linux-kernel - Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical addresses

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Date:	Fri, 05 Oct 2012 09:55:39 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	konrad.wilk@...cle.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, rob@...dley.net, akpm@...ux-foundation.org,
	joerg.roedel@....com, bhelgaas@...gle.com, shuahkhan@...il.com,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	x86@...nel.org, torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical addresses

Alexander Duyck <alexander.h.duyck@...el.com> writes:

> While working on 10Gb/s routing performance I found a significant amount of
> time was being spent in the swiotlb DMA handler.  Further digging found that a
> significant amount of this was due to the fact that virtual to physical
> address translation and calling the function that did it.  It accounted for
> nearly 60% of the total overhead.

Can you find out why that is? Traditionally virt_to_phys was just a
subtraction. Then later on it was a if and a subtraction.

It cannot really be that expensive. Do you have some debugging enabled?

Really virt_to_phys should be fixed. Such fundamental operations
shouldn't slow. I don't think hacking up all the users to work
around this is the r ight way.

Looking at the code a bit someone (crazy) made it out of line.
But that cannot explain that much overhead.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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