Why are uploads maxing out CPU?

Question

Some odd behaviour ... when sending / uploading files, my CPU usage goes to about 100%; Activity Monitor reports 'Kernel Task' as the culprit. I first noticed this when sending emails in Mail, but it is also happening whenever I use any upload protocol:

• Email attachment;
• FTP using Cyberduck or FireFTP (within Firefox);
• Syncing Evernote (suspect this is also FTP?);
• FTP via command line (ftp -u)

Any idea why this happens, or how to stop it?

(OSX Lion 10.7.2, Macbook Pro 2.4 Core Duo; customisations include Growl and Quicksilver)

Update: first line of output from top -o cpu:

PID    COMMAND      %CPU  TIME     #TH  #WQ  #PORT #MREG
0      kernel_task  122.3 20:13.81 73/4 0    2     1907

Update 2: Just decided to try this out using wifi instead of my USB 3G modem ... and the problem disappears! Five samples during ftp upload give:

$ top -l 5 -stats pid,command,cpu,th | grep kernel_task
0      kernel_task      0.0  71/2
0      kernel_task      2.2  71/3
0      kernel_task      2.1  71/2
0      kernel_task      2.1  71/2
0      kernel_task      2.2  71/2

So the issue seems to be any upload with the USB modem ... but would still be good to find a solution!

Answer 1

Ok, let's continue here...

It might related to I/O because I have hard time believing the ftp command would consume such amount of kernel time.
A big network transfer rate could be responsible for that though but you should be sending few dozens Mo/s before you hit 100% of CPU on a CPU like this.

Have a look here for a list of things causing kernel_task to go mad and report if you have any luck with it :)