netdev - RE: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the timecompare method

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Date:	Tue, 27 Mar 2012 18:29:58 +0000
From:	"Keller, Jacob E" <jacob.e.keller@...el.com>
To:	Richard Cochran <richardcochran@...il.com>
CC:	chetan loke <loke.chetan@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: RE: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of
 the timecompare method

> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@...il.com]
> Sent: Tuesday, March 27, 2012 11:05 AM
> To: Keller, Jacob E
> Cc: chetan loke; netdev@...r.kernel.org; e1000-devel@...ts.sourceforge.net;
> Kirsher, Jeffrey T; Ronciak, John; john.stultz@...aro.org; tglx@...utronix.de
> Subject: Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the
> timecompare method
> 
> I really doubt you will see any performance gain from such a change. It
> increases code complexity and size for some dubious, theoretical performance
> gain, for some really whacked use case.
> 
> Richard
> 
> 

It isn't so much about performance gains as it is about preventing poorly written user apps from stalling the clean descriptor routines. I am working on a test case that should prove whether this is even an issue (at least with ixgbe). Once I have data on that it can be determined if the extra lock would alleviate it. The conceptual issue is that spamming get-time could cause the clean tx/rx irq routines to stall inside the interrupt for too long. (thereby not freeing up descriptors on the ring to allow more packets). While performance is an issue, I don't feel that it can be brushed off as "performance loss due to feature" unless I can show that it isn't very easy to trip, or doesn't cause a major issue. Once I know the data, it can be determined what the right approach is.
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