netdev - Re: [PATCH] mlx4_en: map entire pages to increase throughput

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Date:	Tue, 17 Jul 2012 13:20:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	brking@...ux.vnet.ibm.com
Cc:	rick.jones2@...com, cascardo@...ux.vnet.ibm.com,
	netdev@...r.kernel.org, yevgenyp@...lanox.co.il,
	ogerlitz@...lanox.com, amirv@...lanox.com,
	leitao@...ux.vnet.ibm.com, klebers@...ux.vnet.ibm.com
Subject: Re: [PATCH] mlx4_en: map entire pages to increase throughput

From: Brian King <brking@...ux.vnet.ibm.com>
Date: Tue, 17 Jul 2012 15:10:08 -0500

> On 07/17/2012 01:17 PM, Rick Jones wrote:
>> On 07/16/2012 10:29 PM, David Miller wrote:
>>> From: Rick Jones <rick.jones2@...com> Date: Mon, 16 Jul 2012
>>> 10:27:57 -0700
>>> 
>>>> That seems rather extraordinarily low - Power7 is supposed to be
>>>> a rather high performance CPU.  The last time I noticed
>>>> O(3Gbit/s) on 10G for bulk transfer was before the advent of
>>>> LRO/GRO - that was in the x86 space though.  Is mapping really
>>>> that expensive with Power7?
>>> 
>>> Unfortunately, IOMMU mappings are incredibly expensive.  I see
>>> effects like this on Sparc too.
>> 
>> OK, so that has caused some dimm memory to get a small refresh - it
>> ends up being akin to if not actually a PIO yes?  I recall schemes in
>> drivers in other stacks whereby "small" packets were copied because
>> it was cheaper to allocate/copy then it was to remap.
> 
> On Power it ends up being an hcall to the hypervisor

This is true on sparc64 niagara systems as well.
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