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Date:   Mon, 29 Aug 2016 00:20:42 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org, ncardwell@...gle.com, ycheng@...gle.com
Subject: Re: [PATCH net-next] tcp: add tcp_add_backlog()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 27 Aug 2016 07:37:54 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> When TCP operates in lossy environments (between 1 and 10 % packet
> losses), many SACK blocks can be exchanged, and I noticed we could
> drop them on busy senders, if these SACK blocks have to be queued
> into the socket backlog.
> 
> While the main cause is the poor performance of RACK/SACK processing,
> we can try to avoid these drops of valuable information that can lead to
> spurious timeouts and retransmits.
> 
> Cause of the drops is the skb->truesize overestimation caused by :
> 
> - drivers allocating ~2048 (or more) bytes as a fragment to hold an
>   Ethernet frame.
> 
> - various pskb_may_pull() calls bringing the headers into skb->head
>   might have pulled all the frame content, but skb->truesize could
>   not be lowered, as the stack has no idea of each fragment truesize.
> 
> The backlog drops are also more visible on bidirectional flows, since
> their sk_rmem_alloc can be quite big.
> 
> Let's add some room for the backlog, as only the socket owner
> can selectively take action to lower memory needs, like collapsing
> receive queues or partial ofo pruning.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Really nice change, thanks Eric.

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