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tcp: prepare skbs for better sack shifting

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With large BDP TCP flows and lossy networks, it is very important
to keep a low number of skbs in the write queue.

RACK and SACK processing can perform a linear scan of it.

We should avoid putting any payload in skb->head, so that SACK
shifting can be done if needed.

With this patch, we allow to pack ~0.5 MB per skb instead of
the 64KB initially cooked at tcp_sendmsg() time.

This gives a reduction of number of skbs in write queue by eight.
tcp_rack_detect_loss() likes this.

We still allow payload in skb->head for first skb put in the queue,
to not impact RPC workloads.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2016-09-15 09:33:02 -07:00 committed by David S. Miller
parent e812bd905a
commit 3613b3dbd1
1 changed files with 24 additions and 7 deletions
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31
net/ipv4/tcp.c
View File

@ -1020,17 +1020,31 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset,
}
EXPORT_SYMBOL(tcp_sendpage);
static inline int select_size(const struct sock *sk, bool sg)
/* Do not bother using a page frag for very small frames.
* But use this heuristic only for the first skb in write queue.
*
* Having no payload in skb->head allows better SACK shifting
* in tcp_shift_skb_data(), reducing sack/rack overhead, because
* write queue has less skbs.
* Each skb can hold up to MAX_SKB_FRAGS * 32Kbytes, or ~0.5 MB.
* This also speeds up tso_fragment(), since it wont fallback
* to tcp_fragment().
*/
static int linear_payload_sz(bool first_skb)
{
if (first_skb)
return SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER);
return 0;
}
static int select_size(const struct sock *sk, bool sg, bool first_skb)
{
const struct tcp_sock *tp = tcp_sk(sk);
int tmp = tp->mss_cache;
if (sg) {
if (sk_can_gso(sk)) {
/* Small frames wont use a full page:
* Payload will immediately follow tcp header.
*/
tmp = SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER);
tmp = linear_payload_sz(first_skb);
} else {
int pgbreak = SKB_MAX_HEAD(MAX_TCP_HEADER);
@ -1161,6 +1175,8 @@ restart:
}
if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) {
bool first_skb;
new_segment:
/* Allocate new segment. If the interface is SG,
* allocate skb fitting to single page.
@ -1172,10 +1188,11 @@ new_segment:
process_backlog = false;
goto restart;
}
first_skb = skb_queue_empty(&sk->sk_write_queue);
skb = sk_stream_alloc_skb(sk,
select_size(sk, sg),
select_size(sk, sg, first_skb),
sk->sk_allocation,
skb_queue_empty(&sk->sk_write_queue));
first_skb);
if (!skb)
goto wait_for_memory;