Add support for buffering is scripted logs

[Eran Kornblau]

> -----Original Message-----
> From: nginx-devel [mailto:nginx-devel-bounces at nginx.org] On Behalf Of Alexey Ivanov
> Sent: Monday, August 14, 2017 9:25 PM
> To: nginx-devel at nginx.org
> Subject: Re: Add support for buffering is scripted logs
> 
> using syslog for that particular usecase seem way more elegant, customizable, and simple. As a side bonus you won't block event loop on vfs operations (open/write/close).
>
Thanks Alexey. 

Regarding the last point about performance, I ran a quick test, compared -
1. Nginx writing to gzip access log -
access_log /var/log/nginx/access_log.gz main gzip flush=5m;
2. Nginx writing to local rsyslog over UDP, which in turn writes to local file -
access_log syslog:server=127.0.0.1 main;
3. Nginx writing to remote rsyslog on the same LAN over UDP -
access_log syslog:server=192.168.11.94 main;

Ran this command (execute 10 apache bench tests, take the median value):
(for i in `seq 1 10`; do ab -c 1000 -n 100000 127.0.0.1/alive.html 2>/dev/null | grep '^  50%' ; done) | awk '{print $2}' | xargs

The nginx location being tested simply does 'return 200 "hello"'.

Results are:
1. 4 6 5 6 3 5 3 5 7 5
2. 6 7 5 7 6 7 7 7 7 7
3. 5 6 6 6 6 6 6 6 4 6

The numbers fluctuated a bit from one run to another (did it a few times), but the overall trend was the same - 
syslog is slower than having nginx write with gzip. The difference is not dramatic, but it's visible.
This makes sense since with buffered write, nginx only has to perform a syscall once every X messages, 
most messages are written without any syscalls.
Regarding what you wrote about open/write/close, when using variables (even before this patch), 
it is possible to enable open file cache on log files, so you only have 'write'.

Eran