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Date: Fri, 8 Mar 2024 17:24:07 +0100
From: Stefano Brivio <sbrivio@redhat.com>
To: Laurent Vivier <lvivier@redhat.com>
Subject: Re: [PATCH 0/4] Some improvements to the tap send path
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CC: David Gibson <david@gibson.dropbear.id.au>, passt-dev@passt.top
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On Fri, 8 Mar 2024 16:49:20 +0100
Laurent Vivier <lvivier@redhat.com> wrote:
> On 3/8/24 09:34, Stefano Brivio wrote:
> > On Fri, 8 Mar 2024 09:18:48 +0100
> > Laurent Vivier <lvivier@redhat.com> wrote:
> >
> >> On 3/8/24 07:53, David Gibson wrote:
> >>> This series has a handful of small improvements to the tap send path.
> >>> See individual commit messages for the details.
> >>>
> >>> I expect this will conflict with Laurent's upcoming work. I hope the
> >>> conflicts won't be too bad, and indeed will set us up for less
> >>> duplication there in the end.
> >>
> >> I'm working on patch that devides TCP buffers in several buffers pointed out by an IOV
> >> arrays and then provided to tap_send_frames(). I'm going to base my patch on this series.
> >>
> >> The idea is:
> >>
> >> A frame is made with 4 iovecs:
> >>
> >> #define TCP_IOV_VNET 0
> >> #define TCP_IOV_ETH 1
> >> #define TCP_IOV_IP 2
> >> #define TCP_IOV_PAYLOAD 3
> >> #define TCP_IOV_NUM 4
> >> typedef struct iovec tap_iovec_t[TCP_IOV_NUM];
> >
> > Except for the typedef :) (I'm actively trying to discourage them)
> > ...this looks very neat.
> >
> > I would suggest that as soon as you have something barely spitting out
> > pseudo-correct bytes, you give it a try with perf(1). I'm quite
> > concerned that we might end up killing throughput, especially without
> > vhost-user, even though in theory it all sounds nice and byte-saving.
> >
>
> Using iovec improves performance:
>
> iperf3 -c localhost -p 10001 -t 60 -4
>
> iovec
>
> [ 5] 0.00-60.00 sec 32.9 GBytes 4.72 Gbits/sec 0 sender
> [ 5] 0.00-60.06 sec 32.9 GBytes 4.71 Gbits/sec receiver
>
> buf_t
>
> [ 5] 0.00-60.00 sec 15.9 GBytes 2.27 Gbits/sec 0 sender
> [ 5] 0.00-60.06 sec 15.9 GBytes 2.27 Gbits/sec receiver
!
...it must be some alignment matter. Nice. I guess it doesn't happen
with large windows (-w ...) or messages (-l 1M), but it looks promising
enough with small ones.
--
Stefano