[nginx logging module]$Request_time almost show 0.000 with proxy cache configuration

[Lucas Rolff]

 > - a cache hit means that the resource should also be in the linux 
page cache - so no physical disk read needed.

That's a very wrong assumption to make, and only makes sense in very 
small scale setups - and multiple terabytes of memory isn't exactly 
cheap, that's why we have SSD storage to handle such things, and it 
would still be a "HIT" even if it's not within the memory.



Peter Booth wrote:
> This might not be a bug at all. Remember that when nginx logs request
> time it's doing so with millisecond precision. This is very, very 
> coarse-grained when you consider what
> modern hardware is capable of. The Tech Empower benchmarks shwo that 
> an (openresty) nginx on
> a quad-socket host can server more than 800,000 dynamic lua requests 
> per second. We should expect
> that static resources served from ngixn cache to be faster than this.
>
> Remember:
>  - a cache hit means that the resource should also be in the linux 
> page cache - so no physical disk read needed.
> - writing a small png file from memory to the network (on a 10G 
> ethernet ) could take a few microsec. Depending on NIC IRQ 
> consolidation settings this might be as much as 60/70micros.
> - reading the time (gettimeofday()) will itself take about 30 nanoseconds.
>
> These are al intervals that are too small to be visible to the 1ms 
> granularity of the request_time logging.
>
> My experience has been that very busy webservers running on even five 
> year old hardware
> will consistently log 0ms  request time for cache hits. If I saw 
> anything different I'd be wondering
> what was wrong with the environment.
>
> Peter
>
> On Jun 22, 2017, at 05:53 AM, jindov <nginx-forum at forum.nginx.org> wrote:
>
>> Hi guys,
>>
>> I've configured for nginx to cache static like jpeg|png. The problem 
>> is if
>> request with MISS status, it will show a non-zero value request_time, 
>> but if
>> a HIT request, the request_time value is 0.000.
>> This is an nginx bug and is there anyway to resolve it.
>>
>> My log format
>>
>> ```
>> log_format cache '$remote_addr - [$time_local] $upstream_cache_status
>> $upstream_addr '
>> '"$request" $status $body_bytes_sent $request_time
>> ["$upstream_response_time"] "$http_referer" '
>> '"$http_user_agent" "$host" "$server_port"
>> "$connection"';
>> ```
>>
>> I read a topic about this but this is not informational. I've try to set
>> timer_resolution to 0ms but nothing was changed
>>
>> Thanks
>>
>> Posted at Nginx Forum: 
>> https://forum.nginx.org/read.php?2,275053,275053#msg-275053
>>
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