netdev - Re: [PATCH net-next v3] net: use a per task frag allocator

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Message-Id: <20120924.163946.1641999339438679497.davem@davemloft.net>
Date:	Mon, 24 Sep 2012 16:39:46 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	subramanian.vijay@...il.com, netdev@...r.kernel.org,
	bhutchings@...arflare.com, alexander.h.duyck@...el.com
Subject: Re: [PATCH net-next v3] net: use a per task frag allocator

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 24 Sep 2012 11:04:42 +0200

> From: Eric Dumazet <edumazet@...gle.com>
> 
> We currently use a per socket order-0 page cache for tcp_sendmsg()
> operations.
> 
> This page is used to build fragments for skbs.
> 
> Its done to increase probability of coalescing small write() into
> single segments in skbs still in write queue (not yet sent)
> 
> But it wastes a lot of memory for applications handling many mostly
> idle sockets, since each socket holds one page in sk->sk_sndmsg_page
> 
> Its also quite inefficient to build TSO 64KB packets, because we need
> about 16 pages per skb on arches where PAGE_SIZE = 4096, so we hit
> page allocator more than wanted.
> 
> This patch adds a per task frag allocator and uses bigger pages,
> if available. An automatic fallback is done in case of memory pressure.
> 
> (up to 32768 bytes per frag, thats order-3 pages on x86)
> 
> This increases TCP stream performance by 20% on loopback device,
> but also benefits on other network devices, since 8x less frags are
> mapped on transmit and unmapped on tx completion. Alexander Duyck
> mentioned a probable performance win on systems with IOMMU enabled.
> 
> Its possible some SG enabled hardware cant cope with bigger fragments,
> but their ndo_start_xmit() should already handle this, splitting a
> fragment in sub fragments, since some arches have PAGE_SIZE=65536
> 
> Successfully tested on various ethernet devices.
> (ixgbe, igb, bnx2x, tg3, mellanox mlx4)
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

I'm going to apply this, nice work Eric.

I'll also take care of the trailing whitespace pointed out by others.

Thanks again.
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