This tutorial demonstrates both the right ways and wrong ways of benchmarking user Lua code in OpenResty.

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cd ~
mkdir time-lua
cd time-lua/

First of all, make sure our CPU is always at its full speed.

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echo performance | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

Timing Lua Code Correctly in OpenResty_java

It usually takes the value powersave by default and we need to set it toperformance instead.

The simplest way to time some Lua code is to use the time command with the resty command.

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time resty -e 'ngx.re.find("hello, world.", [[\w+\.]], "jo")'

Timing Lua Code Correctly in OpenResty_java_02

But there is a catch. The resty command itself has a startup and exiting overhead.

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time resty -e ''

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We can see there’s overhead of about 11 milliseconds on this machine.

Instead, we should use the ngx.now Lua API function provided by OpenResty.

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restydoc -s ngx.now

Timing Lua Code Correctly in OpenResty_java_04

Let’s put our Lua code into a file named ./bench.lua for better readablility.

We make the following edits:

  1. First of all, we make sure the cached time inside nginx is up to date.
  2. And then we record the begin time which has millisecond precision.
  3. And then put our aforementioned regex matching call.
  4. And then we update our cached time again.
  5. Finally, output the elapsed time by doing a time subtraction.
  6. Let’s save the file.
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ngx.update_time()
local begin = ngx.now()
ngx.re.find("hello, world.", [[\w+\.]], "jo")
ngx.update_time()
ngx.say("elapsed seconds: ", ngx.now() - begin)

Timing Lua Code Correctly in OpenResty_java_05

Then run the resty shell command.

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resty bench.lua

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It records about a latency of about 1 millisecond. But we will soon see it is very inaccurate.

The correct way is to make the following edits in the bench.lua file:

  1. Put the call into a Lua function named target.
  2. And then call this function first for 100 times as a warmup. Now thistarget function should be JIT compiled after this loop is executed.
  3. And then inside the timed code region, we call it repeatedly for 10 million times.
  4. Finally we compute the average time.
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local function target()
    ngx.re.find("hello, world.", [[\w+\.]], "jo")
end

for i = 1, 100 do
    target()
end

ngx.update_time()
local begin = ngx.now()

local N = 1e7
for i = 1, N do
    target()
end

ngx.update_time()
ngx.say("elapsed: ", (ngx.now() - begin) / N)

Timing Lua Code Correctly in OpenResty_java_07

We now run this script again.

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resty bench.lua

Timing Lua Code Correctly in OpenResty_java_08

We can see that it is merely about 30 nanoseconds per call. So many many times faster than the previous result!

Actualy we can further make sure no dead GC objects hanging around before we time the code. Just insert the following line of code before the first ngx.update_time() call.

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collectgarbage()

Timing Lua Code Correctly in OpenResty_java_09

Here we force a full GC cycle before recording the begin time.

It does not help much with our example here, however. This is because our timed code does not create many GC objects anyway.

We can make the target function faster by avoiding unnecessary Lua table lookup operations.

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local re_find = ngx.re.find

local function target()
    re_find("hello, world.", [[\w+\.]], "jo")
end

Timing Lua Code Correctly in OpenResty_java_10

But the difference may not be measurable here.

Timing Lua Code Correctly in OpenResty_java_11

This is what I’d cover today. Hopefully you find it interesting.

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About This Article and Associated Video

This article and its associated video are both generated automatically from a simple screenplay file.

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About The Author

Yichun Zhang is the creator of the OpenResty® open source project. He is also the founder and CEO of the OpenResty Inc. company. He contributed a dozen open source Nginx 3rd-party modules, quite some Nginx and LuaJITcore patches, and designed the OpenResty XRay platform.

Translations

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