Date: Mon, 27 Sep 2010 09:09:25 -0700 From: Julian Elischer <julian@freebsd.org> To: Andre Oppermann <andre@freebsd.org> Cc: Jeff Roberson <jeff@freebsd.org>, Luigi Rizzo <rizzo@iet.unipi.it>, FreeBSD Net <net@freebsd.org> Subject: Re: mbuf changes Message-ID: <4CA0C1B5.2090309@freebsd.org> In-Reply-To: <4CA098BA.2010106@freebsd.org> References: <4C9DA26D.7000309@freebsd.org> <4C9DB0C3.5010601@freebsd.org> <20100925163010.GA76213@onelab2.iet.unipi.it> <4CA09451.7010401@freebsd.org> <20100927131836.GA99909@onelab2.iet.unipi.it> <4CA098BA.2010106@freebsd.org>
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On 9/27/10 6:14 AM, Andre Oppermann wrote: > On 27.09.2010 15:18, Luigi Rizzo wrote: >> On Mon, Sep 27, 2010 at 02:55:45PM +0200, Andre Oppermann wrote: >> ... >>>> my idea was to have an extra field in the mbuf to tell how much room >>>> should be reserved/used for metadata (such as mtags) after >>>> the payload area so you don't need to change the allocator, and >>>> possibly can even modify this on an existing mbuf. >>>> Almost always mbufs have spare room (e.g. incoming pkts have all >>>> data in the cluster and mostly empty mdata; outgoing, except >>>> for rare cases, tend to be in a similar situation. >>>> So this approach would allow to take an already allocated >>>> mbuf and put the mtag in the spare area after the data. >>> >>> For incoming data this approach could work as usually 2K mbuf >>> clusters >>> are used and they have trailing space available, or rather the normal >>> mbuf referencing the cluster doesn't have its own data section >>> unused. >>> >>> When trailing space should be used the M_TAILINGSPACE() needs >>> modifications >>> and a full tree audit is required to make sure that all mbuf >>> consumers are >>> correctly using it and not some own version that directly assumes >>> certain >>> mbuf sizes, etc. A lot of work. >>> >>> For locally generated mbufs and socket buffers we try to use the >>> mbufs to >>> their maximal extent. When the socket buffer data is packetized >>> it normally >>> is referenced then we get the normal mbuf with its data portion >>> unused. So >>> that could work. >>> >>> A complication is the m_tag_free() field and function which puts >>> the memory >>> deallocation into the hands of the mtag user. That means all mtag >>> consumers >>> have to made aware of provided storage w/o having to return the >>> memory >>> directly >>> to the memory allocator (malloc/UMA). >>> >>> So the only way I realistically see is to make use of the mbuf's >>> unused >>> data portion when it has external storage to it. This should >>> probably >>> cover about 98% of all cases. The rest has to malloc() the mtag >>> storage >>> as usual. >> >> so it wouldn't be bad -- i cannot judge the numbers, but definitely >> it would work for all incoming traffic, plus all tcp data packets >> (as the payload is in the cluster), plus all pure acks (which are >> small), >> plus all UDP above some 200 bytes... > > Yes, about that. > >>> I could whip up a prototype for review in the next weeks. >> >> I seem to remember that jeffr had already something done in Perforce. > > That's a more general overhaul of the way mbuf's are structured and > allocated with UMA. I'm not sure it provides for the mtag issue. Will > check though. I'd like to see if we can go over his stuff and any other suggested changes before 9.0 and see if we can agree on a change for 9.0 Jeff, we discussed this a year ago.. do you still have your suggested changes?
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