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Date:   Sat, 23 Mar 2019 21:58:06 -0400 (EDT)
From:   David Miller <>
Subject: Re: [PATCH v3 net-next 0/3] tcp: add rx/tx cache to reduce lock

From: Eric Dumazet <>
Date: Fri, 22 Mar 2019 08:56:37 -0700

> On hosts with many cpus we can observe a very serious contention
> on spinlocks used in mm slab layer.
> The following can happen quite often :
> 1) TX path
>   sendmsg() allocates one (fclone) skb on CPU A, sends a clone.
>   ACK is received on CPU B, and consumes the skb that was in the retransmit
>   queue.
> 2) RX path
>   network driver allocates skb on CPU C
>   recvmsg() happens on CPU D, freeing the skb after it has been delivered
>   to user space.
> In both cases, we are hitting the asymetric alloc/free pattern
> for which slab has to drain alien caches. At 8 Mpps per second,
> this represents 16 Mpps alloc/free per second and has a huge penalty.
> In an interesting experiment, I tried to use a single kmem_cache for all the skbs
> (in skb_init() : skbuff_fclone_cache = skbuff_head_cache =
>                   kmem_cache_create("skbuff_fclone_cache", sizeof(struct sk_buff_fclones),);
> qnd most of the contention disappeared, since cpus could better use
> their local slab per-cpu cache.
> But we can do actually better, in the following patches.
> TX : at ACK time, no longer free the skb but put it back in a tcp socket cache,
>      so that next sendmsg() can reuse it immediately.
> RX : at recvmsg() time, do not free the skb but put it in a tcp socket cache
>    so that it can be freed by the cpu feeding the incoming packets in BH.
> This increased the performance of small RPC benchmark by about 10 % on a host
> with 112 hyperthreads.


Series applied, thanks!

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