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EX SSL-VPN: Error Message: Threads blocked and pool could not be grown
03/26/2020 
8 People found this article helpful
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Description
EX SSL-VPN: Error Message: Threads blocked and pool could not be grown
Resolution
Overview
In the system message log, an administrator sees an error message very similar to the following:
All 256 threads blocked and pool could not be grown, stack traces will be dumped every 60 seconds.
Users may be unable to log in to the appliance, or logins could take a long time.
More Information
The policy service (also referred to as policyserver) is a process which runs on the appliance and is responsible for user authentication and authorization. It is at the heart of the policy model and to remain efficient, it needs to keep certain system resources such as memory or CPU in reserve. If memory usage climbs too much or CPU usage and load are too high, policyserver will not create new threads to handle tasks because of the burden that would put on the system. In the section below, a few scenarios will be outlined to give you an idea of why policyserver may get into this condition.
Causes and Resolutions
Possible causes for this condition, and resolutions will be given here.
Nested Group Lookup
If you are using nested group lookup on your LDAP or AD authentication server, make sure that you are also caching the lookup result; searching the entire directory tree takes time and increases CPU usage. Here is how to enable caching:
- In AMC, go to Authentication Servers and edit the appropriate Active Directory or LDAP server.
- Under the Group lookup section, click the check box next to Cache group checking and then select an appropriate lifetime. 1800 seconds (30 minutes) is the default.
- Save and apply your changes.
In addition, setting your appliance's nested group lookup level to 2 could also help. Here is now to make that change:
- In AMC, go to Authentication Servers and edit the appropriate Active Directory or LDAP server.
- Under the Group lookup section, set the Nested group lookup value to 2.
- Save and apply your changes.
Slow or unavailable backend authentication or DNS
When an appliance is heavily loaded with incoming user requests or authentication requests and the backend authentication server(s) or DNS server(s) are slow to respond or completely unresponsive, the appliance may need to grow its pool of threads to service these requests. Ensure that these backend servers are responding normally to minimize this condition for the appliance. If policyserver reaches a CPU or memory threshold and decides it cannot grow its thread pool, the message above will be logged, and users may notice slower access through the VPN.
Policyserver Bug
As documented in KB item #5329, there is a policyserver bug which has been resolved in a hotfix where tunnel users cannot log into an appliance because policyserver has gotten into a thread blocking condition. There is a link to a hotfix for appliance versions 9.0.0, 9.0.1, and 9.0.2 in that KB article.
Tunnel IP Pool Resource Referenced In Rule
In appliance release 10.0.1 and earlier, if an IP address pool resource is used in an access control rule, policyserver can get itself into a threadblocking condition when that rule is checked by policyserver. This problem is exacerbated when a client system makes a large number of requests to policyserver in a very short period of time (on the order of about 60 per second, for example).
To workaround this issue, a customer can do the following:
- Instead of using an IP address pool in an access control rule, create an IP range resource that references the same IP addresses as the IP address pool resource and then use that IP range resource in the rule. Or,
- Isolate the client system that is making large number of requests and ensure that its behavior is not due to some sort of misconfiguration or software (such as malware).
This problem is fully resolved in appliance release 10.0.2. Customers running that release should not see threadblocking conditions due to the problem described in this section.
Tracking IDs
80725
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