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Congestion Window issues
09-11-2014, 12:06 AM
Post: #3
RE: Congestion Window issues
The test differences they are seeing largely come down to the connectivity. The catchpoint tests are running directly on backbone network links (gigabit with zero latency) which is why they are so much faster.

The webpagetest result is running with a Cable connection profile which limits bandwidth to 5Mbps (and introduces some latency). If you look at the bandwidth chart at the bottom of the waterfall, the bandwidth is pegged at the full 5Mbps while downloading the images so it's not really an origin delivery problem but a problem in trying to get the image bytes through the pipe.

You can also run the WPT tests with no traffic shaping (connectivity selection under advanced settings): http://www.webpagetest.org/result/140910_0R_KDK/ and the images all load really quickly.

At that point the site is mostly CPU constrained and running on the Dulles Thinkpad machines which are a lot faster hardware comes in right around the 500-600ms that they see in Catchpoint: http://www.webpagetest.org/result/140910_AP_KFP/

I didn't look closely enough to see if the congestion window would let you ramp up to the full bandwidth a few round trips faster but at best you're talking < 100ms overall on the images because the parallel loading is saturating the link pretty quickly.

With zero latency they also won't see any impact from congestion window settings so the Catchpoint testing isn't really going to show anything one way or another.
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Messages In This Thread
Congestion Window issues - ewinsmi - 09-04-2014, 11:43 PM
RE: Congestion Window issues - ewinsmi - 09-06-2014, 12:57 AM
RE: Congestion Window issues - pmeenan - 09-11-2014 12:06 AM

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