netdev - Re: [PATCH] tcp: re-enable high throughput for low pacing rate

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Date:   Tue, 28 May 2019 11:40:08 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Sergej Benilov <sergej.benilov@...glemail.com>,
        davem@...emloft.net, kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: re-enable high throughput for low pacing rate



On 5/28/19 11:28 AM, Sergej Benilov wrote:
> Since commit 605ad7f184b60cfaacbc038aa6c55ee68dee3c89 "tcp: refine TSO autosizing",
> the TSQ limit is computed as the smaller of
> sysctl_tcp_limit_output_bytes and max(2 * skb->truesize, sk->sk_pacing_rate >> 10).
> For low pacing rates, this approach sets a low limit, reducing throughput dramatically.
> 
> Compute the limit as the greater of sysctl_tcp_limit_output_bytes and max(2 * skb->truesize, sk->sk_pacing_rate >> 10).
> 
> Test:
> netperf -H remote -l -2000000 -- -s 1000000
> 
> before patch:
> 
> MIGRATED TCP STREAM TEST from 0.0.0.0 () port 0 AF_INET to remote () port 0 AF_INET : demo
> Recv   Send    Send
> Socket Socket  Message  Elapsed
> Size   Size    Size     Time     Throughput
> bytes  bytes   bytes    secs.    10^6bits/sec
> 
>  87380 327680 327680    250.17      0.06
> 
> after patch:
> 
> MIGRATED TCP STREAM TEST from 0.0.0.0 () port 0 AF_INET to remote () port 0 AF_INET : demo
> Recv   Send    Send
> Socket Socket  Message  Elapsed
> Size   Size    Size     Time     Throughput
> bytes  bytes   bytes    secs.    10^6bits/sec
> 
>  87380 327680 327680    1.29       12.54
> 
> Signed-off-by: Sergej Benilov <sergej.benilov@...glemail.com>
> ---
>  net/ipv4/tcp_output.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index e625be56..71efca72 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2054,7 +2054,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
>  		 * One example is wifi aggregation (802.11 AMPDU)
>  		 */
>  		limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 10);
> -		limit = min_t(u32, limit, sysctl_tcp_limit_output_bytes);
> +		limit = max_t(u32, limit, sysctl_tcp_limit_output_bytes);
>  
>  		if (atomic_read(&sk->sk_wmem_alloc) > limit) {
>  			set_bit(TSQ_THROTTLED, &tp->tsq_flags);
> 

NACK to this patch, based on some old linux kernel versions.

The min_t() is here is really what was intended.

You might have an issue on the driver you are using.

Some wifi drivers are now setting a hint, check for sk_pacing_shift_update()

bufferbloat prevention is hard, please do not mess badly with it.