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Author Topic: Interleaved for no reason at all?  (Read 6617 times)

roseway

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Re: Interleaved for no reason at all?
« Reply #15 on: August 30, 2015, 11:19:59 AM »

Quote
Can a web user actually notice a difference in responsiveness due to latency?

A lot of modern web pages make multiple calls to different URLs to load pictures and other data, so a few tens of milliseconds added to each call can add up to a noticeable delay.
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  Eric

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Re: Interleaved for no reason at all?
« Reply #16 on: August 30, 2015, 11:35:12 AM »

@Roseway - of course yes you're right, lots of requests. Is the interleave on VDSL2 much wider than on old-fashioned ADSL? How fast is les-70's pipe?

I would have thought modern web browsers let you issue the second and subsequent HTTP requests in parallel? Originally web browsers were only supposed to issue two requests max in parallel, iirc, but I thought that that had been ignored for a long time now. The dependency could be something like two deep though, html references css, references image, or css references more css through an @import or whatever, it's a long time since I looked at this stuff.

I'm just a bit surprised. There's a lot of good stuff on yahoo's website about page load times and if I remember correctly multiple DNS lookups were performance killers.
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Weaver

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Re: Interleaved for no reason at all?
« Reply #17 on: August 30, 2015, 11:40:25 AM »

If I were designing the web now, first time, there would just be a single zip file delivered  containing a whole web page and also all dependent components using multipart mime, like an email. Later fetches for a changed page would leave out the static components that had already been delivered, to let the client request them if it had cleared its cache or something.

I can't remember, is SPDY a bit like this?
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kitz

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Re: Interleaved for no reason at all?
« Reply #18 on: August 30, 2015, 12:53:17 PM »

Lag, delay and latency is sometimes weird.

For example when the plusnet thingy or MSIL problem is occurring, then I do notice.  Pages can be agonising slow to load, yet a speed test will show 20mbps and you'd think that should be quick enough to load a page.  I don't usually perform speedtests and it's the slow loading of pages that usually lead me to doing them.

Re gaming latency.  The argument is that a couple of milliseconds is faster than the blink of an eye and faster than most human reactions.

The counter argument is that if all things were equal for distance and connection speed bar the fact that you had low level interleaving are.....  If your reactions were quicker and you pulled the trigger 5ms before your opponent, you'd still be the one who got shot in the game.


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Should say those arent my arguments, but are typical of the long running debate which has been ongoing since BTw introduced interleaving about 10yrs ago.
 
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Chrysalis

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Re: Interleaved for no reason at all?
« Reply #19 on: August 30, 2015, 12:57:20 PM »

Can a web user actually notice a difference in responsiveness due to latency? I wouldn't have thought that's the kind of application where you could perceive such a small time difference of a few tens of milliseconds. But then if you can, you can.

answer is yes.

The difference caused by base latency tend to be actually higher than the base latency difference due to how things work.

e.g. lets say a web page needs 20 dns lookups (more common than you think).
lets say each lookup needs latency to dns server + say 30% overhead, that might mean say 13ms on a fast path connection so 260ms waiting for lookups, or 25ms on a light interleaved connection meaning 500ms waiting, or say 50ms on a heavily interleaved connections so 1000ms waiting, hope you get the idea.  Thats also only the tip of it, as there is latency to establish connections to the web server as well.

Finally due to how tcp buffering works, latency also affects throughput and how quick that throughput ramps up.

Browse a website hosted in asia and you should see a very noticeble difference all down to the latency.

Sites hosted in eastern europe might give an idea of the affect of interleaving more realistically.
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Weaver

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Re: Interleaved for no reason at all?
« Reply #20 on: August 30, 2015, 01:08:29 PM »

Thing is, I thought a lot of web page load time things were all about bad web server config and bad webpages that are caching-unfriendly, with no correct cache control headers, no etags, date stamps or even no-cache directives all help to slow things down. Lots of DNS lookups are another example. Even though pipes have got wider, some of these crapness factors have not changed since the days of dial-up. The modern trend of having dynamic, generated webpages for no earthly good reason doesn't help either. Web servers up to be generating pages than storing them then serving up the results are static pages unless they actually need to be changed and recalculated it's possible to do this by putting proxy caches in front in front of the origin server or by using caching modules inside programming environments.

