Re: kernel 2.6 speed

From: Florin Malita
Date: Sun Jul 24 2005 - 16:05:21 EST

On 7/24/05, Ciprian <cipicip@xxxxxxxxx> wrote:
> test /= 10;
> test *= 10;
> test += 10;
> test -= 10;

You're not trying to benchmark the kernel with those arithmetic
operations are you?! That's completely bogus, the kernel is not
involved in any of that.

As it has been already pointed out, the only OS-dependent and (by far)
the most expensive operation in your loop is the time() function call
- so that's the only thing you're really benchmarking there (besides
compiler optimizations).

> In windows were performed about 300 millions cycles,
> while in Linux about 10 millions. This test was run on
> Fedora 4 and Suse 9.2 as Linux machines, and Windows
> XP Pro with VS .Net 2003 on the MS side. My CPU is a
> P4 @3GHz HT 800MHz bus.

I can only speculate as of why the windoze time() call seems so much
faster: maybe it is implemented in userspace and doesn't involve a
system call. Somebody with more knowledge in the area might
confirm/infirm this.

Even in Linux your results will vary a lot depending on whether the
kernel and glibc support vsyscalls. The FC kernels disable vsyscall
because it's incompatible with NX, not sure about the Suse kernels.
Here's what I get on a P4 1.7 with a vsyscall enabled kernel

No. of cycles: 65688977

Check this thread for a FC4 kernel performance discussion:

> Now, can anyone explain this and suggest what other
> optimizations I should use? The 2.4 version was a lot
> faster.

Your bogus test aside, a certain userland performance degradation when
moving from 2.4 to 2.6 is expected as the x86 timer interrupt
frequency has increased from 100Hz to 1KHz (it's about to be lowered
to 250Hz) - so your apps are interrupted more often. But I wouldn't
expect that degradation to be substantial. If you want to dig in and
measure it you should use asm/rdtsc instead of time().
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