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Date:	Fri, 12 Mar 2010 22:28:39 +0100
From:	Eric Dumazet <>
To:	Tom Herbert <>
Subject: Re: [PATCH v7] rps: Receive Packet Steering

Le vendredi 12 mars 2010 à 12:13 -0800, Tom Herbert a écrit :
> This patch implements software receive side packet steering (RPS).  RPS
> distributes the load of received packet processing across multiple CPUs.
> Problem statement: Protocol processing done in the NAPI context for received
> packets is serialized per device queue and becomes a bottleneck under high
> packet load.  This substantially limits pps that can be achieved on a single
> queue NIC and provides no scaling with multiple cores.
> This solution queues packets early on in the receive path on the backlog queues
> of other CPUs.   This allows protocol processing (e.g. IP and TCP) to be
> performed on packets in parallel.   For each device (or each receive queue in
> a multi-queue device) a mask of CPUs is set to indicate the CPUs that can
> process packets. A CPU is selected on a per packet basis by hashing contents
> of the packet header (e.g. the TCP or UDP 4-tuple) and using the result to index
> into the CPU mask.  The IPI mechanism is used to raise networking receive
> softirqs between CPUs.  This effectively emulates in software what a multi-queue
> NIC can provide, but is generic requiring no device support.
> Many devices now provide a hash over the 4-tuple on a per packet basis
> (e.g. the Toeplitz hash).  This patch allow drivers to set the HW reported hash
> in an skb field, and that value in turn is used to index into the RPS maps.
> Using the HW generated hash can avoid cache misses on the packet when
> steering it to a remote CPU.
> The CPU mask is set on a per device and per queue basis in the sysfs variable
> /sys/class/net/<device>/queues/rx-<n>/rps_cpus.  This is a set of canonical
> bit maps for receive queues in the device (numbered by <n>).  If a device
> does not support multi-queue, a single variable is used for the device (rx-0).
> Generally, we have found this technique increases pps capabilities of a single
> queue device with good CPU utilization.  Optimal settings for the CPU mask
> seem to depend on architectures and cache hierarcy.  Below are some results
> running 500 instances of netperf TCP_RR test with 1 byte req. and resp.
> Results show cumulative transaction rate and system CPU utilization.
> e1000e on 8 core Intel
>    Without RPS: 108K tps at 33% CPU
>    With RPS:    311K tps at 64% CPU
> forcedeth on 16 core AMD
>    Without RPS: 156K tps at 15% CPU
>    With RPS:    404K tps at 49% CPU
> bnx2x on 16 core AMD
>    Without RPS  567K tps at 61% CPU (4 HW RX queues)
>    Without RPS  738K tps at 96% CPU (8 HW RX queues)
>    With RPS:    854K tps at 76% CPU (4 HW RX queues)
> Caveats:
> - The benefits of this patch are dependent on architecture and cache hierarchy.
> Tuning the masks to get best performance is probably necessary.
> - This patch adds overhead in the path for processing a single packet.  In
> a lightly loaded server this overhead may eliminate the advantages of
> increased parallelism, and possibly cause some relative performance degradation.
> We have found that masks that are cache aware (share same caches with
> the interrupting CPU) mitigate much of this.
> - The RPS masks can be changed dynamically, however whenever the mask is changed
> this introduces the possibility of generating out of order packets.  It's
> probably best not change the masks too frequently.
> Signed-off-by: Tom Herbert <>
>  include/linux/netdevice.h |   32 ++++-
>  include/linux/skbuff.h    |    3 +
>  net/core/dev.c            |  330 +++++++++++++++++++++++++++++++++++++-------
>  net/core/net-sysfs.c      |  225 ++++++++++++++++++++++++++++++-
>  net/core/skbuff.c         |    2 +
>  5 files changed, 536 insertions(+), 56 deletions(-)

Excellent !

Signed-off-by: Eric Dumazet <>

One last point about placement of rxhash in struct sk_buff, that I
missed in my previous review, sorry...

You put it right before cb[48] which is now aligned to 8 bytes (since
commit da3f5cf1 skbuff: align sk_buff::cb to 64 bit and close some
potential holes), so this adds a 4 bytes hole.

Please put it elsewhere, possibly close to fields that are read in
get_rps_cpu() (skb->queue_mapping, skb->protocol, skb->data, ...) to
minimize number of cache lines that dispatcher cpu has to bring into its
cache, before giving skb to another cpu for IP/TCP processing.

Thanks !

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