Re: Persistent Connection Issue

On Thu, 2006-04-13 at 15:22 -0700, Isaac Cheung wrote:
 
> I finally got persistent connections showing up after I specified
> http_versions = ["1.0"] under the definitions of the server and robot
> in the workload.
>
> Interestingly, when I specified http_versions=["1.1"] for both,
> persistent connections would never be established. Using tcpflow, i
> see that the robot sends out HTTP1.1 header with a "Proxy-Connection:
> close". And, the Server responded with a HTTP1.0 header with a
> "Proxy-Connection: close" (as expected.).

Understood.

> So there are really 2 problems:
>
> 1. Server was using HTTP1.0 even when it had been instructed to use
> 1.1.

This, and the "close" option on the server side could be due to your
proxy sending HTTP/1.0 requests to the server, without the "keep-alive"
connection option. Recall that persistency is managed hop-by-hop and not
end-to-end. Do you know what request the proxy was sending to the
Polygraph server (protocol version and Connection header)?

> 2. HTTP1.1 sent a "Proxy-Connection: close" in the header when
> persistent connection is enabled.

That does not sound good indeed! I would like to investigate this
further.

Bugs notwithstanding, I am not sure what could have caused that unless
you were looking at the last request on a connection: About 25% of
connections will have just one request because of the zipf(16)
distribution properties. Also, all connections will have "Connection:
close" for the last request. The average connection "length" should be
about 5 requests. Thus, if my math is right, the chance that a given
request is the last request on a connection is about 40%, even if the
proxy never closes the connection "prematurely".

BTW, the command below will show you the distribution histogram:

        distr_test --distr 'zipf(16)'

If you have a tcpdump or tcpflow output saved (showing a few complete
connections with multiple requests and responses on all sides) or if you
can make a new one, please send it to me privately along with your
workload file.

Thank you,

Alex.

> Alex Rousskov <rousskov@measurement-factory.com> wrote:
> On Fri, 2006-04-07 at 12:02 -0700, Isaac Cheung wrote:
> > I have previously executed workload with polygraph 2.8.1 on
> Linux
> > 2.4.21-40 having pconn_use_lmt set to zipf(16). But, I found
> out from
> > the SQUID persistent conn utilitization histrograms that
> there were
> > only 1req connections accepted meaning there were no
> persistent conn
> > established.
> >
> > To validate the proxy setup, I further tested it with a
> connected
> > browser and persistent connections were indeed established.
> >
> > With pconn_use_lmt set to zipf(16) and http_versions set to
> the
> > default are there other parameters that I need to set in
> order to
> > enable pconn?
>
> I do not think so. There is also a related persistent
> connection timeout
> parameter (idle_pconn_tout), but it is optional. I would do
> the
> following:
>
> 1. Check that you are setting pconn_use_lmt on the right
> side(s). The
> option is valid for Polygraph robots and servers.
>
> 2. Grab a few HTTP requests and responses (e.g., via
> tcpdump/show or
> ethereal). Check whether Polygraph is _not_ sending
> "Connection: close"
> HTTP header in the HTTP/1.1 request. Check that Squid is
> sending
> "Connection: keep-alive" in the HTTP/1.0 response. If the HTTP
> versions
> or message headers differ from the expected ones, we may be
> able to find
> the culprit.
>
> > Or are there other conditions that will cause persistent
> connections
> > not to be used?
>
> On Polygraph side, errors and similar abnormal conditions
> close a
> persistent connection. The server (including Squid) can signal
> connection close by sending "Connection: close" in an HTTP/1.1
> response
> or sending nothing in an HTTP/1.0 response. On Squid side,
> there may be
> many other factors (e.g., load).
>
> Sorry for the delayed response. Please keep me posted on your
> progress.
>
> Thank you,
>
> Alex.
>
>
>
>
>
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Received on Thu Apr 13 18:24:36 2006