Re: Amazon EC2 CPU Utilization

[Mike Bresnahan <mike(dot)bresnahan(at)bestbuy(dot)com>]

In an attempt to determine whether top(1) is lying about the CPU utilization, I
did an experiment. I fired up a EC2 c1.xlarge instance and ran pgbench and a
tight loop in parallel.

-bash-4.0$ uname -a
Linux domu-12-31-39-00-8d-71.compute-1.internal 2.6.31-302-ec2 #7-Ubuntu SMP Tue
Oct 13 19:55:22 UTC 2009 x86_64 x86_64 x86_64 GNU/Linux

-bash-4.0$ pgbench -S -T 30 -c 16 -h localhost
Password: 
starting vacuum...end.
transaction type: SELECT only
scaling factor: 64
query mode: simple
number of clients: 16
duration: 30 s
number of transactions actually processed: 804719
tps = 26787.949376 (including connections establishing)
tps = 26842.193411 (excluding connections establishing)

While pgbench was running I ran a tight loop at the bash prompt.

-bash-4.0# time for i in {1..10000000}; do true; done

real	0m36.660s
user	0m33.100s
sys	0m2.040s

Then I ran each alone.

-bash-4.0$ pgbench -S -T 30 -c 16 -h localhost
Password: 
starting vacuum...end.
transaction type: SELECT only
scaling factor: 64
query mode: simple
number of clients: 16
duration: 30 s
number of transactions actually processed: 964639
tps = 32143.595223 (including connections establishing)
tps = 32208.347194 (excluding connections establishing)

-bash-4.0# time for i in {1..10000000}; do true; done

real	0m32.811s
user	0m31.330s
sys	0m1.470s

Running the loop caused pgbench to lose about 12.5% (1/8), which is exactly what
I would expect on a 8 core machine. So it seems that top(1) is lying.