From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sergei Shtylyov Subject: Re: [PATCH] xen-netfront: transmit fully GSO-sized packets Date: Thu, 26 Mar 2015 17:14:08 +0300 Message-ID: <55141430.9050309@cogentembedded.com> References: <1427368406-28084-1-git-send-email-jonathan.davies@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: Wei Liu To: Jonathan Davies , netdev@vger.kernel.org, xen-devel@lists.xenproject.org, Konrad Rzeszutek Wilk , Boris Ostrovsky , David Vrabel Return-path: Received: from mail-la0-f43.google.com ([209.85.215.43]:36733 "EHLO mail-la0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752930AbbCZOOL (ORCPT ); Thu, 26 Mar 2015 10:14:11 -0400 Received: by labe2 with SMTP id e2so46434471lab.3 for ; Thu, 26 Mar 2015 07:14:10 -0700 (PDT) In-Reply-To: <1427368406-28084-1-git-send-email-jonathan.davies@citrix.com> Sender: netdev-owner@vger.kernel.org List-ID: Hello. On 3/26/2015 2:13 PM, Jonathan Davies wrote: > xen-netfront limits transmitted skbs to be at most 44 segments in size. However, > GSO permits up to 65536 bytes, which means a maximum of 45 segments of 1448 > bytes each. This slight reduction in the size of packets means a slight loss in > efficiency. > Since c/s 9ecd1a75d, xen-netfront sets gso_max_size to c/s == commit? Please also specify that commit's summary line in parens. > XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER, > where XEN_NETIF_MAX_TX_SIZE is 65535 bytes. > The calculation used by tcp_tso_autosize (and also tcp_xmit_size_goal since c/s > 6c09fa09d) in determining when to split an skb into two is Likewise. > sk->sk_gso_max_size - 1 - MAX_TCP_HEADER. > So the maximum permitted size of an skb is calculated to be > (XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER) - 1 - MAX_TCP_HEADER. > Intuitively, this looks like the wrong formula -- we don't need two TCP headers. > Instead, there is no need to deviate from the default gso_max_size of 65536 as > this already accommodates the size of the header. > Currently, the largest skb transmitted by netfront is 63712 bytes (44 segments > of 1448 bytes each), as observed via tcpdump. This patch makes netfront send > skbs of up to 65160 bytes (45 segments of 1448 bytes each). > Fixes: 9ecd1a75d977 ("xen-netfront: reduce gso_max_size to account for max TCP header") Ah, here's the summary for the first mentioned commit... > Signed-off-by: Jonathan Davies WBR, Sergei