Regarding ngx.var

ajaykemparaj · 2015-08-11T05:20:15+00:00

That's true, was confused because of configuration changes and only after that determined that backend responds with 304 code. Thanks!

Regarding ngx.var

ajaykemparaj

Hi ,

I was going through the documentation of https://github.com/openresty/lua-nginx-module#ngxvarvariable

Here is the use case i just want some understanding on the memory usage and best practice

-- snippet 1 --

local ngx_var = ngx.var

local uri = ngx_var.uri

local args = ngx_var.args

local experssion1 = ngx_var.expression1

local experssion2 = ngx_var.expression2

local experssion3 = ngx_var.expression3

--snippet 2 --

local uri = ngx.var.uri

local args = ngx.var.args

local experssion1 = ngx.var.expression1

local experssion2 = ngx.var.expression2

local experssion3 = ngx.var.expression3

...

the Document Clearly states

WARNING When reading from an Nginx variable, Nginx will allocate memory in the per-request memory pool which is freed only at request termination. So when you need to read from an Nginx variable repeatedly in your Lua code, cache the Nginx variable value to your own Lua variable, for example,

We are currently repeatedly reading from the nginx variable repeatedly as shown in snippet 2 , want to start using snippet 1 , but my team mates think its more of a syntatic sugar than a performance gain , Can you please help me understanding which is better for performance

Thanks,

Ajay

doujiang24

hi,

1. I do think snippet 1 will be a little bit better than snippet 2, but has nothing to do with the WARNING

2. `repeatedly` in the WARNING, means like this:

local experssion = ngx.var._expression_ -- (allocate memory in this step)

for i = 1, n do

-- use experssion here -- (no more allocate)

end

2015-08-11 5:20 GMT+08:00 Ajay k <ajayk...@gmail.com>:

Hi ,

I was going through the documentation of https://github.com/openresty/lua-nginx-module#ngxvarvariable

Here is the use case i just want some understanding on the memory usage and best practice

-- snippet 1 --

local ngx_var = ngx.var
local uri = ngx_var.uri
local args = ngx_var.args

local experssion1 = ngx_var.expression1
local experssion2 = ngx_var.expression2
local experssion3 = ngx_var.expression3

--snippet 2 --

local uri = ngx.var.uri
local args = ngx.var.args

local experssion1 = ngx.var.expression1
local experssion2 = ngx.var.expression2
local experssion3 = ngx.var.expression3
...

the Document Clearly states
WARNING When reading from an Nginx variable, Nginx will allocate memory in the per-request memory pool which is freed only at request termination. So when you need to read from an Nginx variable repeatedly in your Lua code, cache the Nginx variable value to your own Lua variable, for example,

We are currently repeatedly reading from the nginx variable repeatedly as shown in snippet 2 , want to start using snippet 1 , but my team mates think its more of a syntatic sugar than a performance gain , Can you please help me understanding which is better for performance

Thanks,
Ajay

.

ajaykemparaj

Hi ,

Thanks for your response ,

We were reevaluating the ngx.var.* again in couple of code blocks , This might not be the right example but something similar

local experssion = ngx.var._expression_ -- (allocate memory in this step)

for i = 1, n do

-- use experssion here -- (no more allocate)

-- Something like this

local expressionagain = ngx.var._expression_

end

On Mon, Aug 10, 2015 at 7:10 PM, DeJiang Zhu <douj...@gmail.com> wrote:

hi,

1. I do think snippet 1 will be a little bit better than snippet 2, but has nothing to do with the WARNING

2. `repeatedly` in the WARNING, means like this:

local experssion = ngx.var._expression_ -- (allocate memory in this step)
for i = 1, n do
-- use experssion here -- (no more allocate)
end

2015-08-11 5:20 GMT+08:00 Ajay k <ajayk...@gmail.com>:
Hi ,

I was going through the documentation of https://github.com/openresty/lua-nginx-module#ngxvarvariable

Here is the use case i just want some understanding on the memory usage and best practice

-- snippet 1 --

local ngx_var = ngx.var
local uri = ngx_var.uri
local args = ngx_var.args

local experssion1 = ngx_var.expression1
local experssion2 = ngx_var.expression2
local experssion3 = ngx_var.expression3

--snippet 2 --

local uri = ngx.var.uri
local args = ngx.var.args

local experssion1 = ngx.var.expression1
local experssion2 = ngx.var.expression2
local experssion3 = ngx.var.expression3
...

the Document Clearly states
WARNING When reading from an Nginx variable, Nginx will allocate memory in the per-request memory pool which is freed only at request termination. So when you need to read from an Nginx variable repeatedly in your Lua code, cache the Nginx variable value to your own Lua variable, for example,

We are currently repeatedly reading from the nginx variable repeatedly as shown in snippet 2 , want to start using snippet 1 , but my team mates think its more of a syntatic sugar than a performance gain , Can you please help me understanding which is better for performance

Thanks,
Ajay

.

