Re: [Pound Mailing List] strange connection delays

On 07/03/2013 10:25 AM, Pat Erler wrote:
> hi,
>
> we see in our new relic console from time to time periods, where there
> is a delay from the request to pound until the answer is served from the
> (IIS) webserver (between 3 and 7 seconds).
>
> a pound restart solves this.
>
> we suspect it happened the first time after we installed 2.7b (because
> it supports the neccessary security  mechanisms for the PCI security
> test). the machine itself is not the problem, RAM/Disk/CPU/network usage
> is in the single digits to max 20%. all this in ubuntu 64bit.
>
> what could we check if this happens next time?

You might see if the delays correlate with TCP retransmissions recorded 
on the client or pound server or connection request drops on the 
back-end web server.  Three and 7 seconds happen to be along the 
cumulative timeline for a contemporary Linux TCP's sequence of initial 
retransmissions:

Timeout values for successive retransmissions:
1, 2, 4

Cumulative time:
1, 3, 7

If you client kernel is older though, the RTO sequence would be
3, 6, 12 with a cumulative of 3, 9, 17

When this is happening, you might try taking a packet trace on the 
client or the pound server to be able to look at the TCP packets directly.

Here is what a sequence of TCP connection establishment packet (aka TCP 
SYNchronize segment) retransmissions looks like in tcpdump output.  The 
'S' is what identifies it as a SYN segment.  The "EW" you see there is 
simply my system having TCP's use of Explicit Congestion Notification 
enabled:

root@tardy:~# tcpdump -i eth0 host 1.2.3.4
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
15:10:12.536666 IP tardy.50331 > testname.telnet: Flags [SEW], seq 
1436714364, win 14600, options [mss 1460,sackOK,TS val 133075933 ecr 
0,nop,wscale 7], length 0
15:10:13.534479 IP tardy.50331 > testname.telnet: Flags [SEW], seq 
1436714364, win 14600, options [mss 1460,sackOK,TS val 133076183 ecr 
0,nop,wscale 7], length 0
15:10:15.538480 IP tardy.50331 > testname.telnet: Flags [SEW], seq 
1436714364, win 14600, options [mss 1460,sackOK,TS val 133076684 ecr 
0,nop,wscale 7], length 0
15:10:19.546456 IP tardy.50331 > testname.telnet: Flags [SEW], seq 
1436714364, win 14600, options [mss 1460,sackOK,TS val 133077686 ecr 
0,nop,wscale 7], length 0
15:10:27.554480 IP tardy.50331 > testname.telnet: Flags [SEW], seq 
1436714364, win 14600, options [mss 1460,sackOK,TS val 133079688 ecr 
0,nop,wscale 7], length 0

rick jones

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