netdev - Re: [PATCH net-next] tcp: rcvbuf autotuning improvements

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Date:	Thu, 03 Oct 2013 06:03:01 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Francesco Fusco <ffusco@...hat.com>,
	Michael Dalton <mwdalton@...gle.com>, ycheng@...gle.com,
	ncardwell@...gle.com
Subject: Re: [PATCH net-next] tcp: rcvbuf autotuning improvements

On Thu, 2013-10-03 at 09:56 +0200, Daniel Borkmann wrote:
> This is a complementary patch for commit 6ae705323 ("tcp: sndbuf
> autotuning improvements") that fixes a performance regression on
> receiver side in setups with low to mid latency, high throughput,
> and senders with TSO/GSO off (receivers w/ default settings).
> 
> The following measurements in Mbit/s were done for 60sec w/ netperf
> on virtio w/ TSO/GSO off:
> 
> (ms)    1)              2)              3)
>   0     2762.11         1150.32         2906.17
>  10     1083.61          538.89         1091.03
>  25      471.81          313.18          474.60
>  50      242.33          187.84          242.36
>  75      162.14          134.45          161.95
> 100      121.55          101.96          121.49
> 150       80.64           57.75           80.48
> 200       58.97           54.11           59.90
> 250       47.10           46.92           47.31
> 
> Same setup w/ TSO/GSO on:
> 
> (ms)    1)              2)              3)
>   0     12225.91        12366.89        16514.37
>  10      1526.64         1525.79         2176.63
>  25       655.13          647.79          871.52
>  50       338.51          377.88          439.46
>  75       246.49          278.46          295.62
> 100       210.93          207.56          217.34
> 150       127.88          129.56          141.33
> 200        94.95           94.50          107.29
> 250        67.39           73.88           88.35
> 
> Similarly as in 6ae705323, we fixed up power-of-two rounding and
> took cached mss into account, thus bringing per_mss calculations
> closer to each other, the rest stays as is.
> 
> We also renamed tcp_fixup_rcvbuf() to tcp_rcvbuf_expand() to be
> consistent with tcp_sndbuf_expand().
> 
> While we do think that 6ae705323b71 is the right way to go, also
> this follow-up seems necessary to restore performance for
> receivers.

Hmm, I think you based this patch on some virtio requirements.

I would rather fix virtio, because virtio has poor truesize/payload
ratio.

Michael Dalton is working on this right now.

Really I don't understand how 'fixing' initial rcvbuf could explain such
difference in a 60 second transfert.

Normally, if autotuning was working, the first sk_rcvbuf value would
only matter in the very beginning of a flow (maybe one, two or even
three RTT)

It looks like you only need to set sk_rcvbuf to tcp_rmem[2],
so you probably have to fix the autotuning, or virtio to give normal
skbs, not fat ones ;)


Thanks


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