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Hotfix PSE400R01W2K3027 For PS4.0 Fixes The "Black Hole Effect"

Monday, 06 March 2006 by Michel Roth

Hotfix PSE400R01W2K3027 For PS4.0 for Windows Server 2003 fixes a phenomenon in Citrix Environments known as the "Black Hole Effect". In a nutshell, the "Black Hole Effect" is when a Citrix server comes online in the middle of busy login times. This means that all load balancing calculations will point to that server for new logins. If this happens at a busy time, this could result in DoS scenarios.

Thes problem has been around for just as long as Citrix Load Balancing has. Apparantly Citrix has decided to fix this for Presentation Server 4.0 by "Implementing Slow-Start Load Balancing":

"With this Slow-Start Load Balancing fix, logons are given a logarithmic load bias during connection time to limit the number of simultaneous logon requests. This biasing level is used in conjunction with the servers real load to route connections to the least-loaded server. This allows time for servers to gradually increment the number of connections in environments where the server load is well below the farm average load, which is often the case when you restart a server."

"Slow-Start Load Balancing uses Intelligent Load Biasing (ILB). ILB works by giving logons a higher load bias. The default ILB algorithm assigns a bias of ˝ the remaining load capacity. Essentially, the default algorithm is Current Resolution Load += [(Max Load Current Resolution Load) / 2]. The ILB adjusts itself back down after pending logons are complete."

Go to the Hotfix PSE400R01W2K3027 download page here.

For those or you not running PS4 or looking for another approach to dealing with the "Black Hole Effect", take a look at a tool called: DADEPowertools.