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SubjectRe: [PATCH net-next v1] net: use a per task frag allocator
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 19 Sep 2012 18:56:01 +0200

> From: Eric Dumazet <edumazet@google.com>
>
> We currently use a per socket page reserve 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 packets of 64KB, because we need
> about 16 pages per skb on arches where PAGE_SIZE = 4096, so we hit
> page allocator more than wanted.
>
> This patch switches this frag allocator from socket to task structure,
> and uses bigger pages.
>
> (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.
>
> Its possible some TSO 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)
>
> Followup patches can use this infrastructure in two other spots
> and get rid of the socket sk_sndmsg_page.
>
> Open for discussion : Should we fallback to smaller pages
> if order-3 page allocations fail ?
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

I like this a lot and I look forward to your upcoming changes to
convert the other two sk_sndmsg_page users as well, but I can't
apply this to net-next just yet.

The question on fallback is a good one and something we have
to resolve before applying this.

Note in particular that sk_allocation can be set to just about
anything, and this also has potential interaction issues with
SOCK_MEMALLOC.


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