linux-kernel - Re: clock_gettime

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Message-ID: <52280C73.9080209@fb.com>
Date:	Thu, 5 Sep 2013 10:15:39 +0530
From:	Arun Sharma <asharma@...com>
To:	John Stultz <john.stultz@...aro.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Kumar Sundararajan <kumar@...com>,
	Richard Cochran <richardcochran@...il.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: clock_gettime_ns

On 9/5/13 12:47 AM, John Stultz wrote:
> If we're going to add a new interface that uses something other then a
> timespec, we likely need to put some serious thought into that new
> type, and see how it could be used across a number of syscalls. Some
> of the discussion around dealing with the 2038 issue touched on this.

[ I know you're not asking for perf data, but may be useful for new 
readers ]

Here's the benchmarking I did in 2011:

http://thread.gmane.org/gmane.linux.kernel/1233758/focus=1233781

Switching from timespec to s64 was worth 21%. My experience over the 
years is that this performance delta causes userspace guys to implement 
their own TSC based timers, against the advice from kernel developers.

http://code.ohloh.net/search?s=wall%20now%20tsc%20hz&pp=0&fl=C&fl=C%2B%2B&ff=1&mp=1&ml=1&me=1&md=1&filterChecked=true

I worry that trying to solve other clock problems will cause the kernel 
to continue to pass the time in memory instead of registers, giving the 
userspace TSC based implementations a reason to exist.

  -Arun
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