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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