Thanks,

Ajay K

lordnynex

Hello,

I hope someone corrects me if I'm wrong on this.

I believe Snippet1 is less efficient. AFAIK, internally, ngx.var is exposed as a Lua table that surfaces values through a metamethod. So in this case, you would not be avoiding the expensive meta method lookup. I may be thinking about ngx.ctx, or perhaps they're the same. Either way, snippet 2 is more direct and likely more efficient.

On Mon, Aug 10, 2015 at 2:20 PM, Ajay k <ajayk...@gmail.com> wrote:

Hi ,

I was going through the documentation of https://github.com/openresty/lua-nginx-module#ngxvarvariable

Here is the use case i just want some understanding on the memory usage and best practice

-- snippet 1 --

local ngx_var = ngx.var
local uri = ngx_var.uri
local args = ngx_var.args

local experssion1 = ngx_var.expression1
local experssion2 = ngx_var.expression2
local experssion3 = ngx_var.expression3

--snippet 2 --

local uri = ngx.var.uri
local args = ngx.var.args

local experssion1 = ngx.var.expression1
local experssion2 = ngx.var.expression2
local experssion3 = ngx.var.expression3
...

the Document Clearly states
WARNING When reading from an Nginx variable, Nginx will allocate memory in the per-request memory pool which is freed only at request termination. So when you need to read from an Nginx variable repeatedly in your Lua code, cache the Nginx variable value to your own Lua variable, for example,

We are currently repeatedly reading from the nginx variable repeatedly as shown in snippet 2 , want to start using snippet 1 , but my team mates think its more of a syntatic sugar than a performance gain , Can you please help me understanding which is better for performance

Thanks,
Ajay

.

ggroups

Guys, if you really want to know, what's better, benchmark your code!

    location = /bench {
        content_by_lua '
            local ngx_var = ngx.var
            local ngx_var_uri = ngx.var.uri
            local n = 10000000
            
            ngx.update_time()
            local start = ngx.now()
            for i=1,n do
                local tmp = ngx.var.uri
            end
            ngx.update_time()
            local t = ngx.now() - start
            
            ngx.say("ngx.var.uri: ",t)
            
            ngx.update_time()
            local start = ngx.now()
            for i=1,n do
                local tmp = ngx_var.uri
            end
            ngx.update_time()
            local t = ngx.now() - start
            
            ngx.say("ngx_var.uri: ",t)
            
            ngx.update_time()
            local start = ngx.now()
            for i=1,n do
                local tmp = ngx_var_uri
            end
            ngx.update_time()
            local t = ngx.now() - start
            
            ngx.say("ngx_var_uri: ",t)
        ';    

ngx.var.uri: 0.7279999256134
ngx_var.uri: 0.66199994087219
ngx_var_uri: 0.005000114440918

Therefore it doesn't really matter if you use ngx.var.xxx or ngx_var.xxx, it's comparably slow (on my systems the values fluctuate a bit).

Therefore: localize your ngx.var.xxx variables whenever possible at the outermost (out of loops) place!

On Wednesday, August 12, 2015 at 12:07:05 AM UTC+2, Lord Nynex wrote:

Hello,

I hope someone corrects me if I'm wrong on this.

I believe Snippet1 is less efficient. AFAIK, internally, ngx.var is exposed as a Lua table that surfaces values through a metamethod. So in this case, you would not be avoiding the expensive meta method lookup. I may be thinking about ngx.ctx, or perhaps they're the same. Either way, snippet 2 is more direct and likely more efficient.

