linux-kernel - Re: [RFC][PATCH 1/1] cxgb3i: cxgb3 iSCSI initiator

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Date:	Thu, 14 Aug 2008 22:30:11 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	David Miller <davem@...emloft.net>
CC:	open-iscsi@...glegroups.com, rdreier@...co.com, rick.jones2@...com,
	jgarzik@...ox.com, swise@...ngridcomputing.com, kxie@...lsio.com,
	netdev@...r.kernel.org, michaelc@...wisc.edu, daisyc@...ibm.com,
	wenxiong@...ibm.com, bhua@...ibm.com, dm@...lsio.com,
	leedom@...lsio.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/1] cxgb3i: cxgb3 iSCSI initiator

Vladislav Bolkhovitin wrote:
> David Miller wrote:
>> From: Vladislav Bolkhovitin <vst@...b.net>
>> Date: Wed, 13 Aug 2008 22:35:34 +0400
>>
>>> This is because the target sends data in a zero-copy manner, so its
>>> CPU is capable to deal with the load, but on the initiator there are
>>> additional data copies from skb's to page cache and from page cache
>>> to application.
>> If you've actually been reading at all what I've been saying in this
>> thread you'll see that I've described a method to do this copy
>> avoidance in a completely stateless manner.
>>
>> You don't need to implement a TCP stack in the card in order to do
>> data placement optimizations.  They can be done completely stateless.
> 
> Sure, I read what you wrote before writing (although, frankly, didn't 
> get the idea). But I don't think that overall it would be as efficient 
> as full hardware offload. See my reply to Jeff Garzik about that.
> 
>> Also, large portions of the cpu overhead are transactional costs,
>> which are significantly reduced by existing technologies such as
>> LRO.
> 
> The test used Myricom Myri-10G cards (myri10ge driver), which support 
> LRO. And from ethtool -S output I conclude it was enabled. Just in case, 
> I attached it, so you can recheck me.

Also, there wasn't big difference between MTU 1500 and 9000, which is 
another point to think that LRO was working.

> Thus, apparently, LRO doesn't make a fundamental difference. Maybe this 
> particular implementation isn't too efficient, I don't know. I don't 
> have enough information for that.
> 
> Vlad
> 
> 

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