I reproduced this as well. Checking the implementation, we see that "getsockopt" is used for getting the queue length. We can use the "SO_LISTENINCQLEN" option in FreeBSD to get the status of the UNIX domain socket, but I think there is no option in Linux to get it.
https://github.com/php/php-src/blob/php-8.0.6/sapi/fpm/fpm/fpm_sockets.c#L487-L563


The original implementation did not support UNIX domain sockets, so I suppose this is more of an undocumented specification than a but.
http://svn.php.net/viewvc/php/php-src/branches/PHP_5_4/sapi/fpm/fpm/fpm_sockets.c?sortby=date&r1=305266&r2=305267&pathrev=312922&

We can use "netlink" to get the status of all sockets, but I think there are some privilege and performance concerns. Any ideas?

Indeed SO_LISTENINCQLEN seems to not exist on Linux: https://manpages.debian.org/bullseye/manpages/socket.7.en.html#Socket_options

I tried to find out other ways to get current and max queue length on Linux: https://serverfault.com/a/930677/577419

```
# ss -lx | sed -n '1p;/php/p'
Netid State  Recv-Q Send-Q                              Local Address:Port      Peer Address:Port     Process
u_str LISTEN 0      1024                     /run/php/php8.0-fpm.sock 157408202            * 0
```

If I understood it right, this means current queue length is zero and max is 1024? The latter is true, changing "listen.backlog" to 2048 makes "Send-Q" change accordingly. I'll now monitor whether "Recv-Q" is ever different from zero. Checking the internals of "ss" may then provide the options for the FPM as well, I hope.

Best regards,

Micha

Further to Micha's comment:

>> I'll now monitor whether "Recv-Q" is ever different from zero. 

This weekend I wrote a quick-and-dirty set of scripts to see if Micha's plan would show positive results (details below). 

RESULTS:
Day: https://icounsellor.co.uk/phpbugs/phpfpm_queue-day.png
Week: https://icounsellor.co.uk/phpbugs/phpfpm_queue-week.png
Month: https://icounsellor.co.uk/phpbugs/phpfpm_queue-month.png
(These URLs are soft-linked to my live Munin data)

CONCLUSION: Although my PHP-FPM site is of low activity, we do have positive results here showing we can get the value of Receive-Q on Linux

RECOMMENDATION: As Micha suggests earlier, the PHP Development Team will be able to gain knowledge from Linux source code (reference: https://serverfault.com/a/930677/577419 - internals of "ss") to attain the "how to" with respect to gaining Receive-Q data for PHP-FPM.


--------------------------------
FYI - SCRIPTS.

SCRIPT #1: Quick & Dirty Bash Script to collate maximum Receive-Q value for php-fpm7.4.

Purpose: Munin runs only once every 5 minutes so asking it to call ss to show receive-q values for php-fpm once every 5 minutes isn't very detailed. Hence, this script runs (... repeatedly via cron) collating, once a second, the maximum value of phpfpm-7.4 Receive-Q via ss (in periods of 5 minutes).

Source:

#!/bin/bash

FileRoot="php7.4-fpm";
Socket=$FileRoot".sock";
LoopDelay=1;

SharedFile="/dev/shm/"$FileRoot".count";

# Run for 5 minutes... then quit
let RunLoop=300/$LoopDelay;

COUNTER=0
Max=0;

if [ ! -f $SharedFile ]; then
        # https://unix.stackexchange.com/questions/107038/obtain-exclusive-read-write-lock-on-a-file-for-atomic-updates
        flock -x -w 5 $SharedFile echo 0 > $SharedFile;
fi

while [  $COUNTER -lt $RunLoop ]; do

        SendQCount=$(ss -ln | grep $Socket | awk '{print $3}');

        if (( SendQCount > Max )); then
                Max=$SendQCount;
        fi

        sleep $LoopDelay;
        let COUNTER=COUNTER+1;
done
# Write the latest results...
flock -x -w 5 $SharedFile echo $Max > $SharedFile;

-----
SCRIPT #2: Quick and Dirty Munin plugin

Purpose: Munin-node calls this script, reads the data left by the above script (once every 5 minutes), and graphs the information

Source:

#!/bin/bash 
if [ "$1" = "config" ]; then
        MaxQueue=$(ss -ln | grep php7.4-fpm.sock | awk '{print $4}');
        echo 'graph_title PHP-FPM Socket Queue Statistics'
        echo 'graph_vlabel Count'
        echo 'graph_category php'
        echo "graph_info This graph counts the number connections queued. Max queue configured to be: "$MaxQueue
        echo "Current.label Currently Queued Requests";

        exit 0
fi

        CurrentQueue=$(cat /dev/shm/php7.4-fpm.count);
        echo Current.value $CurrentQueue;

exit 0;

Many thanks for providing the scripts. Indeed I can also verify with PHP8.0 that this method works fine and "Recv-Q" indeed shows the current query length.

I did monitoring with this one-liner from shell:
```
while sleep 1; do value=$(ss -lxn | mawk '/php/{print $3}'); (( $value )) && echo "$value"; done
```

However, this is of course everything else than precise as it only takes a snapshot of the current query length at defined interval, which, considering the speed in which PHP requests are handled, will unlikely ever show the real maximum reached.

For the current query length ("listen queue:" on status page) this however works fine. Of course PHP-FPM shouldn't execute any external command but try to obtain the information with the methods "ss" uses ;).

I wonder whether /proc/<PID>/net/netstat contains what we are looking for. But so far I couldn't identify any of the long value list a maximum reached query length. Probably there are other stats provided by the kernel in /sys or /proc somewhere?

Best regards,

Micha

I had a closer look on this. The ss (iproute2) uses netlink to get this info so we could use the same. I created an issue with more info: https://github.com/php/php-src/issues/9943