Hello,
we managed to fix this problem by setting:

net.ipv4.tcp_syncookies=1
net.ipv4.tcp_tw_reuse=1

kernel params. tcp_syscookies is enough but since we had several
connections in TIME_WAIT, we read tcp_tw_reuse helps those be reused
more efficiently.

HTH,
Sandro

Sandro Tosi wrote:
> Hello,
> we have a rather busy Apache web server (~200/300 contemporary
> connections). There are time when Apache is really slow at letting
> client connects to it. For example, with curl, I see:
>
> 02:05:17.885074 == Info: About to connect() to IP_ADDRESS port 80 (#0)
> 02:05:17.885280 == Info:  Trying IP_ADDRESS... 02:05:20.898748 == Info:
> connected
> 02:05:20.898785 == Info: Connected to IP_ADDRESS (IP_ADDRESS) port 80 (#0)
> ...
> 02:05:20.917068 == Info: Closing connection #0
>
> and
>
> 02:06:53.098230 == Info: About to connect() to IP_ADDRESS port 80 (#0)
> 02:06:53.099272 == Info:  Trying IP_ADDRESS... 02:07:02.111596 == Info:
> connected
> 02:07:02.111636 == Info: Connected to IP_ADDRESS (IP_ADDRESS) port 80 (#0)
> 02:07:02.111731 => Send header, 222 bytes (0xde)
> ...
> 02:07:02.422093 == Info: Closing connection #0
>
> as you can see, we see a 3 seconds (first example) and a 9 seconds
> (second example) delay between apache server contact and when the
> connection is actually accepted. The delay is always either 3 or 9
> seconds, that's quite weird and it seems to indicate a sort of
> retry+backoff (3 secs, 3x2 secs (9 secs total) and so on) implemented in
> some Apache layers.
>
> Anyhow, once the client is able to connect to Apache, the request is
> served very fast. Just to be sure it's nothing in the VirtualHosts we've
> setup, we tried also with /server-status but even that page is slow at
> connect.
>
> In addition, sometimes we also see:
>
> 02:10:40.651926 == Info: About to connect() to IP_ADDRESS port 80 (#0)
> 02:10:40.652543 == Info:  Trying IP_ADDRESS... 02:11:00.060879 == Info:
> Connection refused
> 02:11:00.060924 == Info: couldn't connect to host
> 02:11:00.060942 == Info: Closing connection #0
>
> as if the client was not able to be served by any Apache server.
>
> The MPM config we use is:
>
> StartServers 200
> MinSpareServers 150
> MaxSpareServers 300
> ServerLimit 2000
> MaxClients 2000
> MaxRequestsPerChild 100
>
> KeepAlive off
>
> We tried everything that came to our minds to avoid that delay:
>
> - tune the parameters above (StartServer, Min/MaxSpareServer etc)
> - only Listen on a single port (inferred from
> http://httpd.apache.org/docs/2.2/misc/perf-tuning.html)
> - change every possible AcceptMutex values: flock, fcntl, pthread, and
> sysvsem (the default)
>
> but none of them helped.
>
> Does anyone know how to solve this problem? If there are other info
> needed, just let me now and I'll provide them.
>
> Thanks in advance,
> Sandro
>
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