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Message-ID: <88c4f8c4-085c-1934-536b-0fc8a38205c7@intel.com> Date: Fri, 26 May 2023 14:52:57 +0200 From: Alexander Lobakin <aleksander.lobakin@...el.com> To: David Laight <David.Laight@...LAB.COM> CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Magnus Karlsson <magnus.karlsson@...el.com>, Michal Kubiak <michal.kubiak@...el.com>, Larysa Zaremba <larysa.zaremba@...el.com>, "Jesper Dangaard Brouer" <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, Christoph Hellwig <hch@....de>, Paul Menzel <pmenzel@...gen.mpg.de>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH net-next v2 05/12] iavf: always use a full order-0 page From: David Laight <David.Laight@...LAB.COM> 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
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