linux-kernel - Re: [PATCH 0/4] clock_gettime

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Date:	Tue, 27 Dec 2011 08:46:05 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-kernel@...r.kernel.org, Kumar Sundararajan <kumar@...com>,
	john stultz <johnstul@...ibm.com>,
	Arun Sharma <asharma@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/4] clock_gettime_ns and x86-64 optimizations

On Sun, Dec 25, 2011 at 08:50:59AM -0800, Andy Lutomirski wrote:
> On x86-64, clock_gettime is so fast that the overhead converting to and
> from nanoseconds is non-negligible.  clock_gettime_ns is a different and
> faster interface.

But your data contradict this statement. See below.

> Patch 1 adds the syscall and wires it up on x86-64.  Patch 2 implements
> the corresponding vdso entry on x86-64.  Patch 3 optimizes the vdso
> call, and patch 4 is a trivial change that speeds up the vdso
> clock_gettime and clock_gettime_ns implementations.
> 
> The vdso timings are (on an 800MHz Sandy Bridge mobile):

It would be more informative to describe how you made the measurements
in more detail, for example, pseudo code, number of trials, min, max,
mean, std. dev.

> Basic implementation:
> 
> realtime 77.4ns
> monotonic 79.2ns
> realtime_coarse 18.1ns
> monotonic_coarse 22.0ns

In order to better understand what you are reporting, I arranged your
numbers into a table:

1. Basic implementation
2. Optimized implementation
3. Inlined (patch 4)

|---------------------+-------+-------+-------|
|                     |    1. |    2. |    3. |
|---------------------+-------+-------+-------|
| realtime            | 77.40 | 78.50 | 73.40 |
| realtime_ns         | 84.90 | 77.85 | 73.15 |
|---------------------+-------+-------+-------|
| monotonic           | 79.20 | 77.40 | 72.10 |
| monotonic_ns        | 85.10 | 77.75 | 72.10 |
|---------------------+-------+-------+-------|
| realtime_coarse     | 18.10 | 18.40 | 13.20 |
| realtime_coarse_ns  | 19.49 | 18.20 | 14.10 |
|---------------------+-------+-------+-------|
| monotonic_coarse    | 22.00 | 19.40 | 15.80 |
| monotonic_coarse_ns | 27.32 | 18.20 | 15.60 |
|---------------------+-------+-------+-------|

Looking down column 3, it appears that the _ns calls are no faster
than their plain counterparts.

So, while the inline patch does improve performance, the new _ns
functions do not really seem worth the trouble.

Thanks,
Richard


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