Date: Fri, 13 Sep 2013 18:43:23 -0400 (EDT) From: Rick Macklem <rmacklem@uoguelph.ca> To: George Neville-Neil <gnn@neville-neil.com> Cc: "Alexander V. Chernikov" <melifaro@yandex-team.ru>, Luigi Rizzo <luigi@freebsd.org>, Andre Oppermann <andre@freebsd.org>, freebsd-hackers@freebsd.org, FreeBSD Net <net@freebsd.org>, Adrian Chadd <adrian@freebsd.org>, "Andrey V. Elsukov" <ae@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: Network stack changes Message-ID: <221093226.23439826.1379112203059.JavaMail.root@uoguelph.ca> In-Reply-To: <6BDA4619-783C-433E-9819-A7EAA0BD3299@neville-neil.com>
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George Neville-Neil wrote: > > On Aug 29, 2013, at 7:49 , Adrian Chadd <adrian@freebsd.org> wrote: > > > Hi, > > > > There's a lot of good stuff to review here, thanks! > > > > Yes, the ixgbe RX lock needs to die in a fire. It's kinda pointless > > to keep > > locking things like that on a per-packet basis. We should be able > > to do > > this in a cleaner way - we can defer RX into a CPU pinned taskqueue > > and > > convert the interrupt handler to a fast handler that just schedules > > that > > taskqueue. We can ignore the ithread entirely here. > > > > What do you think? > > > > Totally pie in the sky handwaving at this point: > > > > * create an array of mbuf pointers for completed mbufs; > > * populate the mbuf array; > > * pass the array up to ether_demux(). > > > > For vlan handling, it may end up populating its own list of mbufs > > to push > > up to ether_demux(). So maybe we should extend the API to have a > > bitmap of > > packets to actually handle from the array, so we can pass up a > > larger array > > of mbufs, note which ones are for the destination and then the > > upcall can > > mark which frames its consumed. > > > > I specifically wonder how much work/benefit we may see by doing: > > > > * batching packets into lists so various steps can batch process > > things > > rather than run to completion; > > * batching the processing of a list of frames under a single lock > > instance > > - eg, if the forwarding code could do the forwarding lookup for 'n' > > packets > > under a single lock, then pass that list of frames up to > > inet_pfil_hook() > > to do the work under one lock, etc, etc. > > > > Here, the processing would look less like "grab lock and process to > > completion" and more like "mark and sweep" - ie, we have a list of > > frames > > that we mark as needing processing and mark as having been > > processed at > > each layer, so we know where to next dispatch them. > > > > One quick note here. Every time you increase batching you may > increase bandwidth > but you will also increase per packet latency for the last packet in > a batch. > That is fine so long as we remember that and that this is a tuning > knob > to balance the two. > And any time you increase latency, that will have a negative impact on NFS performance. NFS RPCs are usually small messages (except Write requests and Read replies) and the RTT for these (mostly small, bidirectional) messages can have a significant impact on NFS perf. rick > > I still have some tool coding to do with PMC before I even think > > about > > tinkering with this as I'd like to measure stuff like per-packet > > latency as > > well as top-level processing overhead (ie, > > CPU_CLK_UNHALTED.THREAD_P / > > lagg0 TX bytes/pkts, RX bytes/pkts, NIC interrupts on that core, > > etc.) > > > > This would be very useful in identifying the actual hot spots, and > would be helpful > to anyone who can generate a decent stream of packets with, say, an > IXIA. > > Best, > George > > >
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