Date: Wed, 25 Jul 2001 14:08:22 -0700 (PDT) From: Julian Elischer <julian@elischer.org> To: Zhihui Zhang <zzhang@cs.binghamton.edu> Cc: Bosko Milekic <bmilekic@technokratis.com>, Terry Lambert <tlambert2@mindspring.com>, vishwanath pargaonkar <vishubp@yahoo.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: cluster size Message-ID: <Pine.BSF.4.21.0107251404390.4973-100000@InterJet.elischer.org> In-Reply-To: <Pine.SOL.4.21.0107251416550.21505-100000@opal>
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no.. it has to do with the fact that it would be unwise to make a cluster > 1 page size since we have no guarantee that all drivers could handle breaking up a DMA if a cluster spanned 2 physical address ranges. (they can handle a chain of discontinuous mbufs but may assume that a single mbuf will have physically contiguous data. Now since we cannot span a page boundary, we should fit in exacly to get as much room as possible and since (pagesize/3) is too small, the next possibility is (pagesize/2). If pagesize/3 was big enough, we might have used that.. On Wed, 25 Jul 2001, Zhihui Zhang wrote: > > I see. It has something to do with the power-of-two allocator we are > using inside the kernel. > > -Zhihui > > On Wed, 25 Jul 2001, Bosko Milekic wrote: > > > > > On Wed, Jul 25, 2001 at 01:51:51PM -0400, Zhihui Zhang wrote: > > > > > > > > > On Tue, 24 Jul 2001, Terry Lambert wrote: > > > > > > > Zhihui Zhang wrote: > > > > > > Hi, > > > > > > in freebsd can we change the cluster size from 2048 > > > > > > bytes.If yes how can we do that? > > > > > > do we have to configure in some file? > > > > > > > > > > You must be asking why the mbuf cluster size is chosen as 2048, right? It > > > > > is probably a tradeoff between memory efficient and speed. > > > > > > > > Ask yourselves: > > > > > > > > "What is the minimum cluster size I would have to have > > > > to be able to contain the maximum MTU worth of data, > > > > yet remain an even multiple of sizeof(mbuf) -- 256 > > > > bytes?" > > > > > > A dumb question: why even not odd multiple? > > > > > > -Zhihui > > > > It actually has to do with the fact that 2K is the only size equal to > > or greater than the maximum MTU worth of data that can be multiplied to a page > > size without any leftover (in other words, page size modulo 2K is zero). > > > > -- > > Bosko Milekic > > bmilekic@technokratis.com > > > > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > > with "unsubscribe freebsd-hackers" in the body of the message > > > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-hackers" in the body of the message > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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