On 3/22/2019 5:56 PM, Eric Dumazet wrote:
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.
Hi Eric,
Does this have any effect on non tcp traffic?
