We have several machines on Amazon (ec2) of the type c1.xlarge with 16 cpus, running the Amazon AMI.

Details on the machine:
7 GB of memory
20 EC2 Compute Units (8 virtual cores with 2.5 EC2 Compute Units each)
1690 GB of instance storage
64-bit platform
I/O Performance: High
API name: c1.xlarge

One out of the several machines is showing a high load average, since we have run the last yum upgrade a couple of weeks a go. We did not yet update the other machines, and everything looks normal on them.

The strange thing is that the top command not showing any hint for the cause of the load. CPUs are 4.8%us, 1.1%sy, 0.0%ni, 94.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st(see below). Mem is about 1.5GB free.

Any idea what could it be, or where else can we check? Many thanks for the help.

#
# top
#
top - 07:57:42 up  4:18,  1 user,  load average: 1.36, 1.45, 1.47
Tasks: 131 total,   1 running, 130 sleeping,   0 stopped,   0 zombie
Cpu(s):  4.8%us,  1.1%sy,  0.0%ni, 94.1%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   7120092k total,  5644920k used,  1475172k free,   532888k buffers
Swap:        0k total,        0k used,        0k free,  3463936k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 1557 mysql     20   0 1829m 374m 6448 S 14.3  5.4  11:15.09 mysqld
 6655 apache    20   0  416m  49m 3744 S  9.3  0.7   0:04.85 httpd
27683 apache    20   0  421m  54m 3708 S  9.0  0.8   0:00.99 httpd
 6682 apache    20   0  424m  57m 3788 S  8.3  0.8   0:03.81 httpd
16816 apache    20   0  419m  51m 3760 S  4.3  0.7   0:04.09 httpd
22182 apache    20   0  417m  50m 3756 S  1.7  0.7   0:06.34 httpd
  219 root      20   0     0    0    0 S  0.3  0.0   0:00.34 kworker/7:1
  699 root      20   0     0    0    0 S  0.3  0.0   0:00.40 kworker/3:1
    1 root      20   0 19376 1508 1212 S  0.0  0.0   0:00.29 init
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthreadd
    3 root      20   0     0    0    0 S  0.0  0.0   0:00.71 ksoftirqd/0