From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH net-next] udp: under rx pressure, try to condense skbs Date: Thu, 08 Dec 2016 13:26:02 -0500 (EST) Message-ID: <20161208.132602.1437771867201682270.davem@davemloft.net> References: <1481131173.4930.36.camel@edumazet-glaptop3.roam.corp.google.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, pabeni@redhat.com To: eric.dumazet@gmail.com Return-path: Received: from shards.monkeyblade.net ([184.105.139.130]:36102 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752727AbcLHS0E (ORCPT ); Thu, 8 Dec 2016 13:26:04 -0500 In-Reply-To: <1481131173.4930.36.camel@edumazet-glaptop3.roam.corp.google.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Eric Dumazet Date: Wed, 07 Dec 2016 09:19:33 -0800 > From: Eric Dumazet > > Under UDP flood, many softirq producers try to add packets to > UDP receive queue, and one user thread is burning one cpu trying > to dequeue packets as fast as possible. > > Two parts of the per packet cost are : > - copying payload from kernel space to user space, > - freeing memory pieces associated with skb. > > If socket is under pressure, softirq handler(s) can try to pull in > skb->head the payload of the packet if it fits. > > Meaning the softirq handler(s) can free/reuse the page fragment > immediately, instead of letting udp_recvmsg() do this hundreds of usec > later, possibly from another node. > > > Additional gains : > - We reduce skb->truesize and thus can store more packets per SO_RCVBUF > - We avoid cache line misses at copyout() time and consume_skb() time, > and avoid one put_page() with potential alien freeing on NUMA hosts. > > This comes at the cost of a copy, bounded to available tail room, which > is usually small. (We might have to fix GRO_MAX_HEAD which looks bigger > than necessary) > > This patch gave me about 5 % increase in throughput in my tests. > > skb_condense() helper could probably used in other contexts. > > Signed-off-by: Eric Dumazet This is isolated to UDP, and would be easy to revert if it causes problems. So applied, thanks Eric.