Re: [RFC PATCH net-next] tcp: reduce cpu usage under tcp memory pressure when SO_SNDBUF is set
On 08/10/2015 10:47 AM, Eric Dumazet wrote:
On Fri, 2015-08-07 at 18:31 +, Jason Baron wrote:
From: Jason Baron jba...@akamai.com
When SO_SNDBUF is set and we are under tcp memory pressure, the effective
write
buffer space can be much lower than what was set using SO_SNDBUF. For
example,
we may have set the buffer to 100kb, but we may only be able to write 10kb.
In
this scenario poll()/select()/epoll(), are going to continuously return
POLLOUT,
followed by -EAGAIN from write() in a very tight loop.
Introduce sk-sk_effective_sndbuf, such that we can track the 'effective'
size
of the sndbuf, when we have a short write due to memory pressure. By using
the
sk-sk_effective_sndbuf instead of the sk-sk_sndbuf when we are under memory
pressure, we can delay the POLLOUT until 1/3 of the buffer clears as we
normally
do. There is no issue here when SO_SNDBUF is not set, since the tcp layer
will
auto tune the sk-sndbuf.
In my testing, this brought a single threaad's cpu usage down from 100% to 1%
while maintaining the same level of throughput when under memory pressure.
I am not sure we need to grow socket for something that looks like a
flag ?
So I added a new field because I needed to store the new 'effective'
sndbuf somewhere and then restore the original value that was set via
SO_SNDBUF. So its really b/c of SO_SNDBUF. We could perhaps use the fact
that we are in memory pressure to signal wakeups differently, but I'm
not sure exactly how.
Also you add a race in sk_stream_wspace() as sk_effective_sndbuf value
can change under us.
+ if (sk-sk_effective_sndbuf)
+ return sk-sk_effective_sndbuf - sk-sk_wmem_queued;
+
thanks. better?
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -798,8 +798,10 @@ static inline int sk_stream_min_wspace(const struct
sock *sk)
static inline int sk_stream_wspace(const struct sock *sk)
{
- if (sk-sk_effective_sndbuf)
- return sk-sk_effective_sndbuf - sk-sk_wmem_queued;
+ int effective_sndbuf = sk-sk_effective_sndbuf;
+
+ if (effective_sndbuf)
+ return effective_sndbuf - sk-sk_wmem_queued;
return sk-sk_sndbuf - sk-sk_wmem_queued;
}
Thanks,
-Jason
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Re: [RFC PATCH net-next] tcp: reduce cpu usage under tcp memory pressure when SO_SNDBUF is set
On Fri, 2015-08-07 at 18:31 +, Jason Baron wrote: From: Jason Baron jba...@akamai.com When SO_SNDBUF is set and we are under tcp memory pressure, the effective write buffer space can be much lower than what was set using SO_SNDBUF. For example, we may have set the buffer to 100kb, but we may only be able to write 10kb. In this scenario poll()/select()/epoll(), are going to continuously return POLLOUT, followed by -EAGAIN from write() in a very tight loop. Introduce sk-sk_effective_sndbuf, such that we can track the 'effective' size of the sndbuf, when we have a short write due to memory pressure. By using the sk-sk_effective_sndbuf instead of the sk-sk_sndbuf when we are under memory pressure, we can delay the POLLOUT until 1/3 of the buffer clears as we normally do. There is no issue here when SO_SNDBUF is not set, since the tcp layer will auto tune the sk-sndbuf. In my testing, this brought a single threaad's cpu usage down from 100% to 1% while maintaining the same level of throughput when under memory pressure. I am not sure we need to grow socket for something that looks like a flag ? Also you add a race in sk_stream_wspace() as sk_effective_sndbuf value can change under us. + if (sk-sk_effective_sndbuf) + return sk-sk_effective_sndbuf - sk-sk_wmem_queued; + -- To unsubscribe from this list: send the line unsubscribe netdev in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC PATCH net-next] tcp: reduce cpu usage under tcp memory pressure when SO_SNDBUF is set
On Mon, 2015-08-10 at 13:29 -0400, Jason Baron wrote:
+
thanks. better?
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -798,8 +798,10 @@ static inline int sk_stream_min_wspace(const struct
sock *sk)
static inline int sk_stream_wspace(const struct sock *sk)
{
- if (sk-sk_effective_sndbuf)
- return sk-sk_effective_sndbuf - sk-sk_wmem_queued;
+ int effective_sndbuf = sk-sk_effective_sndbuf;
+
+ if (effective_sndbuf)
+ return effective_sndbuf - sk-sk_wmem_queued;
return sk-sk_sndbuf - sk-sk_wmem_queued;
}
You need to use instead :
int effective_sndbuf = READ_ONCE(sk-sk_effective_sndbuf);
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[RFC PATCH net-next] tcp: reduce cpu usage under tcp memory pressure when SO_SNDBUF is set
From: Jason Baron jba...@akamai.com
When SO_SNDBUF is set and we are under tcp memory pressure, the effective write
buffer space can be much lower than what was set using SO_SNDBUF. For example,
we may have set the buffer to 100kb, but we may only be able to write 10kb. In
this scenario poll()/select()/epoll(), are going to continuously return POLLOUT,
followed by -EAGAIN from write() in a very tight loop.
