RE: ~5x greater CPU load for a networked application when using 2.6.15-rt15-smpvs. 2.6.12-1.1390_FC4

[Jonathan Walsh]

As a follow up to previous emails (Gautam Thaker, Ingo Molnar, Ted Tso, et. al.) on the subject of large CPU overhead by the RT kernel when under heavy network load, I ran the following test in order to get more reasonable data.  I have 19 nodes with 20 "virtual" node processes sending UDP messages to a single host at a rate of 100Hz for 38,000 packets per second.  Using cyclesoak to determine cpu usage (over 240 samples, 1 sample per second), I found the following results:
 
RT kernel: linux-2.6.17-rt1-uni
Mean: 48.9%
Variance: 5.91
Standard Deviation: 2.43
 
Standard kernel: Standard Fedora Core 4
Mean: 23.2%
Variance: 0.237
Standard Deviation: 0.4867
 
Thus I found the average cpu load on the RT kernel to be 2.11 times that of the standard kernel.  Hopefully this information will be of some use.

-Jonathan Walsh
Distributed Processing Lab; Lockheed Martin Adv. Tech. Labs

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