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Re: Timing overhead and Linux clock sources

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Timing overhead and Linux clock sources
Date: 2012-08-27 16:39:02
Message-ID: CA+TgmoZMJRL=ru0KKq8OyvkScpc14FDdEWUp4MBz5MWzXoSNPg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sat, Aug 25, 2012 at 10:48 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Mon, Aug 20, 2012 at 03:11:51PM -0400, Robert Haas wrote:
>> On Thu, Aug 16, 2012 at 10:28 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>> > FYI, I am planning to go ahead and package this tool in /contrib for PG
>> > 9.3.
>>
>> Isn't this exactly what we already did, in 9.2, in the form of
>> contrib/pg_test_timing?
>
> Sorry, not sure how I missed that commit.  Anyway, I am attaching a
> patch for 9.3 that I think improves the output of the tool, plus adds
> some C comments.
>
> The new output has the lowest duration times first:
>
>         Testing timing overhead for 3 seconds.
>         Per loop time including overhead: 41.31 nsec
>         Histogram of timing durations:
>         < usec   % of total      count
>              1     95.87135   69627856
>              2      4.12759    2997719
>              4      0.00086        628
>              8      0.00018        133
>             16      0.00001          5
>             32      0.00000          1
>
> This should make the output clearer to eyeball for problems --- a good
> timing has a high percentage on the first line, rather than on the last
> line.

I guess I'm not sure the output format is an improvement.  I wouldn't
care much one way or the other if we had made this change at the time
in AS92, but I'm not sure it's really worth breaking compatibility for
a format that may or may not be any better.  The person who wrote the
original code presumably preferred it way it already is.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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