So many websites are painfully slow simply because sysadmins are bought cheap and server software stacks are badly configure or outdated. I've seen a number of shockers recently where using the back button within the site is completely painful for no reason at all.

Since DSL links have got 10 times faster, for the haves at least, then latency due to interleave should have gone right down. But no wait, perhaps my logic is wrong, perhaps the length of time of an interleave period should have to remain the same so that it covers the time length of a noise burst and then some, regardless of what the data rate is?
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Weaver

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Re: Interleaved for no reason at all?
« Reply #21 on: August 30, 2015, 01:15:11 PM »

@Chrysalis - I realise now that the multiplication problem is the key. About the number of DNS lookups,  I thought that say eight or so ought to occur in parallel so that means three lots of latency for 20 lookups.
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Chrysalis

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Re: Interleaved for no reason at all?
« Reply #22 on: August 30, 2015, 01:18:55 PM »

possibly 'some' may be done in parallel depending on the browser and how page is laid out.  But never all in parallel as the extra hostnames arent even known by the browser until the page is processed.

Also now days many websites deliberately avoid keepalive so you get the overheads of establishing connections, e.g. a web page with lots of images may deliberately split those images over multiple hostnames, which again penalizes higher latency.

http 2.0 improves a lot of this stuff but is years away yet from mass adoption.

also some browsers adopt things like dns prefetching (lookups done for all links in background), and speculative connecting (doing all http connections in background for links on page).  Both practices are frowned upon by server admin's as it creates extra server load, in addition they can be considered anti privacy.  But these are thing browsers are doing to try and speed up responsiveness.

Also for about 6 months in 2014 firefox were doing 2 dns lookups per hostname to get round an issue they had, this is now fixed tho after the bug report was flooded by angry server admin's :) was made even worse by the fact the 2nd dns lookup was never cached.
« Last Edit: August 30, 2015, 01:27:48 PM by Chrysalis »
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Weaver

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Re: Interleaved for no reason at all?
« Reply #23 on: August 30, 2015, 01:44:23 PM »

There's a name for this technique of splitting a set of images across multiple domain names as mentioned on the Yahoo website, but I forget. I think the idea is to get another allowance of so many HTTP connections per hostname. Yahoo recommend this trick as a good design technique.
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loonylion

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Re: Interleaved for no reason at all?
« Reply #24 on: August 30, 2015, 02:22:10 PM »

There's a name for this technique of splitting a set of images across multiple domain names as mentioned on the Yahoo website, but I forget. I think the idea is to get another allowance of so many HTTP connections per hostname. Yahoo recommend this trick as a good design technique.

it's called pipelining. The reason it's done is because browsers by default can only have two simultaneous connections per hostname, so while those two connections are downloading large files (i.e images) they're blocking the rest of the page content from being downloaded. By offloading images onto separate hostnames, the initial two connections are free to continue downloading html, css and script files while the images are being downloaded, and having multiple hostnames means image heavy pages can download more than two images at once.
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Chrysalis

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Re: Interleaved for no reason at all?
« Reply #25 on: August 31, 2015, 01:27:06 AM »

yeah google recommend the same but I actually disagree with both of them.

That recommendation was started in the era when browsers didnt do keepalive, keepalive is faster than bombarding servers with dozens of connections.

However of course many web servers do disable keepalive, so when keepalive is off it can help.
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loonylion

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Re: Interleaved for no reason at all?
« Reply #26 on: August 31, 2015, 02:16:27 AM »

yeah google recommend the same but I actually disagree with both of them.

That recommendation was started in the era when browsers didnt do keepalive, keepalive is faster than bombarding servers with dozens of connections.

However of course many web servers do disable keepalive, so when keepalive is off it can help.

keepalive just prevents the connection being closed, it doesn't address the two connections per hostname issue or the fact that downloading images blocks other content.
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Chrysalis

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Re: Interleaved for no reason at all?
« Reply #27 on: August 31, 2015, 03:02:16 PM »

keepalive allows for continuous stream of requests to be made without the overhead of any new dns lookups or establishing connections.
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Weaver

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Re: Interleaved for no reason at all?
« Reply #28 on: August 31, 2015, 10:29:30 PM »

Keepalive avoids slow-start.

I was under the impression that some web browsers now completely ignore the low limits on the number of HTTP connections?
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