Blocking is not showing correctly

I see in the performance monitor multiple blocked sessions. I click on Current and see the currently blocked sessions have a number. I click on the link show blocked sessions but when it goes to that page it shows that no sessions are blocking.

When I go to SSMS and check, I find that there are indeed blocked sessions, but DPA is not displaying the information under sessions.

blocked sessions.png

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brianflynn

I just spent a bit talking to support and architecture. Don't have a sure answer yet. Could you open a ticket with support so we can start gathering more info?

Thanks!

brianflynn

One theory... Since you're able to identify the blocking processes in SSMS, are all of the SPIDs under 50? If so, those are system spids which are filtered from some views. That might explain this.

cwong

There were multiple processes and none of the SPIDs were under 50. These were actual connections from the clients to a particular DB. I will open a ticket so you can get more info.

spaceman

Hi, We're seeing a similar issue..

In our case whenever this occurs the cause appears spids blocked by themselves (parallel processing threads). It appears to be inconsistent in the GUI if this is case - and (personally) i'd rather they were ignored on the currently blocked sessions area of Current activity as it's normal behaviour.

(i think the self blocking spids are also included in Blocked Sessions graph)

brianflynn

Hey spaceman & cwong,

I spoke to our support team manager. He says he thinks these things can be timing/refresh issues. The currently blocked sessions don't update instantly. It only updates when the current page refreshes. So it could say 2 blocking sessions for the interval between refreshes when in reality when they click on blocking, it will do a real-time ad hoc query to look, then see nothing.

If you feel this is not what's happening for your case, I encourage you to open a support case. We'd love to get a look at this and it may require more information than we should share on a public forum.

spaceman

Thanks Brian, i've run a script checking what i consider to be blockers polling every second for a minute and refreshed DPA several times in the same window... DPA showed various numbers blocked but these weren't reflected by my script.

I'll raise a case