Those gray bars indicate the Major and Minor GC Time.
That is how much time in milliseconds it took to perform the major and minor GC.
Good value for minor GC is below 5 sec and major is below 20-60 sec. (Controversial statement! It depends upon application ergonomics!)
Thanks Rajesh. Although I'm not totally convinced that the GC measures in your example correlate to those in my screenshot. As you can see in mine there is no legend reference to the gray bars and they appear much lighter in shade. Also, GC activity is in blue.
If you hover on the gray areas (I hesitate to call them bars since they always occupy the full Y-axis) I think you will see there is no tool tip pop up like you get for the actual bars. Also, notice on the Survivor chart, there is no Y-axis on the right, which further confirms this is a lack of data rather than a bar chart showing GCs. Also, I don't think we usually (ever?) show GC on the Survivor chart. Also, I have a session file that has missing data for the beginning of the time period and it shows this with a light gray area on the charts. For these reasons, I think it is due to lack of data.
Of course, it would be really nice if someone familiar with the code could confirm this as I am just using logic to refine my guesswork as I have found no documentation yet that explains it.
Thanks all for your responses on this topic. It seems you are correct Dave, support confirmed that the gray bars indicate that there is no data from the agent for that time period. So we can rule out GC cycle activity.
However, this doesn't explain why i'm seeing passing transactions for the same periods of gray.
Fyi support also confirmed this is not documented anywhere.
@Dave: I agree, going by my dashboards i don't believe we ever see GC on the Survivor charts.
My guess is the specific GC measures were either unavailable or dropped during the time periods with the gray areas. You could check agent logs and the skipped events in the server health dashboard if you want to track it down maybe.