ATOP shows red line vmcom and vmlim. What does it mean?
Answer to main query is further below - but first a warning regarding Mirra's suggestion:
Be careful with this:
In : /proc/sys/vm/overcommit_memory I try put there 2.
In that case, even when physical memory is available, all processes requesting memory from OS when vmcom greater than vmlim will receive errors (I`ve got a lot of errors and fails with basic system applications like compiz). And because of:
vmlim = SWAP_size + 0.5 * RAM_size.
where 0.5 (50%) is the default value for /proc/sys/vm/overcommit_ratio parameter you can easily get a lot of errors like me.
Answer to main question: it is better to revert changes in overcommit_memory parameter back to the default value (0 for me (Ubuntu 12.04 LTS), but can be 1 for other OS`es). According to the great article we can calculate memory actually used by processes:
MemoryUsed ~ tot - (cache + buff + free) ~ 23.6G - ( 6.7G + 0.067G + 0.136G) ~ 16.7G
So only 16.7G is actually used by processes (from 23.6G installed RAM) and red line in atop output may be ignored.