- From: Steve Souders <steve@souders.org>
- Date: Wed, 17 Dec 2014 21:53:38 -0800
- To: Ilya Grigorik <igrigorik@google.com>, Patrick Meenan <pmeenan@webpagetest.org>
- CC: Peter Lepeska <bizzbyster@gmail.com>, Nic Jansma <nic@nicj.net>, Yoav Weiss <yoav@yoav.ws>, public-web-perf <public-web-perf@w3.org>
- Message-ID: <54926BE2.6080705@souders.org>
Hi, Ilya.
The use cases were CDNs, RUM providers, and website owners using
Resource Timing's duration to measure (what they thought was) download
time of resources. In fact, one of the RUM providers (Buddy from SOASTA)
did a preso at WebPerfdays showing code to track "duration" and captured
it in a property called "downloadtime" - so everyone in that audience
now things "duration" means "download time". Bummer!
For the (2b) case (different origin & you don't control it so can't add
TAO header), you're right that sometimes there's no action the website
owner can take. For example, if the Twitter widget loads other scripts &
images dynamically, there's not much the website owner can do. But there
are *numerous* situations where the timing of (2b) content IS
actionable. If the website owner was able to distinguish blocking time
from download time they'd be able to make the right decision and take
action. For example:
- fonts - These are blocking the page from rendering. If it's
because the fonts are slow to download, then I might want to switch font
providers. If it's because of blocking, then I might want to preload or
prefetch the fonts.
- ads - I moved the ad in my page and clickthroughs dropped off
significantly. Is that because the ad content is blocked or slow? Or
something else?
- JS libs - I might want to find out if
https://code.jquery.com/jquery-2.1.2.min.js is loading slow on my site
because it's blocked or just slow to download. Again, there are many
actions the website owner can take - load it async, prefetch it, host it
locally, get it from Google CDN.
Choosing a name is hard because I assume we do NOT want to reveal
whether the object was read from cache for cross-origin resources. Thus,
"networkDuration" could actually not involve any network requests at
all. I thought about calling it "loadtime" since that covers loading it
over the network or from cache. Again, I'm not insistent on
"networkDuration" and would love better name brainstorming.
I see that a new draft of Resource Timing came out today. Darn. How do
we get this added to the next draft?
-Steve
On 12/4/14 9:13 AM, Ilya Grigorik wrote:
> On Mon, Nov 24, 2014 at 4:34 PM, Steve Souders <steve@souders.org
> <mailto:steve@souders.org>> wrote:
>
> LONG: A few weeks ago I discovered that "duration" includes
> blocking time, so "duration" is greater than the actual network
> time needed to download the resource. Since then I've been at
> Velocity and WebPerfDays where many people have shown their
> Resource Timing code. Everyone I spoke to (~5 different teams)
> assumed that "duration" was just the network time. When I explain
> that it also includes blocking they were surprised, admitted they
> hadn't known that, and agreed it is NOT the metric they were
> trying to capture.
>
>
> Steve, can you elaborate on the use case a bit more? Who's measuring
> what here, and for what purpose? Are we benchmarking CDN performance?
>
> In terms of getting access to the data, we have the following cases:
> 1) same origin resources: full access to timing data.
> 2) different origin:
> a) if you control it, add TAO header for full access to timing data.
> b) if you don't control it, you only have "duration"
>
> For (1) and (2a), I can see why you may want or need to get low-level
> "network duration" data: you want to track your provider's DNS
> performance, latency to your CDN, TTFB, total response time, and so
> on. You care about this because this is something *you can affect*.
> However, for (2b)... this same data falls into interesting but not
> actionable bucket? Further, it seems like if you are actually
> interested in benchmarking your CDN, then you really should be looking
> deeper than just total time: you want to decompose DNS, TCP, TLS, HTTP
> req>resp cycles. At which point.. you need the full timing object anyway.
>
> I propose we add a new property to Resource Timing that reflects
> the time to actually load the resource excluding blocking time.
> I'm flexible about the name but for purposes of this discussion
> let's call it "networkDuration". The important piece of this
> proposal is that "networkDuration" should be available for all
> resources, similar to "duration". In other words, it should be
> available for same origin as well as cross origin resources as
> part of the PerformanceEntry
> <http://www.w3.org/TR/performance-timeline/#performanceentry> interface.
>
>
> Note that "blocking time" is a thing of the past for SPDY and HTTP/2,
> as this demo demonstrates really well: http://www.httpvshttps.com/
>
> I'm skeptical of above definition: if you want "network duration", you
> should also exclude cache time; it's a computed metric that you can
> access today with TAO and a redundant one with http/2; if you really
> care about "network duration" you should probably decompose it
> further, but at that point it becomes a conversation about removing
> the TAO restriction.
>
> ig
>
> P.S. "networkDuration = dns + tcp + waiting + content" ... don't
> forget the https handshake!
>
> On Wed, Nov 26, 2014 at 9:01 AM, Patrick Meenan
> <pmeenan@webpagetest.org <mailto:pmeenan@webpagetest.org>> wrote:
>
> Would be great to see it either as a high-level duration or as an
> unblocking of the redirectStart time for cross-origin (though it
> may still not be clear to people that that is the time they really
> care about).
>
> I expect the current logic was the easiest and didn't require any
> privacy reviews because it's quite literally the exact same detail
> that you get if you do it manually in javascript by creating an
> element and listening to the onload. Even if the more-granular
> detail doesn't really expose anything you couldn't figure out
> before it does provide additional detail that wouldn't otherwise
> be measurable and is probably going to require reviews by privacy
> and security teams.
