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

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Date:	Wed, 24 Dec 2008 17:37:51 +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 12:38 AM wrote:
> On Tue, Dec 23, 2008 at 10:16:25PM +0300, Vladislav Bolkhovitin (vst@...b.net) wrote:
>> Actually, there's another way, which seems to be a lot simpler. Alexey 
>> Kuznetsov privately suggested it to me.
>>
>> In skb_shared_info new pointer transaction_token would be added, which 
>> would point on:
>>
>> struct sk_transaction_token
>> {
>> 	atomic_t			io_count;
>> 	struct sk_transaction_token	*next;
>> 	unsigned long			token;
>> 	unsigned long			private;
>> 	void				(*finish_callback)(struct 
>> 	sk_transaction_token *);
>> };
>>
>> When skb is translated, transaction_token inherited. If 2 skb are merged 
>> (the same places where I put net_get_page's in my patch), the *older* 
>> token is inherited. This is the main point of this idea.
>>
>> Before starting new asynchronous send a client would open a new token. 
>> Everything sent then would receive that token. Finish_callback() would 
>> be called and the corresponding token freed, when io_count == 0 *AND* 
>> all previous tokens closed.
>>
>> This idea seems to be simpler, than even what Rusty implemented. Correct 
>> me, if I wrong. But, unfortunately, in the near future I will have no 
>> time to develop it.. :-(
> 
> 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.

Thanks,
Vlad


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