On 04/30/2018 09:36 AM, Eric Dumazet wrote:
>
>
> On 04/30/2018 09:14 AM, Ben Greear wrote:
>> On 04/27/2018 08:11 PM, Steven Rostedt wrote:
>>>
>>> We'd like this email archived in netdev list, but since netdev is
>>> notorious for blocking outlook email as spam, it didn't go through. So
>>> I'm replying here to help get it into the archives.
>>>
>>> Thanks!
>>>
>>> -- Steve
>>>
>>>
>>> On Fri, 27 Apr 2018 23:05:46 +0000
>>> Michael Wenig <mwe...@vmware.com> wrote:
>>>
>>>> As part of VMware's performance testing with the Linux 4.15 kernel,
>>>> we identified CPU cost and throughput regressions when comparing to
>>>> the Linux 4.14 kernel. The impacted test cases are mostly TCP_STREAM
>>>> send tests when using small message sizes. The regressions are
>>>> significant (up 3x) and were tracked down to be a side effect of Eric
>>>> Dumazat's RB tree changes that went into the Linux 4.15 kernel.
>>>> Further investigation showed our use of the TCP_NODELAY flag in
>>>> conjunction with Eric's change caused the regressions to show and
>>>> simply disabling TCP_NODELAY brought performance back to normal.
>>>> Eric's change also resulted into significant improvements in our
>>>> TCP_RR test cases.
>>>>
>>>>
>>>>
>>>> Based on these results, our theory is that Eric's change made the
>>>> system overall faster (reduced latency) but as a side effect less
>>>> aggregation is happening (with TCP_NODELAY) and that results in lower
>>>> throughput. Previously even though TCP_NODELAY was set, system was
>>>> slower and we still got some benefit of aggregation. Aggregation
>>>> helps in better efficiency and higher throughput although it can
>>>> increase the latency. If you are seeing a regression in your
>>>> application throughput after this change, using TCP_NODELAY might
>>>> help bring performance back however that might increase latency.
>>
>> I guess you mean _disabling_ TCP_NODELAY instead of _using_ TCP_NODELAY?
>>
>
> Yeah, I guess auto-corking does not work as intended.
I would try the following patch :
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index
44be7f43455e4aefde8db61e2d941a69abcc642a..c9d00ef54deca15d5760bcbe154001a96fa1e2a7
100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -697,7 +697,7 @@ static bool tcp_should_autocork(struct sock *sk, struct
sk_buff *skb,
{
return skb->len < size_goal &&
sock_net(sk)->ipv4.sysctl_tcp_autocorking &&
- skb != tcp_write_queue_head(sk) &&
+ !tcp_rtx_queue_empty(sk) &&
refcount_read(&sk->sk_wmem_alloc) > skb->truesize;
}