>
> On Wed, Nov 26, 2014 at 9:36 AM, Peter Lepeska
> <bizzbyster@gmail.com <mailto:bizzbyster@gmail.com>> wrote:
>
> +1
>
> On Tue, Nov 25, 2014 at 12:31 PM, Nic Jansma <nic@nicj.net
> <mailto:nic@nicj.net>> wrote:
>
> Good point! Hadn't considered that, so yes I would agree
> it's a very valuable addition to consider.
>
> As far as what interface to put it on, I'm not sure
> networkDuration would make sense for UserTiming, for
> example. While it could sit on PerformanceEntry and just
> be "0" for interfaces that aren't applicable, we could
> also create a PerformanceNetworkEntry interface (with
> networkDuration) that PerformanceResourceTiming inherits
> from, while PerformanceUserTiming only inherits from
> PerformanceEntry.
>
> That's all minor details though. Really depends on the
> browser privacy teams OK'ing the addition.
>
> - Nic
> http://nicj.net/
> @NicJ
>
> On 11/25/2014 12:16 PM, Steve Souders wrote:
>> Nic -
>>
>> You can *not* calculate networkDuration from other
>> attributes for *cross-origin* resources. That's why I'm
>> suggesting adding this to PerformanceEntry (rather than
>> PerformanceResourceTiming).
>>
>> And as mentioned, about 50% of resources are cross-origin
>> so it's important to provide a means for *accurate*
>> download time measurements.
>>
>> -Steve
>>
>>
>> On 11/25/14, 8:02 AM, Nic Jansma wrote:
>>> Steve,
>>>
>>> The only downside I see is that we're adding a new
>>> attribute that can be entirely calculated via other
>>> attributes.
>>>
>>> One alternate (or additional thing) would be to
>>> highlight this point in the description for "duration"
>>> in the spec.
>>> - Nic
>>> http://nicj.net/
>>> @NicJ
>>> On 11/25/2014 3:04 AM, Yoav Weiss wrote:
>>>>
>>>> On Tue, Nov 25, 2014 at 1:34 AM, Steve Souders
>>>> <steve@souders.org <mailto:steve@souders.org>> wrote:
>>>>
>>>> SHORT: I propose we add the "networkDuration"
>>>> property to PerformanceEntry
>>>> <http://www.w3.org/TR/performance-timeline/#performanceentry>
>>>> objects.
>>>>
>>>> LONG: A few weeks ago I discovered that "duration"
>>>> includes blocking time, so "duration" is greater
>>>> than the actual network time needed to download the
>>>> resource. Since then I've been at Velocity and
>>>> WebPerfDays where many people have shown their
>>>> Resource Timing code. Everyone I spoke to (~5
>>>> different teams) assumed that "duration" was just
>>>> the network time. When I explain that it also
>>>> includes blocking they were surprised, admitted
>>>> they hadn't known that, and agreed it is NOT the
>>>> metric they were trying to capture.
>>>>
>>>> I propose we add a new property to Resource Timing
>>>> that reflects the time to actually load the
>>>> resource excluding blocking time. I'm flexible
>>>> about the name but for purposes of this discussion
>>>> let's call it "networkDuration". The important
>>>> piece of this proposal is that "networkDuration"
>>>> should be available for all resources, similar to
>>>> "duration". In other words, it should be available
>>>> for same origin as well as cross origin resources
>>>> as part of the PerformanceEntry
>>>> <http://www.w3.org/TR/performance-timeline/#performanceentry>
>>>> interface.
>>>>
>>>> Same origin resources can calculate
>>>> "networkDuration" as follows (assume "r" is a
>>>> PerformanceResourceTiming
>>>> <http://?ui=2&ik=b493d86064&view=att&th=149e4608a5dad0d6&attid=0.1.1&disp=emb&zw&atsh=0>
>>>> object):
>>>>
>>>> dns = r.domainLookupEnd - r.domainLookupStart;
>>>> tcp = r.connectEnd - r.connectStart; //
>>>> includes ssl negotiation
>>>> waiting = r.responseStart - r.requestStart; // aka
>>>> "TTFB"
>>>> content = r.responseEnd - r.responseStart;
>>>> networkDuration = dns + tcp + waiting + content;
>>>>
>>>> I've discussed this with a few people and the only
>>>> concern I've heard is with regard to privacy along
>>>> the lines of "if we exclude blocking we've added
>>>> the ability to distinguish cache reads from network
>>>> fetches". This isn't an issue for two reasons:
>>>>
>>>> 1. Even with the exclusion of blocking time, it's
>>>> still possible for "networkDuration" to have a
>>>> non-zero value for resources read from cache
>>>> due to disk access time, etc. Therefore,
>>>> excluding blocking time does not necessarily
>>>> provide a clear means of determining resources
>>>> read from cache.
>>>> 2. This concern assumes that adding
>>>> "networkDuration" lessens privacy because
>>>> removing blocking time provides additional
>>>> information that is not available today.
>>>> However, it's possible to exclude blocking time
>>>> today by loading a cross-origin resource after
>>>> window.onload, when there is no blocking
>>>> contention.
>>>>
>>>> Therefore, individuals who have JavaScript access
>>>> to a page and can measure duration also have enough
>>>> access to load resources after window.onload and
>>>> can thus determine the duration excluding blocking
>>>> time. Adding "networkDuration" does not give these
>>>> individuals additional information beyond what is
>>>> measurable today.
>>>>
>>>> What "networkDuration" provides is additional
>>>> information for the normal case of resources that
>>>> are loaded as part of the main page when blocking
>>>> contention may occur. This will give current web
>>>> developers the metric they want for cross-origin
>>>> resources, and will provide it more simply for same
>>>> origin resources.
>>>>
>>>>
>>>> Assuming that the privacy concerns are in fact
>>>> non-existent, a big +1.
>>>>
>>>
>>
>
>
>
>
Received on Thursday, 18 December 2014 05:54:16 UTC