On 05/31/12 01:02, Adrian Chadd wrote:
I've re-run the test with powerd and sleep state stuff disabled - lo
and behold, UDP tests are now up around 240-250MBit, what I'd expect
for this 2 stream 11n device.
So why is it that I lose roughly 80MBit of throughput with powerd and
C2/C3 enabled, when there's plenty of CPU going around? The NIC
certainly isn't going to sleep (I've not even added that code.)
I've seen penalties from both of them myself on latency-sensitive
single-threaded disk benchmark: 17K IOPS instead of 30K IOPS without.
Problem with powerd was that CPU load during the test was below powerd
idle threshold and it decided to drop frequency, that proportionally
increased I/O handling latency. powerd can't know that while average CPU
load is now, the request handling latency is critical for the test.
About C-states, I've noticed on my tests on Core2Duo system that while
ACPI reports equal exit latency for C1 and C2 states of 1us there, they
are not really equal -- C2 exit is measurably slower. On newer
generations of systems (Core i) I've never seen C2 latency reported as
1us, but instead it has much higher values. Having real big value there
system should automatically avoid entering those states under the high
interrupt rates to not get penalties.
But that is all about latency-sensitive test. I am surprised to see such
results for the network benchmarks. Handling packets in bursts should
hide that latency. Unless you are loosing packets because of some
overflows during these delays.
--
Alexander Motin
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