What does open_file_cache_events do?

Server NGINX
#1

Hello,

I’m looking to document directives that are missing from the Wiki.
I found loads in the source code, especially thanks to Manlio P…

I’ve stumbled upon “open_file_cache_events” declared in
ngx_http_core_module.c.
I investigated what this does, here is what I found:

  • it’s a on/off directive
  • it’s off by default
  • its main purpose is from 1 file, ngx_open_file_cache.c

That’s where it’s mostly used:

/*
 * we ignore any possible event setting error and
 * fallback to usual periodic file retests
 */

static void
ngx_open_file_add_event(ngx_open_file_cache_t *cache,
    ngx_cached_open_file_t *file, ngx_open_file_info_t *of, ngx_log_t 
*log)
{
    ngx_open_file_cache_event_t  *fev;

    if (!(ngx_event_flags & NGX_USE_VNODE_EVENT)
        || !of->events
        || file->event
        || of->fd == NGX_INVALID_FILE
        || file->uses < of->min_uses)
    {
        return;
    }

basically of->events takes the value of open_file_cache_events before
this function is called.
When called, if of->events is false, the flow returns immediately and
the function is not processed.
So it prevents Nginx from “adding an event”, I presume for the specified
file.

Now I understand what the open file cache does, but I don’t understand
what exactly what this directive does.
When the directive is enabled, does Nginx process some kind of
event-based cache invalidation?
Eg. Nginx would subscribe to events for this file, when the file is
deleted, Nginx gets notified with an event, and then updates the cache
entry?

Thanks a lot in advance for helping me clear it out!

Best regards,
Costello

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,85785,85785#msg-85785

#2

Hello!

On Wed, May 12, 2010 at 06:57:24AM -0400, Costello wrote:

I’m looking to document directives that are missing from the Wiki.
I found loads in the source code, especially thanks to Manlio P…

I’ve stumbled upon “open_file_cache_events” declared in ngx_http_core_module.c.
I investigated what this does, here is what I found:

  • it’s a on/off directive
  • it’s off by default
  • its main purpose is from 1 file, ngx_open_file_cache.c

[…]

Now I understand what the open file cache does, but I don’t
understand what exactly what this directive does.
When the directive is enabled, does Nginx process some kind of
event-based cache invalidation?
Eg. Nginx would subscribe to events for this file, when the file
is deleted, Nginx gets notified with an event, and then updates
the cache entry?

Yes, it activates event based watching for file descriptor changes
instead of periodic checks; available with kqueue event method.

It’s intentionally left undocumented. Don’t use it, it’s
unfinished code (there are known race which leads to SIGSEGV).

Maxim D.

#3

Oh, thanks a lot for the update Maxim, so helpful as always!

Though I think it would be better to document the directive, making sure
to mention that it is not finished/not recommended to use!
With no documentation, people might try to use it and have problems with
it…

Again, thank you for the help.

Best regards,
Costello

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,85785,85821#msg-85821