netdev - Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen

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Date:	Wed, 15 Apr 2015 10:52:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	George Dunlap <george.dunlap@...citrix.com>
Cc:	Jonathan Davies <Jonathan.Davies@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Paul Durrant <paul.durrant@...rix.com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Felipe Franciosi <felipe.franciosi@...rix.com>,
	linux-arm-kernel@...ts.infradead.org,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance
 regression on Xen

On Wed, 2015-04-15 at 18:41 +0100, George Dunlap wrote:

> So you'd be OK with a patch like this?  (With perhaps a better changelog?)
> 
>  -George
> 
> ---
> TSQ: Raise default static TSQ limit
> 
> A new dynamic TSQ limit was introduced in c/s 605ad7f18 based on the
> size of actual packets and the amount of data being transmitted.
> Raise the default static limit to allow that new limit to actually
> come into effect.
> 
> This fixes a regression where NICs with large transmit completion
> times (such as xennet) had a 30% hit unless the user manually tweaked
> the value in /proc.
> 
> Signed-off-by: George Dunlap <george.dunlap@...citrix.com>
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 1db253e..8ad7cdf 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -50,8 +50,8 @@ int sysctl_tcp_retrans_collapse __read_mostly = 1;
>   */
>  int sysctl_tcp_workaround_signed_windows __read_mostly = 0;
> 
> -/* Default TSQ limit of two TSO segments */
> -int sysctl_tcp_limit_output_bytes __read_mostly = 131072;
> +/* Static TSQ limit.  A more dynamic limit is calculated in
> tcp_write_xmit. */
> +int sysctl_tcp_limit_output_bytes __read_mostly = 1048576;
> 
>  /* This limits the percentage of the congestion window which we
>   * will allow a single TSO frame to consume.  Building TSO frames
> 

Have you tested this patch on a NIC without GSO/TSO ?

This would allow more than 500 packets for a single flow.

Hello bufferbloat.

So my answer to this patch is a no.



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