I noticed the same when I was viewing db2top output and trying to compare it with what db2pd captured.
I just tried again and got:
*** End of input stream reached, size was 896.4K...
Exiting...
The size matches the output file:
-rw------- 1 db2inst1 db2grp1 917932 Feb 02 15:02 db2top.out.20120202.150220
So, it gives this message when it reaches the end of file.
But the time it stays up is not consistent. When viewing the same file, 1st time it stayed up for ~1 min and then existed, other times it stayed up longer. Not sure why that is.
I use the following command to collect db2top output:
db2top -d xxxxx -C -i 60 -m 1 -f $SNAP/db2top.out
And use "db2top -f <file name>" to view it.
Check this article, you might find some helpful info:
DB2 problem determination using db2top utility