Re: [RFC] Patch to add Software/Generic Segmentation Offload (GSO) support in FreeBSD

From: Adrian Chadd <adrian_at_freebsd.org>
Date: Wed, 17 Sep 2014 10:27:51 -0700
Hi!

Cool!

How many flows were you testing with? Just one or two?

It's for outbound, so it's not _as_ big a deal as it is for inbound,
but it'd still be nice to know.


-a


On 17 September 2014 01:27, Stefano Garzarella
<stefanogarzarella_at_gmail.com> wrote:
> Hi all,
> I have recently worked, during my master’s thesis with the supervision
> of Prof. Luigi Rizzo, on a project to add GSO (Generic Segmentation
> Offload) support in FreeBSD. I will present this project at EuroBSDcon
> 2014, in Sofia (Bulgaria) on September 28, 2014.
>
> Following is a brief description of our project:
>
> The use of large frames makes network communication much less
> demanding for the CPU. Yet, backward compatibility and slow links
> requires the use of 1500 byte or smaller frames.  Modern NICs with
> hardware TCP segmentation offloading (TSO) address this problem.
> However, a generic software version (GSO) provided by the OS has
> reason to exist, for use on paths with no suitable hardware, such
> as between virtual machines or with older or buggy NICs.
>
> Much of the advantage of TSO comes from crossing the network stack only
> once per (large) segment instead of once per 1500-byte frame.
> GSO does the same both for segmentation (TCP) and fragmentation (UDP)
> by doing these operations as late as possible. Ideally, this could be done
> within the device driver, but that would require modifications to all
> drivers.
> A more convenient, similarly effective approach is to segment
> just before the packet is passed to the driver (in ether_output()).
>
> Our preliminary implementation supports TCP and UDP on IPv4/IPv6;
> it only intercepts packets large than the MTU (others are left unchanged),
> and only when GSO is marked as enabled for the interface.
>
> Segments larger than the MTU are not split in tcp_output(),
> udp_output(), or ip_output(), but marked with a flag (contained in
> m_pkthdr.csum_flags), which is processed by ether_output() just
> before calling the device driver.
>
> ether_output(), through gso_dispatch(), splits the large frame as needed,
> creating headers and possibly doing checksums if not supported by
> the hardware.
>
> In experiments agains an LRO-enabled receiver (otherwise TSO/GSO
> are ineffective) we have seen the following performance,
> taken at different clock speeds (because at top speeds the
> 10G link becomes the bottleneck):
>
>
>     Testing enviroment (all with Intel 10Gbit NIC)
>     Sender: FreeBSD 11-CURRENT - CPU i7-870 at 2.93 GHz + Turboboost
>     Receiver: Linux 3.12.8 - CPU i7-3770K at 3.50GHz + Turboboost
>     Benchmark tool: netperf 2.6.0
>
>     --- TCP/IPv4 packets (checksum offloading enabled) ---
>     Freq.      TSO       GSO     none     Speedup
>     [GHz]     [Gbps]   [Gbps]   [Gbps]   GSO-none
>     2.93       9347      9298      8308     12 %
>     2.53       9266      9401      6771     39 %
>     2.00       9408      9294      5499     69 %
>     1.46       9408      8087      4075     98 %
>     1.05       9408      5673      2884     97 %
>     0.45       6760      2206      1244     77 %
>
>
>     --- TCP/IPv6 packets (checksum offloading enabled) ---
>     Freq.      TSO       GSO     none     Speedup
>     [GHz]     [Gbps]   [Gbps]   [Gbps]   GSO-none
>     2.93       7530      6939      4966     40 %
>     2.53       5133      7145      4008     78 %
>     2.00       5965      6331      3152    101 %
>     1.46       5565      5180      2348    121 %
>     1.05       8501      3607      1732    108 %
>     0.45       3665      1505        651    131 %
>
>
>     --- UDP/IPv4 packets (9K) ---
>     Freq.      GSO      none     Speedup
>     [GHz]     [Gbps]   [Gbps]   GSO-none
>     2.93       9440      8084     17 %
>     2.53       7772      6649     17 %
>     2.00       6336      5338     19 %
>     1.46       4748      4014     18 %
>     1.05       3359      2831     19 %
>     0.45       1312      1120     17 %
>
>
>     --- UDP/IPv6 packets (9K) ---
>     Freq.      GSO      none     Speedup
>     [GHz]     [Gbps]   [Gbps]   GSO-none
>     2.93       7281      6197     18 %
>     2.53       5953      5020     19 %
>     2.00       4804      4048     19 %
>     1.46       3582      3004     19 %
>     1.05       2512      2092     20 %
>     0.45         998        826     21 %
>
> We tried to change as little as possible the network stack to add
> GSO support. To avoid changing API/ABI, we temporarily used spare
> fields in struct tcpcb (TCP Control Block) and struct ifnet to store
> some information related to GSO (enabled, max burst size, etc.).
> The code that performs the segmentation/fragmentation is contained
> in the file gso.[h|c] in sys/net.  We used 4 bit in m_pkthdr.csum_flags
> (CSUM_GSO_MASK) to encode the packet type (TCP/IPv4, TCP/IPv6, etc)
> to prevent access to the TCP/IP/Ethernet headers of each packet.
> In ether_output_frame(), if the packet requires the GSO
> ((m->m_pkthdr.csum_flags & CSUM_GSO_MASK) != 0), it is segmented
> or fragmented, and then they are sent to the device driver.
>
> At https://github.com/stefano-garzarella/freebsd-gso
> you can find the kernel patches for FreeBSD-current, FreeBSD
> 10-stable, FreeBSD 9-stable, a simple application (gso-stats.c)
> that prints the GSO statistics and picobsd images with GSO support.
>
> At https://github.com/stefano-garzarella/freebsd-gso-src
> you can get the FreeBSD source with GSO patch (various branch for
> FreeBSD current, 10-stable, 9-stable).
>
> Any feedbacks, comments, questions are welcome.
>
> Thank you very much,
> Stefano Garzarella
>
> ------------------------------------------------------------------------------------------------------------
> How to use GSO:
>
> - Apply the right kernel patch.
>
> - To compile the GSO support add ‘ options GSO ' to your kernel config file
> and
>    rebuild a kernel.
>
> - To manage the GSO parameters there are some sysctls:
>      - net.inet.tcp.gso - GSO enable on TCP communications (!=0)
>      - net.inet.udp.gso - GSO enable on UDP communications (!=0)
>
>      - for each interface:
>           - net.gso.dev."ifname”.max_burst - GSO burst length limit
>                 [default: IP_MAXPACKET=65535]
>           - net.gso.dev."ifname”.enable_gso - GSO enable on “ifname”
> interface (!=0)
>
> - To show statistics:
>      - make sure that the GSO_STATS macro is defined in sys/net/gso.h
>      - use the simple gso-stats.c application to access the sysctl
> net.gso.stats
>        that contains the address of the gsostats structure (defined in
> gso.h)
>        which records the statistics.  (compile with
> -I/path/to/kernel/src/patched/)
> ------------------------------------------------------------------------------------------------------------
>
> --
> *Stefano Garzarella*
> stefano.garzarella_at_gmail.com
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Received on Wed Sep 17 2014 - 15:27:54 UTC

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