Add Documentation describing batching API. Signed-off-by: Krishna Kumar <[EMAIL PROTECTED]> --- Batching_skb_API.txt | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 82 insertions(+)
diff -ruNp ORG/Documentation/networking/Batching_skb_API.txt NEW/Documentation/networking/Batching_skb_API.txt --- ORG/Documentation/networking/Batching_skb_API.txt 1970-01-01 05:30:00.000000000 +0530 +++ NEW/Documentation/networking/Batching_skb_API.txt 2007-08-07 22:41:55.000000000 +0530 @@ -0,0 +1,82 @@ + HOWTO for batching skb API support + ----------------------------------- + +Section 1: What is batching skb API ? +Section 2: How batching API works vs the original API ? +Section 3: How drivers can support this API ? +Section 4: How users can work with this API ? + + +Introduction: Kernel support for batching skb +---------------------------------------------- + +A new xmit API - hard_start_xmit_batch() is provided in the netdevice layer +similar to the existing hard_start_xmit() API. Drivers which export this +API can implement it similar to the hard_start_xmit handler. The new API +should process multiple skbs (or even one) in a single call while the +existing hard_start_xmit processes one skb. It is possible for the driver +writer to re-use most of the code from the existing API in the new API +without having code duplication. + + +Section 1: What is batching skb API ? +------------------------------------- + + This API is optionally exported by a driver. The pre-requisite for a + driver to use this API is that it should have a reasonably sized + hardware queue that can process multiple skbs. + + +Section 2: How batching API works vs the original API ? +------------------------------------------------------- + + The networking stack gets called from upper layer protocols with a + single skb to transmit. This skb is first enqueue'd and an attempt is + made to transmit it immediately (via qdisc_run). However, events like + tx lock contention, tx queue stopped, etc, can result in the skb not + getting sent out and it remains in the queue. When the next xmit is + called or when the queue is re-enabled, qdisc_run could potentially + find multiple packets in the queue, and iteratively send them all out + one-by-one. + + The batching skb API was added to exploit this situation where all + skbs can be passed in one shot to the device. This reduces driver + processing, locking at the driver (or in stack for ~LLTX drivers) + gets amortized over multiple skbs, and in case of specific drivers + where every xmit results in a completion processing (like IPoIB), + optimizations can be made in the driver to request a completion for + only the last skb that was sent which results in saving interrupts + for every (but the last) skb that was sent in the same batch. + + Batching can result in significant performance gains for systems that + have multiple data stream paths over the same network interface card. + + +Section 3: How drivers can support this API ? +--------------------------------------------- + + The new API - dev->hard_start_xmit_batch(struct net_device *dev), + simplistically, can be written almost identically to the regular + xmit API except that multiple skbs should be processed by the driver + instead of one skb. The new API doesn't get a skb as an argument, + instead it picks up all the skbs from dev->skb_blist, where it was + added by the core stack, and tries to send them out. + + Batching requires the driver to set dev->hard_start_xmit_batch to the + new API implemented for that driver. + + +Section 4: How users can work with this API ? +--------------------------------------------- + + Batching could be disabled for a particular device, e.g. on desktop + systems if only one stream of network activity for that device is + taking place, since performance could be slightly affected due to + extra processing that batching adds (unless packets are getting + sent fast resulting in stopped queue's). Batching can be enabled if + more than one stream of network activity per device is being done, + e.g. on servers; or even desktop usage with multiple browser, chat, + file transfer sessions, etc. + + Per device batching can be enabled/disabled using ethtool, where + passing 1 enables batching and passing 0 disables batching. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
