Date: Tue, 08 Jul 2014 06:31:47 +0200 From: Hans Petter Selasky <hps@selasky.org> To: pyunyh@gmail.com Cc: freebsd-net@freebsd.org, freebsd-current@FreeBSD.org Subject: Re: [RFC] Allow m_dup() to use JUMBO clusters Message-ID: <53BB7433.2010306@selasky.org> In-Reply-To: <20140708021439.GA3965@michelle.fasterthan.com> References: <53BA5657.8010309@selasky.org> <20140708021439.GA3965@michelle.fasterthan.com>
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On 07/08/14 04:14, Yonghyeon PYUN wrote: > On Mon, Jul 07, 2014 at 10:12:07AM +0200, Hans Petter Selasky wrote: >> Hi, >> >> I'm asking for some input on the attached m_dup() patch, so that >> existing functionality or dependencies are not broken. The background >> for the change is to allow m_dup() to defrag long mbuf chains that >> doesn't fit into a specific hardware's scatter gather entries, typically >> when doing TSO. >> >> In my case the HW limit is 16 entries of length 4K for doing a 64KByte > > I wonder how HW can handle a full-sized TSO packet(64KB + Ethernet > header + VLAN tag). > >> TSO packet. Currently m_dup() is at best producing 32 entries of each 2K >> for a 64Kbytes TSO packet. >> >> By allowing m_dup() to get JUMBO clusters when allocating mbufs, we >> avoid creating a new function, specific to the hardware, to defrag some >> rare-occurring very long mbuf chains into a mbuf chain below 16 entries. >> > > I think m_dup() was used to get a copy of writable mbuf chains. If > m_dup() starts to allocate jumbo mbufs it will eventually fail on > long running boxes. This will break firewall(ipfw divert, pf/ipf > dup-to) rules and several ethernet drivers. > > I don't know how many TSO requests could be queued by HW but if the > number is very small, the driver may be able to pre-allocate that > number of buffers (N * (64KB + Ethernet header + VLAN tag)) in > driver. Upper stack will almost always generate more than 16 mbufs > for TSO packets. When driver knows the length of mbuf chain of TSO > packet is more than 16, you can copy the mbuf chain to the > pre-allocated buffer. > > I recall I didn't implement TSO on txp(4) because the firmware of > txp(4) controller does not support more than 16 fragment > descriptors. Hi, Would it be better if my patch used the PAGE_SIZE clusters instead of the 16K ones? Then it should not be affected by memory defragmentation. Thanks for shedding some light into this area? --HPS
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