Why named shared memory zones

[Marcus Clyne]

Igor Sysoev wrote:
> On Mon, Sep 28, 2009 at 06:25:37PM +0300, Marcus Clyne wrote:
>
>   
>> Hi,
>>
>> Igor Sysoev wrote:
>>     
>>> On Mon, Sep 28, 2009 at 03:28:44PM +0300, Marcus Clyne wrote:
>>>
>>>   
>>>       
>>>> Hi,
>>>>
>>>> What's the purpose/benefit of naming shared memory zones in config files?
>>>>     
>>>>         
>>> Its names are used in other directives. For example, you may have
>>> several proxy_cache's.
>>>
>>>   
>>>       
>> I understand their use in other directives, but I was just wondering why 
>> you actually need them in the directives.
>>
>> For example, if you define several proxy caches, then each one would 
>> automatically use a different shared memory section.  It seems 
>> unnecessary to me to use names in the config file, since they'd always 
>> be different, and from what I gather, if you use the same name (and tag) 
>> for two shared memory sections, then you'll get a conf error (correct me 
>> if I'm wrong, though). 
>>
>> I feel that having the config like
>>
>> proxy_cache_path   /data/nginx/cache   levels=1:2  10m;
>>
>> or even
>>
>> proxy_cache_path   /data/nginx/cache   1:2  10m;
>>
>> would be much neater than
>>
>> proxy_cache_path   /data/nginx/cache   levels=1:2  key_zone=one:10m;
>>
>> And overall wouldn't lose any information that couldn't be generated in 
>> the background.
>>     
>
> No, suppose the following:
>
>  proxy_cache_path   /data/nginx/cache1   levels=1:2  keys_zone=ONE:10m;
>  proxy_cache_path   /data/nginx/cache2   levels=1:2  keys_zone=TWO:10m;
>
>    location / {
>       proxy_cache  ONE;
>    }
>
>    location /one/ {
>       proxy_cache  ONE;
>    }
>
>    location /two/ {
>       proxy_cache  TWO;
>    }
>
>   
I see.

What about offering an alternative, clearer syntax?  e.g.:

proxy_cache_path   /data/nginx/cache1  1:2  ONE  10m;

>>> The second reason is Win32 uses named shared memory mapping.
>>> However, it's lamost impossible to use shared memory in Win32 due to
>>> Vista ASLR.
>>>   
>>>       
>> If you need to have named sections, it would be easy enough to generate 
>> them sequentially (e.g. ngx_shms1, ngx_shms2...) whilst reading the 
>> config file.
>>     
>
> The zone name is also logged when zone is out of space.
>   
Ok, that's useful.

Thanks,

Marcus.