Date: Wed, 12 Nov 2003 16:17:35 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: Daniel Ellard <ellard@eecs.harvard.edu> Cc: freebsd-hackers@freebsd.org Subject: Re: Confused about HyperThreading and Performance Message-ID: <20031112221735.GE37293@dan.emsphone.com> In-Reply-To: <20031112103358.S11644@bowser.eecs.harvard.edu> References: <20031112103358.S11644@bowser.eecs.harvard.edu>
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In the last episode (Nov 12), Daniel Ellard said: > When I run a kernel using the default configuration (no SMP, no > APIC), the application takes an average of 8.45 seconds (wall-clock > time) to run, and this number is consistent from run to run. When I > run a kernel built with either SMP or SMP+APIC, it takes an average > of 13.25 seconds, and this number is also quite consistent. (It's > not the Alteon; using the Intel Pro/1000 XT server adapter with the > em driver gives a similar difference, although the intel NIC is > slower.) > > Interestingly, the CPU utilization for the default kernel is about > 75-80%, while for the APIC kernel it is over 95%. My guess would be > that something is spinning on a lock in the APIC kernel, but that's > just a hunch. You may just be seeing the overhead due to having SMP enabled. A single-processor kernel doesn't need to lock structures against simultaneous access by another CPU. -- Dan Nelson dnelson@allantgroup.com
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