On Mon, Aug 10, 2015 at 2:20 PM, Ajay k <ajayke...@gmail.com> wrote:
Hi ,

I was going through the documentation of https://github.com/openresty/lua-nginx-module#ngxvarvariable

Here is the use case i just want some understanding on the memory usage and best practice

-- snippet 1 --

local ngx_var = ngx.var
local uri = ngx_var.uri
local args = ngx_var.args

local experssion1 = ngx_var.expression1
local experssion2 = ngx_var.expression2
local experssion3 = ngx_var.expression3

--snippet 2 --

local uri = ngx.var.uri
local args = ngx.var.args

local experssion1 = ngx.var.expression1
local experssion2 = ngx.var.expression2
local experssion3 = ngx.var.expression3
...

the Document Clearly states
WARNING When reading from an Nginx variable, Nginx will allocate memory in the per-request memory pool which is freed only at request termination. So when you need to read from an Nginx variable repeatedly in your Lua code, cache the Nginx variable value to your own Lua variable, for example,

We are currently repeatedly reading from the nginx variable repeatedly as shown in snippet 2 , want to start using snippet 1 , but my team mates think its more of a syntatic sugar than a performance gain , Can you please help me understanding which is better for performance

Thanks,
Ajay

.

anorov.vorona

Shouldn't LuaJIT cache table lookups like those in many cases? I was under the impression that tight loops like those would cause it to typically hoist it out of the loop and cache the lookup automatically.

On Tuesday, August 11, 2015 at 6:50:25 PM UTC-4, bjoe2k4 wrote:

Guys, if you really want to know, what's better, benchmark your code!

location = /bench { content_by_lua ' local ngx_var = ngx.var local ngx_var_uri = ngx.var.uri local n = 10000000 ngx.update_time() local start = ngx.now() for i=1,n do local tmp = ngx.var.uri end ngx.update_time() local t = ngx.now() - start ngx.say("ngx.var.uri: ",t) ngx.update_time() local start = ngx.now() for i=1,n do local tmp = ngx_var.uri end ngx.update_time() local t = ngx.now() - start ngx.say("ngx_var.uri: ",t) ngx.update_time() local start = ngx.now() for i=1,n do local tmp = ngx_var_uri end ngx.update_time() local t = ngx.now() - start ngx.say("ngx_var_uri: ",t) ';
ngx.var.uri: 0.7279999256134 ngx_var.uri: 0.66199994087219 ngx_var_uri: 0.005000114440918
Therefore it doesn't really matter if you use ngx.var.xxx or ngx_var.xxx, it's comparably slow (on my systems the values fluctuate a bit).

Therefore: localize your ngx.var.xxx variables whenever possible at the outermost (out of loops) place!

On Wednesday, August 12, 2015 at 12:07:05 AM UTC+2, Lord Nynex wrote:
Hello,

I hope someone corrects me if I'm wrong on this.

I believe Snippet1 is less efficient. AFAIK, internally, ngx.var is exposed as a Lua table that surfaces values through a metamethod. So in this case, you would not be avoiding the expensive meta method lookup. I may be thinking about ngx.ctx, or perhaps they're the same. Either way, snippet 2 is more direct and likely more efficient.

On Mon, Aug 10, 2015 at 2:20 PM, Ajay k <ajayke...@gmail.com> wrote:
Hi ,

I was going through the documentation of https://github.com/openresty/lua-nginx-module#ngxvarvariable

Here is the use case i just want some understanding on the memory usage and best practice

-- snippet 1 --

local ngx_var = ngx.var
local uri = ngx_var.uri
local args = ngx_var.args

local experssion1 = ngx_var.expression1
local experssion2 = ngx_var.expression2
local experssion3 = ngx_var.expression3

--snippet 2 --

local uri = ngx.var.uri
local args = ngx.var.args

local experssion1 = ngx.var.expression1
local experssion2 = ngx.var.expression2
local experssion3 = ngx.var.expression3
...

the Document Clearly states
WARNING When reading from an Nginx variable, Nginx will allocate memory in the per-request memory pool which is freed only at request termination. So when you need to read from an Nginx variable repeatedly in your Lua code, cache the Nginx variable value to your own Lua variable, for example,

We are currently repeatedly reading from the nginx variable repeatedly as shown in snippet 2 , want to start using snippet 1 , but my team mates think its more of a syntatic sugar than a performance gain , Can you please help me understanding which is better for performance

Thanks,
Ajay

.

agentzh

Hello!

On Thu, Aug 13, 2015 at 9:27 AM, Anorov wrote:
> Shouldn't LuaJIT cache table lookups like those in many cases? I was under
> the impression that tight loops like those would cause it to typically hoist
> it out of the loop and cache the lookup automatically.
>

This metamethod for ngx.var.foo has side-effects like allocating
memory in nginx's request pool and it's never safe for the LuaJIT
compiler to assume that it is a constant and side-effect free.

Regards,
-agentzh