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Eric Dumazet says:
====================
tcp: add rx/tx cache to reduce lock contention
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.
v2 : - Solved a race condition : sk_stream_alloc_skb() to make sure the prior
clone has been freed.
- Really test rps_needed in sk_eat_skb() as claimed.
- Fixed rps_needed use in drivers/net/tun.c
v3: Added a #ifdef CONFIG_RPS, to avoid compile error (kbuild robot)
====================
Signed-off-by: David S. Miller <davem@davemloft.net>
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Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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There are various text files in the Documentation/ subdirectory,
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Please read the Documentation/process/changes.rst file, as it contains the
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the problems which may result by upgrading your kernel.