> How does the P4 machine compare against itself with hyperthreading turned > off?
The latency is fine (when booting into a up kernel). I did not run generic benchmarks for testing speed. There is a gain but it is not that much (from benchmarks I've seen on the web). > I'm not surprised that hyperthreading is slower than a true dual CPU setup. > Two CPUs sharing their cache? That's got to increase cache misses. The problem is not speed but latency glitches. It certainly should be slower than a "real" dual cpu system but it could be slightly faster than the same uniprocessor machine with ht turned off. Of course if you have latency problems it is not very useful. -- Fernando > -------Original Message------- > From: Fernando Pablo Lopez-Lezcano <[EMAIL PROTECTED]> > Sent: 06/02/03 09:05 PM > To: [EMAIL PROTECTED] > Subject: [linux-audio-dev] latency and P4 hyperthreading? > > > Hi all, has anyone tested latency on a P4 machine with Hyperthreading > support? (HT simulates two processors on one processor using - I guess - > the spare time available in the cpu when it is waiting for, for example, > memory accesses). > > Anyway, on a: > P4 2.4G 800MHz fsb, 875P chipset, 1G ram, Seagate Barracuda V 60G drive: > > Booting into an smp kernel with HT enabled in the bios (kernel based on > 2.4.21-rc6 with the usual low latency, preemptive and full acpi kernel > patches) I see both processors, but the latency in the disk tests (using > latencytest 0.42) has spikes that go up to 10/15 msecs, specially in the > read/write test. > > If I boot into a up kernel, or if I boot with acpi=off I see only one > processor and the latency for the disk tests is fine. > > Same kernel, same alsa on a dual Athlon MP 1800+ machine does not show > any abnormal latency behavior so this seems to be confined to HT > machines... :-( > > I guess "made up" processors are not as good as real ones :-) > -- Fernando
