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You don't have any tailroom at all when you split 4k page into two halves on x86_64. From those 512 bytes, 320+ are used for skb_shared_info. And then you're left with 192 bytes or even less (with increased MAX_SKB_FRAGS, which becomes a new trend thanks to Eric and Big TCP). XDP requires 256 bytes of headroom -- and you already don't have them even with the default number of frags. > It is used to append data to packets being sent - but that isn't > really relevant here. > > While the unused memory is moderate for 4k pages, it is horrid > for anything with large pages - think 64k and above. But hey, there's always unused space and it's arbitrary whether to treat it "horrid". For example, imagine a machine mostly handling 64-byte traffic. From that point of view, even splitting pages into 2 halves still is "horrid" -- 2048 bytes of truesize only to receive 64 bytes :> > IIRC large pages are common on big PPC and maybe some arm cpus. Even in that case, the percentage of 4k-page machines running those NICs are much higher than > 4k. I thought we usually optimize things for the most common usecases. Adding a couple hundred lines, branches on hotpaths, limitations etc. only to serve a particular architecture... Dunno. I have 16k pages on my private development machines and I have no issues with using 1 page per frame in their NIC drivers :s > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) > Thanks, Olek From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from smtp1.osuosl.org (smtp1.osuosl.org [140.211.166.138]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8FA60C77B73 for ; Fri, 26 May 2023 12:53:27 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp1.osuosl.org (Postfix) with ESMTP id 1CF5C8434B; Fri, 26 May 2023 12:53:27 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org 1CF5C8434B DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=osuosl.org; s=default; t=1685105607; 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a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1685105603; x=1716641603; h=message-id:date:subject:to:cc:references:from: in-reply-to:content-transfer-encoding:mime-version; bh=LqzVE0lU/1GvWumVv/gU63LLKqWsOobFk8T/wLOvU50=; b=X+JuRVUsaPUVwdnye4CoygqaNjycBZDluA4rAxOybatdodCfR+icBsZF 1BWH4LmPRCBYTux4FhzThF8gxYMFLSCGm044NmSN1hdV0r8cg0yriPm2c +DG+g89IH8WD9U8IRGRYRt47j+9uvwx0XRivOiwF5p971AvVoq9gq7mW/ hQgoQy9H1YkL+ApXzAi5SffkHkDNIWncPMhDkD2VCnqoVIJJoYrSrwVY7 zZXCbYNsNq4e3HIxvtu9vWmKEuw/dvmtO8jadrkN94cj9ZtIWxz09COui gJuE7v5I/kwnYw2vbX7jL0godP4E3WoloGJl6OsLldbwzBletOIP5dfdx Q==; X-Mailman-Original-Authentication-Results: smtp3.osuosl.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.a=rsa-sha256 header.s=Intel header.b=X+JuRVUs X-Mailman-Original-Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=intel.com; Subject: Re: [Intel-wired-lan] [PATCH net-next v2 05/12] iavf: always use a full order-0 page X-BeenThere: intel-wired-lan@osuosl.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel Wired Ethernet Linux Kernel Driver Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paul Menzel , Jesper Dangaard Brouer , Larysa Zaremba , "netdev@vger.kernel.org" , Ilias Apalodimas , "linux-kernel@vger.kernel.org" , Christoph Hellwig , Eric Dumazet , Michal Kubiak , "intel-wired-lan@lists.osuosl.org" , Jakub Kicinski , Paolo Abeni , "David S. Miller" , Magnus Karlsson Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: intel-wired-lan-bounces@osuosl.org Sender: "Intel-wired-lan" From: David Laight Date: Fri, 26 May 2023 08:57:31 +0000 > From: Alexander Lobakin >> Sent: 25 May 2023 13:58 [...] >> 4096 page >> 64 head, 320 tail >> 3712 HW buffer size >> 3686 max MTU w/o frags > > I'd have thought it was important to pack multiple buffers for > MTU 1500 into a single page. > 512 bytes split between head and tail room really ought to > be enough for most cases. > > Is much tailroom ever used for received packets? You don't have any tailroom at all when you split 4k page into two halves on x86_64. From those 512 bytes, 320+ are used for skb_shared_info. And then you're left with 192 bytes or even less (with increased MAX_SKB_FRAGS, which becomes a new trend thanks to Eric and Big TCP). XDP requires 256 bytes of headroom -- and you already don't have them even with the default number of frags. > It is used to append data to packets being sent - but that isn't > really relevant here. > > While the unused memory is moderate for 4k pages, it is horrid > for anything with large pages - think 64k and above. But hey, there's always unused space and it's arbitrary whether to treat it "horrid". For example, imagine a machine mostly handling 64-byte traffic. From that point of view, even splitting pages into 2 halves still is "horrid" -- 2048 bytes of truesize only to receive 64 bytes :> > IIRC large pages are common on big PPC and maybe some arm cpus. Even in that case, the percentage of 4k-page machines running those NICs are much higher than > 4k. I thought we usually optimize things for the most common usecases. Adding a couple hundred lines, branches on hotpaths, limitations etc. only to serve a particular architecture... Dunno. I have 16k pages on my private development machines and I have no issues with using 1 page per frame in their NIC drivers :s > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) > Thanks, Olek _______________________________________________ Intel-wired-lan mailing list Intel-wired-lan@osuosl.org https://lists.osuosl.org/mailman/listinfo/intel-wired-lan