Re: [squid-users] Caching in forward proxy mode

On 24/04/2012 10:32 p.m., Ahmed Talha Khan wrote:
> Hey All,
>
> Does squid cache web-pages when configured in the forward interception
> proxy mode i.e http_port x:y intercept.? If so how can it be turned
> off.?

"cache deny all" does what you ask.

>
> I actually want to bench-mark squid performance in terms of
> requests-per-second that squid can support.

Disabling caching will not tell you that. It will tell you the average
speed of Squid under DoS or worst-case traffic conditions. ie the
minimum capacity.

> I do not want squid to
> cache the web pages on which i am doing the test. The reason is
> obvious that i do not want squid to service that page from its
> cache.:). I want it to get the pages from the server every time. I am
> using AB(apache-benchmark) to do a comparison between request process
> times when squid is inline vs when squid is not there. Looking at the
> access.log file gives this for my test page.
>
> 1335266704.412 0 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
> 1335266704.415 0 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
> 1335266704.416 1 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
> 1335266704.418 1 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
> 1335266704.418 0 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
> 1335266704.420 0 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
> 1335266704.423 0 192.168.8.39 TCP_REFRESH_UNMODIFIED/200 601 GET
> http://158.1.4.2/elastica.html - DIRECT/158.1.4.2 text/html
>
>
>
> The TCP_REFRESH_UNMODIFIED request status makes me think that squid is
> serving me the page from its cache and not retrieving it from its
> actual source. Am i right?

Yes and no.
  REFRESH --> origin has been contacted to ensure the response is fresh.
  UNMODIFIED --> the origin said no change.
  200 --> full cached object was returned to client.

Squid is both servicing out of cache, AND contacting the origin as you
require. All that is being optimized there is that the body data is not
taking up bandwidth (and byte transfer time) between origin and Squid.

Performance and capacity of a proxy is not as simple as how much lag it
adds. A proxy can both add or subtract lag, in multiple diffferent ways.

You can break down the tests into three basc performance levels. From
worst to best they are:
  * non-cacheable: every request goes all the way to the origin and
every response comes back in full from the origin.
  * validation (stale client cache): every request goes all the way to
the origin, only a revalidate response (no body data) is transmitted
origin->Squid, full body squid->client.
  * validation (fresh client cache): every request goes all the way to
the origin, only a revalidate response (no body data) is transmitted
origin->Squid->client.
  * cachable: requests only go as far as Squid. Serviced out of cache
with full body data.
  * cachable revalidate: requests only go as far as Squid. Serviced out
of cache with only a revalidate response (no body data) transmitted
Squid->client.

Across these levels Squid speed varies from hundreds of request per
second (non-cacheable) up to hundred of thousands per second
(cache+validate).

Squid caching types complicate things even further, by being extremely
fast (RAM cache) or variably slow (disk cache) on retrieval I/O cycles
and thus time to push out to the client for the latter 4 cases.

Amos
Received on Tue Apr 24 2012 - 12:37:08 MDT