Status of shared memory zones

[Eirik Øverby]

On 8/20/25 18:08, Maxim Dounin wrote:
> On Wed, Aug 20, 2025 at 01:06:33PM +0200, Eirik Øverby via nginx wrote:
>> In the commercial F5 nginx, there has for some time been ways to
>> monitor the status/usage of shared memory zones and (presumably)
>> some other run-time stats that remain opaque to users of the
>> community version. I'm wondering if there are any such
>> facilities in freenginx - implemented or planned? It would be
>> nice if the VTS plugin could emit some statistics, for example.
>>
>> We would be quite interested in such a feature, but realise it
>> will be a fair amount of work for a relatively narrow audience.
>> Is there any way we can "encourage" such functionality to be
>> added, e.g. through financial support or some other means? We
>> might even be able to produce code to do it, but would prefer if
>> it was done by someone who is already familiar with (free)nginx
>> and its architecture - we have no such people on-hand.
> 
> Providing better status information might indeed be an interesting
> area for improvements.
> 
> Right now, the only status information provided by freenginx is
> that shown by stub_status, which is quite limited and basically
> only shows connections in various states and total
> accepts/requests counters.  Everything else needs to be monitored
> either via logs or with 3rd party modules, like VTS you've
> mentioned.
> 
> It would be great if you could share which stats you use/monitor,
> and how.  In particular, it would be interesting to know preferred
> data formats.

We use the VTS extension to monitor a number of things, including connection counts, latencies, etc. We use Prometheus for this, so the data is collected by the extension and then scraped by Prometheus at regular intervals. The VTS extensions emits a lot of information, we are not even close to making use of all of it, but collecting it allows us to drill into the data e.g. after an incident or when trying to understand application behaviour.

Simplified example config we use:

--
vhost_traffic_status_zone;
vhost_traffic_status_histogram_buckets 0.005 0.01 0.05 0.1 0.2 0.5 1 5 10;

...

location ~ ^/foo/(.*)/bar {
     set $loc $1;
     # Get stats for individual locations and server ports
     vhost_traffic_status_filter_by_set_key $server_port $loc:$request_method;
     ....
}

location /status {
     vhost_traffic_status_display;
}
--

We are currently not using the nginx-internal status endpoint, as I believe VTS gives us everything it provides - but if the shared memory zone info would be available there, we'd of course start using it. :)


> It would also be interesting to know how you expect to use
> information about shared memory zones usage.

I'm mostly concerned about the (apparent?) total lack of information available to guide sizing of shared memory zones. For rate limiting based purely on source IP it's "easy" to calculate a worst-case scenario, but in our case we do rate limiting on a number of different keys - like an md5 of the request string (hard lesson learned there..), combinations of IP and other request properties, etc.

As for what might be interesting to know, off the top of my head:
  - information about utilization (%/bytes) vs max size
  - information about number of entries
  - if possible the age distribution of entries (to know if entries likely to be evicted next could be too young, therefore suggesting a larger zone is needed)
  - something to see/predict possible fragmentation problems (not sure exactly what that would be, but knowing what the max size of a new element is at any given time might be an indicator. If it's low, it might be a problem; if it fluctuates a lot, that might be a different problem..)


> As for improving the VTS module, I don't think it's specific to
> freenginx.  Information about shared memory zone usage is in the
> ngx_slab_pool_t structure since nginx 1.11.7, and it's more or
> less a question of showing this information properly.  Just in
> case, relevant commits are:
> 
> changeset:   6828:99770a5ea14f
> user:        Ruslan Ermilov <ru at nginx.com>
> date:        Wed Dec 07 22:25:37 2016 +0300
> summary:     Slab: slots statistics.
> 
> changeset:   6829:6e757036e588
> user:        Ruslan Ermilov <ru at nginx.com>
> date:        Wed Dec 07 22:25:37 2016 +0300
> summary:     Slab: free pages statistics.
> 
> https://freenginx.org/hg/nginx/rev/99770a5ea14f
> https://freenginx.org/hg/nginx/rev/6e757036e588

Are you saying I should speak to the VTS author/project? I can try that of course, but it would be good to know what data is available first, so I don't send anyone (else) on a wild goose chase :)

Thanks,
/Eirik