Date: Thu, 21 Oct 2004 14:23:01 +1000 (EST) From: lukem.freebsd@cse.unsw.edu.au To: freebsd-performance@freebsd.org Subject: CPU utilisation cap? Message-ID: <Pine.LNX.4.61.0410211419480.8238@wagner.orchestra.cse.unsw.EDU.AU>
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I have been doing some benchmarking as a part of some driver development work, and have encountered a phenomenon I can't explain. I am using FreeBSD 5.2.1-RELEASE with SMP and IO-APIC disabled. I am using a dual 2.8GHz xeon box, but only one CPU without hyperthreading. The box in question has three em interfaces, and one fxp. Two of the em's are 133Mhz/64bit, and one is 33MHz/32bit. I have verified these values by modifying the em driver to print out what it detects. em0: MAC type:82546 r3 Bus speed:133MHz Bus width:64bit Bus type:PCI-X em1: MAC type:82546 r3 Bus speed:133MHz Bus width:64bit Bus type:PCI-X em2: MAC type:82540 Bus speed:33MHz Bus width:32bit Bus type:PCI The particular benchmark I have been using is a UDP echo test, where I have a number of linux boxes sending UDP packets to the freebsd box, which the freebsd box echoes at user-level (think inetd udp echo, though in fact I have also used an optimised server which gets higher throughput). Throughput is measured on the boxes which generate the UDP packets. I am measuring idle time using a CPU soaker process which runs at a very low priority. Top seems to confirm the output it gives. What I see is strange. CPU utilisation always peaks (and stays) at between 80 & 85%. If I increase the amount of work done by the UDP echo program (by inserting additional packet copies), CPU utilisation does not rise, but rather, throughput declines. The 80% figure is common to both the slow and fast PCI cards as well. This is rather confusing, as I cannot tell if the system is IO bound or CPU bound. Certainly I would not have expected the 133/64 PCI bus to be saturated given that peak throughput is around 550Mbit/s with 1024-byte packets. (Such a low figure is not unexpected given there are 2 syscalls per packet). -- Luke
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