Date: Wed, 21 Aug 2013 17:23:24 +0200 From: Andre Oppermann <andre@freebsd.org> To: Julian Elischer <julian@freebsd.org> Cc: svn-src-head@freebsd.org, freebsd-net@freebsd.org, Navdeep Parhar <np@FreeBSD.org>, Peter Grehan <grehan@freebsd.org> Subject: Re: M_NOFREE removal (was Re: svn commit: r254520 - in head/sys: kern sys) Message-ID: <5214DB6C.6060207@freebsd.org> In-Reply-To: <5212DEE7.6060804@freebsd.org> References: <201308191116.r7JBGsc6065793@svn.freebsd.org> <521256CE.6070706@FreeBSD.org> <5212587A.2080202@freebsd.org> <52128937.1010407@freebsd.org> <52129E55.30803@freebsd.org> <5212DEE7.6060804@freebsd.org>
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On 20.08.2013 05:13, Julian Elischer wrote: > On 8/20/13 6:38 AM, Peter Grehan wrote: >> Hi Andre, >> >> (moving to the more appropriate freebsd-net) >> >>> I'm sorry for ambushing but this stuff has to be done. I have provided >>> an alternative way of handling it and I'm happy to help you with your >>> use case to make it good for you and to prevent the mbuf system from >>> getting bloated and hackish again. >> >> Sure. I'm not really upset since my code wasn't too far along, but with any API, you never know >> who consumers might be so it's always worth being proactive about announcing it's removal. >> >>> Can you please describe your intended use of M_NOFREE to better understand >>> the shortcomings of the current mbuf systems and the additional advantages >>> of the M_NOFREE case? >> >> I was looking at something similar to Linux's vhost-net, where a guest's virtio ring would be >> processed in-kernel. An mbuf chain with external buffers would be used to pass guest tx buffer/len >> segments directly into FreeBSD drivers. >> >> The intent of M_NOFREE was to avoid small mbuf allocations/frees in what is a hot path. This code >> was intended to run at 10/40G. >> >> Note this code isn't really generic - it would require interfaces to be 'owned' by the guest, >> except that direct PCI-level pass-through wouldn't be needed. >> >> If there's an alternative to M_NOFREE, I'd be more than happy to use that. > > I think an alternative would be a reference counted version. we used to have that and NetBSD had a > quite sophisticated mbuf system where there were multiple owners of mbufs.. > they wouldn't be freed until the last one freed it but I don't remember the details. I just had a glance at the NetBSD mbuf system and it doesn't look convincing either. There may be some serious scalability issues with this "ownership" thing. -- Andre
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