2.6: Load average calculation?

[Russell King <rmk+lkml@arm.linux.org.uk>]

Hi,

2.6.11 based FC3 kernel.

One of the servers being used to download FC5 has a rather high load
average, which is hardly surprising.  What is, is the results from top
and the fact that the machine is still _very_ responsive via ssh.

top - 11:05:22 up 206 days, 22:31, 49 users,  load average: 145.78, 140.34, 133
Tasks: 1221 total,  10 running, 1209 sleeping,   2 stopped,   0 zombie
Cpu(s):  1.4% us,  0.6% sy,  0.0% ni, 93.0% id,  4.4% wa,  0.5% hi,  0.0% si
Mem:   2075852k total,  2025220k used,    50632k free,    10152k buffers
Swap:  2249060k total,      576k used,  2248484k free,  1032408k cached

Note the high load average and the mostly idle (not io wait) CPU %age.

The load average seems to be coming from the apache / vsftpd:

  PID USER     STAT COMMAND          WCHAN
  818 apache   D    httpd            sync_buffer
 6517 apache   D    httpd            sync_buffer
 6527 apache   D    httpd            sync_page
 6575 apache   D    httpd            sync_page
 1774 ftp      D    vsftpd           sync_page
... about 128 more vsftpds in the same state ...
 9335 ftp      D    vsftpd           sync_page

and if we look closer, from sysrq-t (note that the dump is far larger
than the system log buffer, so I will only give a couple of examples):

 vsftpd        D D72C6DF4  2452 12309  12306              (NOTLB)
 d72c6e20 00000082 c013d4c9 d72c6df4 d72c6df4 f7c01d48 c028a0cc f7c01d48
        c028a13a e5bd0db0 c013d4c9 d72c6df4 d72c6df4 00000202 00000246 00000000
        7adc5940 004ecb04 e5bd0f18 d72c6e70 d72c6e78 c200da20 d72c6e28 c0364c53
 Call Trace:
  [<c013d4c9>] autoremove_wake_function+0x0/0x37
  [<c028a0cc>] __generic_unplug_device+0x16/0x31
  [<c028a13a>] generic_unplug_device+0x53/0x158
  [<c013d4c9>] autoremove_wake_function+0x0/0x37
  [<c0364c53>] io_schedule+0xe/0x16
  [<c014afec>] sync_page+0x36/0x42
  [<c0364f57>] __wait_on_bit_lock+0x3e/0x5e
  [<c014afb6>] sync_page+0x0/0x42
  [<c014b7b9>] __lock_page+0x90/0x98
  [<c013d500>] wake_bit_function+0x0/0x3c
  [<c013d500>] wake_bit_function+0x0/0x3c
  [<c01a5a55>] mpage_readpage+0x39/0x3f
  [<c014c4c0>] do_generic_mapping_read+0x3ae/0x63d
  [<c014cbed>] generic_file_sendfile+0x5e/0x70
  [<c014cb2d>] file_send_actor+0x0/0x62
  [<c01759fd>] do_sendfile+0x1d3/0x28e
  [<c014cb2d>] file_send_actor+0x0/0x62
  [<c0175b6f>] sys_sendfile+0xb7/0xc2
  [<c0103903>] syscall_call+0x7/0xb

 vsftpd        D 00000100  2452 12353  12351              (NOTLB)
 c98d9e20 00000082 cdfe7780 00000100 ea4bce80 f7c01d48 c028a0cc f7c01d48
        c028a13a ea4bce80 00000000 c032368a c98d9df4 00000202 00000246 00000000
        2b782400 004ecb05 f4d7cc98 c98d9e70 c98d9e78 c2016240 c98d9e28 c0364c53
 Call Trace:
  [<c028a0cc>] __generic_unplug_device+0x16/0x31
  [<c028a13a>] generic_unplug_device+0x53/0x158
  [<c032368a>] do_tcp_sendpages+0x3ce/0xa25
  [<c0364c53>] io_schedule+0xe/0x16
  [<c014afec>] sync_page+0x36/0x42
  [<c0364f57>] __wait_on_bit_lock+0x3e/0x5e
  [<c014afb6>] sync_page+0x0/0x42
  [<c014b7b9>] __lock_page+0x90/0x98
  [<c013d500>] wake_bit_function+0x0/0x3c
  [<c013d500>] wake_bit_function+0x0/0x3c
  [<c014c4b4>] do_generic_mapping_read+0x3a2/0x63d
  [<c014cbed>] generic_file_sendfile+0x5e/0x70
  [<c014cb2d>] file_send_actor+0x0/0x62
  [<c01759fd>] do_sendfile+0x1d3/0x28e
  [<c014cb2d>] file_send_actor+0x0/0x62
  [<c0175b6f>] sys_sendfile+0xb7/0xc2
  [<c0103903>] syscall_call+0x7/0xb

The disk subsystem is coping very well with this load.  However, the
network interface through which all the ftp and http traffic is flowing
is running at around 92mbps (timed over 10 seconds), and is therefore
probably close to saturation.  (Note that this is the same network
interface through which ssh is connected, which remains responsive.)

So far so good.

However, programs such as MTAs make decisions about delivery based on
the load average, so a high induced (but apparantly ficticious) load
average denies service to other parts of the system.

So, the question becomes - should a lot of network activity contribute
to the system load average, thereby denying other services from
performing their usual business.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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