Date: Sun, 21 Mar 2010 08:21:25 +0300 From: Alexander Bubnov <alexander.bubnov@gmail.com> To: Bruce Simpson <bms@incunabulum.net> Cc: freebsd-net@freebsd.org Subject: Re: why zero-copy sockets(9) are not popular? Message-ID: <c3e287ff1003202221p15fe6da8gce813c5889e095e0@mail.gmail.com> In-Reply-To: <4BA50BA1.5060401@incunabulum.net> References: <c3e287ff1003200306l162800bfgfc0eecec04401917@mail.gmail.com> <4BA50BA1.5060401@incunabulum.net>
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Bruce, many thanks for comprehensive answer! 2010/3/20 Bruce Simpson <bms@incunabulum.net> > On 03/20/10 10:06, Alexander Bubnov wrote: > >> Hello, all! >> Anybody knows why zero copy is not popular although this technique allows >> to increase performance of servers? It is very hard to find any examples >> of >> zero-copy for FreeBSD. >> >> > > Transmit is easy. Receive is hard. > > The whole concept of zero-copy revolves around being able to use > page-flipping to map buffers in user and kernel space, to amortize the cost > of copies across that system boundary. > > The compromise usually taken is to use the sendfile() API, or rely on TCP > Segmentation Offload (TSO), much like Microsoft's Chimney stack does in > Windows 7. Unfortunately, sendfile() only covers transmit. TSO only offloads > up to the point where sockets hit the card; TSO can offload TCP stream > reassembly, but you still have to copy from the kernel buffers into > userland. > > True zero-copy sockets generally require scatter/gather DMA engine support, > and TCP/IP header splitting, to do zero-copy recieve. > > S/G PCI DMA cores are often custom designed, and you tend not to find them > in off-the-shelf VHDL libraries. That IP (as in intellectual property) still > has cost. > > Historically the only cards in FreeBSD which supported this, were the > Tigon-II, which got bought by Broadcom (bge is the Tigon-III). Modified > firmware was required to do this. > > > _______________________________________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org" > -- /BR, Alexander
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