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Re: Performanceproblem Squid with one URL - strange behaviour ...

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On 12.01.2012 04:25, Andreas Schulz wrote:
Hi folks,

we have a very special problem with our proxy environment. It concerns only ONE uri http://www.mediaassetbox.com/client/escada. Other uris are
working properly.

Unfortunately this is a very bad uri because it works only with flash. Nevertheless our customer is working with it and we have a performance
issue.

If page starts to load it need approx. 60-70 seconds until the blue
progress bar under the login field disappears.

If I use another proxy product - eg. IWSS - the page loads in about 30
seconds. Also with direct internet connection we have this value ...

So far so good - strange behaviour starts after working on the problem. Starting strace on the squid process - the performance increases to direct
internet connection speed.

Next we started debugging in squid itself - ALL,3 - without strace - the performance increases again. Starting with debug section 0 we found out that 'debug_options 5,3' (or 5,5 ...) increases the performance as fast
as a direct connection.

So doing I/O to a disk log somehow speeds up TCP throughput? strange

This sounds a bit like the speed problems we see with very low traffic rates. When the I/O loops get very few requests through they end up pausing in 10ms time chunks each processing cycle to prevent CPU overload doing lots of processing on very small amounts of bytes.

Taking a wild guess; the debug log I/O might be raising the number of total I/O being handled each second over that low-speed bump. Unlikely but possible.


What we already did without success
- disable ipv6 in os
- strip configuration to minimum
- using a cache_peer parent configuration (the IWSS proxy)
- tried to find out, which systemcalls 'increases' the squid (see
  statistics below)

Now some details about the system:
- OS - Debian Squeeze - Linux xxx 2.6.32-5-amd64 #1 SMP Thu Nov 3
03:41:26 UTC 2011 x86_64 GNU/Linux
- Squid - 3.1.6-1.2+squeeze2

This release is getting a bit old now and has a few I/O buffering bugs in it that may be related. Please try the 3.1.18 Debian package from Wheezy / testing repositories (may require some dependency updates as well).


We can also provide HTTPFox (Firefox extension) lines for fast and slow
connections.

We searched the mailing list and found
http://www.mail-archive.com/squid-users@xxxxxxxxxxxxxxx/msg33267.html
-
but there was no really helpful information. Other entries doesn't
match.

We collected the strace statistics only for this session:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 97.66    0.004015           1      3209           epoll_wait
  1.24    0.000051           0       145       145 connect

145 connect() calls in 0.05 ms, all failing? does not seem right at all.

Given the time measure I don't think its related, but probably worth knowing and fixing. Did the section 5 trace show what was going on here?


  0.71    0.000029           0       870           recvmsg
  0.22    0.000009           0       912           epoll_ctl
  0.17    0.000007           0       299           getsockname
  0.00    0.000000           0       484         2 read
  0.00    0.000000           0       494           write
  0.00    0.000000           0       444           close
  0.00    0.000000           0       435           socket
  0.00    0.000000           0        16         7 accept
  0.00    0.000000           0       290           sendto
  0.00    0.000000           0       290           bind
  0.00    0.000000           0       290           setsockopt
  0.00    0.000000           0       145           getsockopt
  0.00    0.000000           0       616           fcntl
  0.00    0.000000           0         1           getrusage
------ ----------- ----------- --------- --------- ----------------
100.00    0.004111                  8940       154 total

Our squid config:

***********
pid_filename /var/run/squid3-special.pid
http_port 8081

hierarchy_stoplist cgi-bin ?
acl QUERY urlpath_regex cgi-bin \?
no_cache deny QUERY

QUERY can die.


refresh_pattern ^ftp:        1440    20%    10080
refresh_pattern ^gopher:    1440    0%    1440

Add here:
  refresh_pattern -i (/cgi-bin/|\?)  0 0% 0

refresh_pattern .        0    20%    4320

<snip>

http_access allow manager localhost
http_access deny manager

http_access allow purge localhost
http_access deny purge

http_access deny CONNECT !SSL_ports
http_access deny !CONNECT !Safe_ports

http_access allow all

Eeek! nearly unlimited access to the whole Internet. Why?

icp_access deny all

#debug_options 5,5
***********

network layout is:

client -> firewall -> proxy -> firewall -> internet

Does anyone has an idea what could be the cause for this strange
behaviour?

--
Andreas Schulz



HTH
Amos


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