all syscalls initially taking 4usec on a P4? Re: nonblocking UDPv4 recvfrom() taking 4usec @ 3GHz?

bert hubert
2007-02-20T16:27:21Z

On Tue, Feb 20, 2007 at 11:50:13AM +0100, Andi Kleen wrote:
> P4s are pretty slow at taking locks (or rather doing atomical operations)
> and there are several of them in this path. You could try it with a UP
> kernel. Actually hotunplugging the other virtual CPU should be sufficient
> with recent kernels.

This is on a UP kernel, on a single CPU. It does have hyperthreading, but
the kernel is uniprocessor, non-preempt. No frequency scaling. Linux
2.6.20-rc4, 2.6.19, 2.6.18, P4, P-M, Athlon 64. Ubunty Edgy Eft on the P4.

> Also BTW RDTSC on P4 is not very accurate for small measurements
> because it has a quite high overhead by itself, i would suggest
> running it in a loop.

I've done so, with some interesting results. Source on
http://ds9a.nl/tmp/recvtimings.c - be careful to adjust the '3000' divider
to your CPU frequency if you care about absolute numbers!

These are two groups, each consisting of 10 consecutive nonblocking UDP
recvfroms, with 10 packets preloaded. Reported is the number of microseconds
per recvfrom call which yielded a packet:

$ ./recvtimings
4.142333
2.237667
1.927333
1.580000
1.770000
1.632333
1.712667
1.685000
1.620000
2.415000
1.347333
1.545000
1.492667
1.902333
1.485000
1.532667
1.460000
1.517667
1.492333
1.580000

This in a nearly quiet P4 - I've removed the first line:
$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 0 290064 307036 296036 0 0 0 0 124 58 0 0 100 0
0 0 0 289972 307036 296036 0 0 0 4 139 95 0 0 100 0
0 0 0 289972 307036 296036 0 0 0 0 119 55 0 0 100 0
1 0 0 289972 307036 296036 0 0 0 0 135 71 0 0 100 0

HZ is clearly 100. If I usleep in between, timings for each recvfrom call
become higher. If I sleep for a full second, I get nearly flat results:
4.250000
5.317667
3.525000
4.147333
3.360000
3.552667
3.087667

Various differing CPUs report more or less the same results. Now I know we
have caching effects, but these effects are HUGE.

Is this supposed to be the case? I'm on an up to date system, glibc 2.4.

Bert

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