  end


Results:

atable["foo"]: 1.125 (100%)
atable.foo: 1.141 (101%)

Conclusion:

No difference.


TEST 11: Buffered Table Item Access

Code 1:

  for i=1,1000000 do
    for n=1,100 do
      a[n].x=a[n].x+1
    end
  end


Code 2:

  for i=1,1000000 do
    for n=1,100 do
      local y = a[n]
      y.x=y.x+1
    end
  end


Results:

'a[n].x=a[n].x+1': 1.453 (127%)
'local y=a[n]; y.x=y.x+1': 1.140 (100%)

Conclusion:

Buffering can speed up table item access.


TEST 12: Adding Table Items (table.insert vs. [ ])

Code 1:

  local tinsert = table.insert
  for i=1,1000000 do
    tinsert(a,i)
  end


Code 2:

  for i=1,1000000 do
    a[i]=i
  end


Code 3:

  for i=1,1000000 do
    a[#a+1]=i
  end


Code 4:

  local count = 1
  for i=1,1000000 do
    d[count]=i
    count=count+1
  end


Results:

table.insert: 1.250 (727%)
a[i]: 0.172 (100%)
a[#a+1]=x: 0.453 (263%)
a[count++]=x: 0.203 (118%)

Conclusion:

Don't use table.insert!!! Try to save the table-size somewhere and use a[count+1]=x!


TEST 12: Adding Table Items (mytable ={} vs. mytable={...})

When you write {true, true, true} , Lua knows beforehand that the table will need three slots in its array part, so Lua creates the table with that size. Similarly, if you write {x = 1, y = 2, z = 3}, Lua will create a table with four slots in its hash part.

As an example, the next loop runs in 2.0 seconds:


for i = 1, 1000000 do

local a = {}

a[1] = 1; a[2] = 2; a[3] = 3

end

If we create the tables with the right size, we reduce the run t ime to 0.7 seconds:

for i = 1, 1000000 do

local a = {true, true, true}

a[1] = 1; a[2] = 2; a[3] = 3

end

If you write something like {[1] = true, [2] = true, [3] = true}, however, Lua is not smart enough to detect that the given expressions (literal numbers, in this case) describe array indices, so it creates a table with four slots in its hash part, wasting memory and CPU time

[1]

Test 13

Table initialisation with local members and static intialisation.

local x = os.clock()
T={}
print(string.format("start time: %.2f\n", os.clock() - x))
local CachedTable= {"abc","def","ghk"}
for i=1 , 10000000, 1 do
T[i] = CachedTable
end
print(string.format("elapsed time: %.2f\n", os.clock() - x))
local x = os.clock()
A={}
print(string.format("start time: %.2f\n", os.clock() - x))
for i=1 , 10000000, 1 do
A[i] = {"abc","def","ghk"}
end
print(string.format("elapsed time: %.2f\n", os.clock() - x))

Result: local table initializer = 100 % static intializer = 500 %

Analysis: Lua sucks at identifying static initializers, do not use them, use local initalizations instead.

Lua Garbage Collection

Forum thread about Lua Garbage collection info