This adds two new concurrency limits in the throttle plugin:
1. Maximum number of open files per unique entity (which might be a unique identifier from a token).
2. Maximum number of "active connections" per entity.
The intent is to provide some per-"user" limits on resource consumption. (1) should be self-evident; open file handles are expensive for some filesystems and this prevents exhaustion by a single user.
(2) is less obvious. I define an "active connection" to be a thread with at least one open file handle. It's meant to approximate "open connections" as each connection currently costs an OS thread and these are definitely finite resources. I see the throttle plugin as a reasonable place to do this because (a) it's self-contained (no changes outside the plugin) and (b) for connections not associated with a particular entity (e.g., tokens), an open file might be the only reasonable place where the "owner" is defined. This will *not* be useful for limiting idle connections (but the reaper can help with these) or connections where files aren't being opened (e.g., a client only doing stat's). This *would* cover the instances that @juztas has described where a user with many concurrent jobs overloads his server; I just wanted to point out it's not a general-purpose solution (and, after a few tries, I've concluded that a solution without any tradeoffs is not obviously in reach...).
This introduces two new configuration parameters and two new debug levels. Here's what I was using in testing:
```
xrootd.fslib ++ throttle
throttle.max_open_files 3
throttle.max_active_connections 2
throttle.trace debug files connections
```
You can view, comment on, or merge this pull request online at:
https://github.com/xrootd/xrootd/pull/1582
-- Commit Summary --
* Add max file open tracking.
* Add support for connection tracking in the throttle plugin.
-- File Changes --
M src/XrdThrottle/XrdThrottle.hh (8)
M src/XrdThrottle/XrdThrottleFile.cc (35)
M src/XrdThrottle/XrdThrottleFileSystemConfig.cc (56)
M src/XrdThrottle/XrdThrottleManager.cc (188)
M src/XrdThrottle/XrdThrottleManager.hh (18)
M src/XrdThrottle/XrdThrottleTrace.hh (2)
-- Patch Links --
https://github.com/xrootd/xrootd/pull/1582.patch
https://github.com/xrootd/xrootd/pull/1582.diff
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