Update information about Linux's overcommit memory behavior.

Andrew Dunstan

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index d74cf993d983d324bcf7b5e99910bddf8a34ec24..eabc67c646f85414aa0090f9272f7af319efbe2e 100644 (file)
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1,5 +1,5 @@
 
 
 
@@ -3081,14 +3081,61 @@ kernel.shmmax = 134217728
        
 
        
-        Linux has poor default memory overcommit behavior.  Rather than
-        failing if it can not reserve enough memory, it returns success, 
-        but later fails when the memory can't be mapped and terminates 
-        the application with kill -9.  To prevent unpredictable 
-        process termination, use:
+   Linux kernel version 2.4.* has poor default memory overcommit 
+   behavior, which can result in the postmaster being killed by the 
+   kernel due to memory demands by another process if the system 
+   runs out of memory.
+   
+
+       
+   The symptom of this occuring is a kernel message looking like 
+   this (consult your system documentation and configuration on 
+   where to look for such a message):
+
+ Out of Memory: Killed process 12345 (postmaster). 
+
+   
+
+       
+   To avoid this situation, run PostgreSQL 
+   on a machine where you 
+   can be sure that other processes will not run the machine out 
+   of memory. If your kernel supports strict and/or paranoid modes 
+   of overcommit handling, you can also relieve this problem by 
+   altering the system's default behaviour. This can be determined 
+   by examining the function vm_enough_memory
+   in the file mm/mmap.cin the kernel source. 
+   If this file reveals that strict and/or paranoid modes are 
+   supported by your kernel, turn one of these modes on by using
+
+sysctl -w vm.overcommit_memory=2
+
+    for strict mode or
 
 sysctl -w vm.overcommit_memory=3
 
+   for paranoid mode, or placing an equivalent entry in 
+   /etc/sysctl.conf.
+   
+
+       
+   
+    Warning: using these settings in a kernel which does not support 
+    these modes will almost certainly increase the danger of the 
+    kernel killing the postmaster, rather than reducing it. 
+    If in any doubt, consult a kernel expert or your kernel vendor.
+   
+       
+
+       
+   These modes are expected to be supported in all 2.6 and later 
+   kernels. Some vendor 2.4 kernels may also support these modes. 
+   However, it is known that some vendor documents suggest that 
+   they support them while examination of the kernel source reveals 
+   that they do not.
+   
+
+       
         Note, you will need enough swap space to cover all your memory needs.