netdev - Re: [PATCH] net-tcp: extend tcp_tw_reuse sysctl to enable loopback only optimization

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Message-ID: <c7f41437-6184-1552-9587-80fe16240cfa@gmail.com>
Date:   Mon, 4 Jun 2018 10:18:03 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>,
        Maciej Żenczykowski <maze@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     netdev@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>, Wei Wang <weiwan@...gle.com>
Subject: Re: [PATCH] net-tcp: extend tcp_tw_reuse sysctl to enable loopback
 only optimization



On 06/03/2018 10:41 AM, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <maze@...gle.com>
> 
> This changes the /proc/sys/net/ipv4/tcp_tw_reuse from a boolean
> to an integer.
> 
> It now takes the values 0, 1 and 2, where 0 and 1 behave as before,
> while 2 enables timewait socket reuse only for sockets that we can
> prove are loopback connections:
>   ie. bound to 'lo' interface or where one of source or destination
>   IPs is 127.0.0.0/8, ::ffff:127.0.0.0/104 or ::1.
> 
> This enables quicker reuse of ephemeral ports for loopback connections
> - where tcp_tw_reuse is 100% safe from a protocol perspective
> (this assumes no artificially induced packet loss on 'lo').
> 
> This also makes estblishing many loopback connections *much* faster
> (allocating ports out of the first half of the ephemeral port range
> is significantly faster, then allocating from the second half)
> 
> Without this change in a 32K ephemeral port space my sample program
> (it just establishes and closes [::1]:ephemeral -> [::1]:server_port
> connections in a tight loop) fails after 32765 connections in 24 seconds.
> With it enabled 50000 connections only take 4.7 seconds.
> 
> This is particularly problematic for IPv6 where we only have one local
> address and cannot play tricks with varying source IP from 127.0.0.0/8
> pool.
> 
> Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Wei Wang <weiwan@...gle.com>

This seems fine, thanks Maciej

Signed-off-by: Eric Dumazet <edumazet@...gle.com>