FAQ: Cannot Fork: No Swap Space

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There are times when you run into uncharted territory, when you least expect it. A host was failing to execute commands, giving a no swap space error even though swap space was only 9% used. Commands fails to execute but work intermitently.

This happened on an IBM server running AIX 5.3.

All of our efforts to resolve this issue failed, so we called the vendor -- IBM. The error manifests itself as:

user@host# uptime 
ksh: cannot fork: no swap space 
user@host# lsps -s 
Total Paging Space   Percent Used 
4096MB               9% 

Here is a solution to the problem as provided from IBM. It is a known issue from APAR IY89941 fixed at 5.3 TL5 (bos.mp 5.3.0.53) and above. The root cause is that you are running out of 64K pages in the VMM and there is code bug when this type of scenario occurs.

The work around is to disable 64K page size support (until upgrade to higher levels of AIX). To disable run command:

root@host# vmo -r -o vmm_mpsize_support=0

This change is not dynamic. A reboot is required after making this change.