linux-kernel - Re: [PATCH] shrink printk timestamp field

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Message-Id: <20080829163540.634e86d9.akpm@linux-foundation.org>
Date:	Fri, 29 Aug 2008 16:35:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joe Korty <joe.korty@...r.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shrink printk timestamp field

On Wed, 27 Aug 2008 11:17:59 -0400
Joe Korty <joe.korty@...r.com> wrote:

> Shrink the printk timestamp field.
> 
> Keep the printk timestamp from occupying more of the
> scarce, 80-column console line space than it really needs.
> 
> We eliminate the excessive whitespace the field added to
> the line, and reduce timestamp precision from six digits
> (usecs) to three digits (msecs).  msecs seems adequate
> for the purpose of tracking boot sequence timing issues.
> 
> Signed-off-by: Joe Korty <joe.korty@...r.com>
> 
> Index: 2.6.27-rc4-git4/kernel/printk.c
> ===================================================================
> --- 2.6.27-rc4-git4.orig/kernel/printk.c	2008-08-26 18:23:03.000000000 -0400
> +++ 2.6.27-rc4-git4/kernel/printk.c	2008-08-26 18:23:49.000000000 -0400
> @@ -739,9 +739,9 @@
>  
>  				t = cpu_clock(printk_cpu);
>  				nanosec_rem = do_div(t, 1000000000);
> -				tlen = sprintf(tbuf, "[%5lu.%06lu] ",
> +				tlen = sprintf(tbuf, "[%3lu.%03lu] ",
>  						(unsigned long) t,
> -						nanosec_rem / 1000);
> +						nanosec_rem / 1000000);
>  
>  				for (tp = tbuf; tp < tbuf + tlen; tp++)
>  					emit_log_char(*tp);

This is a significant loss in utility.

[   16.817285] md: Autodetecting RAID arrays.
[   16.817288] md: Scanned 0 and added 0 devices.
[   16.817290] md: autorun ...
[   16.817292] md: ... autorun DONE.

This not-terribly-fast machine can emit printks into the log buffer
within two microseconds.  That's a pretty useful ad-hoc timing
factility.

This patch will reduce the precision by a factor of five hundred.
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