Introduce sk-sk_effective_sndbuf, such that we can track the 'effective' size
of the sndbuf, when we have a short write due to memory pressure. By using the
sk-sk_effective_sndbuf instead of the sk-sk_sndbuf when we are under memory
pressure, we can delay the POLLOUT until 1/3 of the buffer clears as we normally
do. There is no issue here when SO_SNDBUF is not set, since the tcp layer will
auto tune the sk-sndbuf.
In my testing, this brought a single threaad's cpu usage down from 100% to 1%
while maintaining the same level of throughput when under memory pressure.
Signed-off-by: Jason Baron jba...@akamai.com
---
include/net/sock.h | 12
net/core/sock.c| 1 +
net/core/stream.c | 1 +
net/ipv4/tcp.c | 10 +++---
4 files changed, 21 insertions(+), 3 deletions(-)
diff --git a/include/net/sock.h b/include/net/sock.h
index 43c6abc..ca49415 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -380,6 +380,7 @@ struct sock {
atomic_tsk_wmem_alloc;
atomic_tsk_omem_alloc;
int sk_sndbuf;
+ int sk_effective_sndbuf;
struct sk_buff_head sk_write_queue;
kmemcheck_bitfield_begin(flags);
unsigned intsk_shutdown : 2,
@@ -779,6 +780,14 @@ static inline bool sk_acceptq_is_full(const struct sock
*sk)
return sk-sk_ack_backlog sk-sk_max_ack_backlog;
}
+static inline void sk_set_effective_sndbuf(struct sock *sk)
+{
+ if (sk-sk_wmem_queued sk-sk_sndbuf)
+ sk-sk_effective_sndbuf = sk-sk_sndbuf;
+ else
+ sk-sk_effective_sndbuf = sk-sk_wmem_queued;
+}
+
/*
* Compute minimal free write space needed to queue new packets.
*/
@@ -789,6 +798,9 @@ static inline int sk_stream_min_wspace(const struct sock
*sk)
static inline int sk_stream_wspace(const struct sock *sk)
{
+ if (sk-sk_effective_sndbuf)
+ return sk-sk_effective_sndbuf - sk-sk_wmem_queued;
+
return sk-sk_sndbuf - sk-sk_wmem_queued;
}
diff --git a/net/core/sock.c b/net/core/sock.c
index 193901d..4fce879 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2309,6 +2309,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
sk-sk_allocation = GFP_KERNEL;
sk-sk_rcvbuf = sysctl_rmem_default;
sk-sk_sndbuf = sysctl_wmem_default;
+ sk-sk_effective_sndbuf = 0;
sk-sk_state= TCP_CLOSE;
sk_set_socket(sk, sock);
diff --git a/net/core/stream.c b/net/core/stream.c
index d70f77a..7c175e7 100644
--- a/net/core/stream.c
+++ b/net/core/stream.c
@@ -32,6 +32,7 @@ void sk_stream_write_space(struct sock *sk)
if (sk_stream_is_writeable(sk) sock) {
clear_bit(SOCK_NOSPACE, sock-flags);
+ sk-sk_effective_sndbuf = 0;
rcu_read_lock();
wq = rcu_dereference(sk-sk_wq);
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 45534a5..9e7f0a5 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -845,6 +845,7 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int
size, gfp_t gfp,
sk-sk_prot-enter_memory_pressure(sk);
sk_stream_moderate_sndbuf(sk);
}
+ sk_set_effective_sndbuf(sk);
return NULL;
}
@@ -939,9 +940,10 @@ new_segment:
tcp_mark_push(tp, skb);
goto new_segment;
}
- if (!sk_wmem_schedule(sk, copy))
+ if (!sk_wmem_schedule(sk, copy)) {
+ sk_set_effective_sndbuf(sk);
goto wait_for_memory;
-
+ }
if (can_coalesce) {
skb_frag_size_add(skb_shinfo(skb)-frags[i - 1], copy);
} else {
@@ -1214,8 +1216,10 @@ new_segment:
copy = min_t(int, copy, pfrag-size - pfrag-offset);
- if (!sk_wmem_schedule(sk, copy))
+ if (!sk_wmem_schedule(sk, copy)) {
+ sk_set_effective_sndbuf(sk);
goto wait_for_memory;
+ }
err = skb_copy_to_page_nocache(sk, msg-msg_iter, skb,
pfrag-page,
--
1.8.2.rc2
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