Conditional backend requests on repository branch experimental-ims

[Geoff Simmons]

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On 6/9/11 2:13 AM, Paulo Paracatu wrote:
> 
> Yesterday I finally had the time to download this and make the RPM to
> install in my caches servers.

Thanks for trying it out, and sorry for the late response. The
experimental-ims branch in the repository is now up to date with the
current master branch, including the 3.0 release.

> *      252531       416.87       344.05 fetch_304 - Fetch no body (304)*
> 
> Ah, before the patch I was hitting much more RX than now, so I guess it
> is really working and helping.
> 
> If you have any tips, I'd be glad to test.

Since a few people have asked how to make sure that the conditional
requests are working, I added a section about verification to the Wiki page:

http://www.varnish-cache.org/trac/wiki/BackendConditionalRequests#Verification

Non-zero values of fetch_304 definitely indicate that Varnish has sent
conditional requests. Watching varnishlog is the best way to know
exactly what's going on.

As for testing: any exposure to a live web site will be very helpful.
All of my testing has been with varnishtest test cases and synthetic
performance tests. I haven't been able to break it for a while using
load generators for heavy load and high concurrency, but nothing beats
the real thing.

I haven't tested it with ESI, which has been improved for 3.0. Varnish
with the IMS feature should work smoothly with conditional requests for
ESI includes, but I'm not familiar with ESI and haven't confirmed that.
(I have confirmed that conditional requests work correctly with objects
that are compressed in the cache.)

The feature introduces some additional copying -- Varnish always creates
a new object from a backend fetch, and the IMS function copies the body
and headers from the stale object to the new object when a 304 is
returned. It would be good to test it with large objects to see if there
are signs of performance loss.

Thanks again for the feedback!


Geoff
- -- 
UPLEX Systemoptimierung
Schwanenwik 24
22087 Hamburg
http://uplex.de/
Mob: +49-176-63690917
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