From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoffer Dall <christoffer.dall@linaro.org>
Subject: Re: [PATCH v3 2/2] arm64: add micro test
Date: Wed, 24 Jan 2018 09:17:55 +0100
Message-ID: <20180124081755.GO21802@cbox>
References: <1516225596-27071-1-git-send-email-shihwei@cs.columbia.edu>
 <1516225596-27071-3-git-send-email-shihwei@cs.columbia.edu>
 <20180118100958.j4iiq3ep2uruikss@yury-thinkpad>
 <20180118103737.jiwf5ly3up6w7f7r@kamzik.brq.redhat.com>
 <20180118112519.vzkfwkgqavvxqzdn@yury-thinkpad>
 <CAO+sbHHKyYHjGqmS5uM=C6bCnUQ2CbHmmFBTAb=WanQsWdLU2Q@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Yury Norov <ynorov@caviumnetworks.com>,
        Andrew Jones <drjones@redhat.com>, kvm@vger.kernel.org,
        kvmarm@lists.cs.columbia.edu, Marc Zyngier <marc.zyngier@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Christoffer Dall <cdall@cs.columbia.edu>
To: Shih-Wei Li <shihwei@cs.columbia.edu>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mail-wm0-f42.google.com ([74.125.82.42]:43016 "EHLO
        mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752314AbeAXIR6 (ORCPT <rfc822;kvm@vger.kernel.org>);
        Wed, 24 Jan 2018 03:17:58 -0500
Received: by mail-wm0-f42.google.com with SMTP id g1so6657976wmg.2
        for <kvm@vger.kernel.org>; Wed, 24 Jan 2018 00:17:58 -0800 (PST)
Content-Disposition: inline
In-Reply-To: <CAO+sbHHKyYHjGqmS5uM=C6bCnUQ2CbHmmFBTAb=WanQsWdLU2Q@mail.gmail.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

Hi Shih-Wei,

On Fri, Jan 19, 2018 at 04:57:55PM -0500, Shih-Wei Li wrote:
> Thanks for the feedback about the mistakes in math and some issues in
> naming, print msg, and coding style. I'll be careful and try to avoid
> the same problems the next patch set. Sorry for all of the confusion.
> 
> So we now skip the test when "sample == 0" happens over 1000 times.
> This is only due to the case that "cost is < 1/cntfrq" since it's not
> possible for the tick to overflow for that many times. Did I miss
> something here? I do agree that we should output better msgs to tell
> users that the cost of a certain test is constantly smaller than a
> tick.
> 

I think for things like vmexit counts, it's very likely that all the
samples will result in 0 ticks on many systems (fast CPU and slow arch
counter; the architecture doesn't give us any guarantees here).  For
example, a system with a 2 GHz CPU and a 1 MHz counter will give you a
granularity of 2000 cycles for each counter tick, which is not that
useful for low-level tuning of KVM.

So what I thought we were going to do was:

main_test_function()
{
	long ntimes = NTIMES;
	long cost, total_cost;

	cnt1 = read_cnt();
	do {
		run_test();
	} while(ntimes--);
	cnt2 = read_cnt();

	if (verify_sane_counter(cnt1, cnt2))
		return;

	total_cost = to_nanoseconds(cnt2 - cnt1);
	cost = total_cost / NTIMES;
	printf("testname: %l (%l)\n", cost, total_cost);
}

And in that way amortize the potential lack of precision over all the
iterations.  Did I miss some prior discussion about why that was a bad
idea?

It would also be possible to have two functions, one that does the above
and one that does a per-run measurement, in case the user wants to know
min/max/stddev and is running on a system with sufficient precision.
The method could be chosen via an argument.

Thanks,
-Christoffer