netdev - Re: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user space data

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Date:	Wed, 24 Dec 2008 20:46:56 +0300
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Evgeniy Polyakov <zbr@...emap.net>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-scsi@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Jeff Garzik <jeff@...zik.org>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, scst-devel@...ts.sourceforge.net,
	Bart Van Assche <bart.vanassche@...il.com>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	netdev@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>,
	David Miller <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Subject: Re: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user
 space data

Evgeniy Polyakov, on 12/24/2008 05:44 PM wrote:
> On Wed, Dec 24, 2008 at 05:37:51PM +0300, Vladislav Bolkhovitin (vst@...b.net) wrote:
>>> Yes, it is simpler and cleaner, but it requires additional allocation.
>>> This is additional (and quite noticeble) overhead.
>> Not necessary requires. For instance, in iscsi-scst sk_transaction_token 
>> can (and should) be part of iSCSI cmd structure, so no additional 
>> allocations would be needed.
> 
> This is special case, I'm not sure it is always possible to cache that
> token and attach to every skb, but if it can be done, then of course
> this does not end up with additional overhead.

I think in most cases there would be possibility to embed 
sk_transaction_token to some higher level structure. E.g. Xen apparently 
should have something to track packets passed through host/guest 
boundary. From other side, kmem cache is too well polished to have much 
overhead. I doubt, you would even notice it in this application. In most 
cases allocation of such small object in it using SLUB is just about the 
same as a list_del() under disabled IRQs.

Vlad



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