CPU Mismatch between Tagged Web Request and its purepaths

Vaibhav,

I'm don't understand the distinction you're making between "a specific tagged web request" (first paragraph) and "each tagged web request" (second paragraph).

Could you include a screenshot or two to illustrate the problem?

-- Graeme

Hi Graeme, I work with Vaibhav (different time zone).  Here are some more details.  We're using jmeter to push some traffic to our app, and we tag the requests in jmeter.  So here's what the TaggedWebRequest dashlet looks like after we've stored our results.  We ran 20 loops of this particular test.. We'll focus on the ADC_CardActivity transaction.  You can see from this that dynaTrace is telling us that the avg / total CPU for this web request is 103.73 / 2074.59.  

If I right-click this line and choose to drill down into purepaths, I see the following.  Here are our 20 purepaths. Just eyeballing the CPU numbers, they are about 300ms on average, which is of course much higher than we saw in the TaggedWebRequest dashlet (the actual CPU total for all 20 purepaths is about 6109ms).  If I drill down from here into the Methods dashlet, the CPU numbers agree with the purepaths.

So it would seem that the purepaths include a lot of activity over and above what is in the web requests.  But we can't figure out what that is.  If I select all the purepaths and then drill down into the WebRequest dashlet, I see numbers that agree with the TaggedWebRequests dashlet.

I can't drill down from WebRequests into Methods, so I can't compare that way.  Just in case, I added a filter to the Purepath dashlet to make sure we're limiting the display to a single Agent, but that made no difference.  We're stumped.

Vaibhav and Glenn,

Thanks for the additional data.  The CPU sum shown in the Tagged Web Requests dashboard is the CPU sum of the first tier, not the whole PurePath.  You can verify this by drilling down to PurePaths, selecting them all and then drilling down to Agent Breakdown.

If you can verify this on your system, I'll see if I can chase down whether this is a bug or works-as-designed.

-- Graeme

Glenn,

Is there any chance you can send us a PurePath so we can take a look at it?

If not, you might consider opening a support ticket.

-- Graeme

Here's a PurePath  APT20_ALL_CAVS [2015-07-20 17-45-45].dts

Glenn,

The Tagged Web Request dashboard is taking the CPU sum value from the CPU total of the first thread.  You can see what's going on in the PP dashboard:

This is correct – Dynatrace is pretty consistent is using "total" to mean "including children" and "sum" to mean "all threads".  You can find the remainder of the PP CPU sum in the other threads lower down in the PP.

The question is, why is the Tagged Web Requests dashboard displaying the value for the first thread?

Could someone from R&D say whether this is a bug or works-as-designed, please?

-- Graeme

Ok, this is interesting.  The PurePath contains 3 asynchronous threads, which are included in the Purepath's CPU Sum, but not in the Web Request's (or Tagged Web Request's) CPU Sum.  This is great information...it explains the sporadic differences we have been seeing.  Maybe the CPU Sum column for the Web Request and Tagged Web Request dashlets should be renamed to CPU Total?

Glenn,

It's a bug.  If you'd like a patch, you'll need to submit a support ticket.

Otherwise, you can wait for the fix to be rolled out in a regular release.  If you post here in a week or so, I can probably let you know when.

-- Graeme

Thanks, Graeme.  There's no urgency, now that we understand the issue.  What exactly is the fix?  Is it just labeling, or the calculation itself?

Glenn,

I expect they'll fix the calculation – there's no precedent for displaying the current value anywhere other than the PP tree.

The issue also applies to the ordinary Web Request dashboard, so you got a twofer!

-- Graeme