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Message-Id: <20070822082858.11964.87377.sendpatchset@localhost.localdomain> Date: Wed, 22 Aug 2007 13:58:58 +0530 From: Krishna Kumar <krkumar2@...ibm.com> To: johnpol@....mipt.ru, herbert@...dor.apana.org.au, hadi@...erus.ca, kaber@...sh.net, shemminger@...ux-foundation.org, davem@...emloft.net Cc: jagana@...ibm.com, Robert.Olsson@...a.slu.se, peter.p.waskiewicz.jr@...el.com, kumarkr@...ux.ibm.com, xma@...ibm.com, gaagaan@...il.com, netdev@...r.kernel.org, rdreier@...co.com, rick.jones2@...com, mcarlson@...adcom.com, jeff@...zik.org, general@...ts.openfabrics.org, mchan@...adcom.com, tgraf@...g.ch, Krishna Kumar <krkumar2@...ibm.com>, sri@...ibm.com Subject: [PATCH 1/10 Rev4] [Doc] HOWTO Documentation for batching Add Documentation describing batching skb xmit capability. Signed-off-by: Krishna Kumar <krkumar2@...ibm.com> --- batching_skb_xmit.txt | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 78 insertions(+) diff -ruNp org/Documentation/networking/batching_skb_xmit.txt new/Documentation/networking/batching_skb_xmit.txt --- org/Documentation/networking/batching_skb_xmit.txt 1970-01-01 05:30:00.000000000 +0530 +++ new/Documentation/networking/batching_skb_xmit.txt 2007-08-22 10:21:19.000000000 +0530 @@ -0,0 +1,78 @@ + HOWTO for batching skb xmit support + ----------------------------------- + +Section 1: What is batching skb xmit +Section 2: How batching xmit works vs the regular xmit +Section 3: How drivers can support batching +Section 4: How users can work with batching + + +Introduction: Kernel support for batching skb +---------------------------------------------- + +A new capability to support xmit of multiple skbs is provided in the netdevice +layer. Drivers which enable this capability should be able to process multiple +skbs in a single call to their xmit handler. + + +Section 1: What is batching skb xmit +------------------------------------- + + This capability is optionally enabled by a driver by setting the + NETIF_F_BATCH_SKBS bit in dev->features. The pre-requisite for a + driver to use this capability is that it should have a reasonably + sized hardware queue that can process multiple skbs. + + +Section 2: How batching xmit works vs the regular xmit +------------------------------------------------------- + + The network 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. + + Batching skb xmit is a mechanism 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 batching +--------------------------------------------- + + Batching requires the driver to set the NETIF_F_BATCH_SKBS bit in + dev->features. + + The driver's xmit handler should be modified to process multiple skbs + instead of one skb. The driver's xmit handler is called either with a + skb to transmit or NULL skb, where the latter case should be handled + as a call to xmit multiple skbs. This is done by sending out all skbs + in the dev->skb_blist list (where it was added by the core stack). + + +Section 4: How users can work with batching +--------------------------------------------- + + Batching can 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 by passing 'on' or 'off' + respectively to ethtool. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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